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GEOG 3: Contemporary Geographical Issues
C
hapter 1: Weather and Climate Associated Hazards
1. Introduction: Why are the British obsessed with the weather?
The nature of the climatic region that we inhabit as human populations plays a large part of determining
lifestyle. It affects the activities we can undertake, when we do them, the food we
eat, the clothes we wear, how we travel the sports we play and the activities we
chose to pass our leisure time, all of which in part allow us to define specific cultures.
The British are renowned for discussing the apparently ‗unpredictable‘ and
changeable nature of British weather that we witness on a daily basis. So much is our
obsession with the weather, it has become a British pastime that appears odd to the
rest of the World who Inhabit different climate zones. It is no surprise the reason
for this obsession, no other country in the world has such varied weather conditions,
with the same power to rule people‘s lives as we have. Therefore the real question
the rest of the world should ask is “Why is the British weather so unpredictable and changeable in
nature?” not why are we so obsessed with it! In this part of the module you will find out the answer to
this question and be enlightened (hopefully) by the other questions that it will allow you to answer!
Public awareness and interest in the climate has increased dramatically over the last 30 years as has our
ability to understand the atmosphere and predict weather patterns using computer models which allow
us to suggest future trends. However climate is complicated and chaotic in nature and its link to
weather phenomena is poorly understood at the moment, making long term predictions difficult. Issues
such as anthropogenic (man made) climate change may pose in the future to be one of the chief threats
to the well being of the planet and to society, therefore the issue deserves its newsworthy status and
relevance to day to day lives!
Key Terms:
‗Climate change should be seen as the greatest challenge to face
Weather: The state of the atmosphere at a
man and treated as a much bigger priority in the United Kingdom
Prince Charles‘
particular point at a specific time. Weather can
be described in terms of temperature,
precipitation, wind speed, wind direction, cloud
type, humidity and visibility.
Climate: the mean atmospheric conditions of
2. Major Climate Types
an area measured over a substantial period of
time. Different parts of the world have
recognisable climate characteristics with
distinctive seasonal patterns.
Climate is defined as an area‘s long term weather pattern. The link between climate and weather
conditions is complicated but good examples of long term weather phenomena observed in different
climates would be: Precipitation, temperature, hours of sunshine, wind speed etc… There are
numerous types:
Polar = Cold + Dry
Continental = Cold + Humid
Temperate = Mild + Humid
Dry or Arid = Hot Deserts
Tropical = Hot + Humid
Mountainous = Semi
Arid or Alpine: Cold
Winters, mild summers
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3. Major
Climatic
Controls
Key Terms:
Atmosphere: the mixture of transparent gases that surround the earth and are held in place
by gravity. It contains mainly Nitrogen (78%) and Oxygen (21%) as well as other minor gases
such as carbon dioxide, methane, water vapour, argon and other traces.
Humidity: the amount of water vapour in the atmosphere which varies with latitude, virtually
zero at poles but can be over 5% at tropics. It is dependant mainly on temperature the warmer
the air the more water vapour it can hold. Absolute humidity is total amount of water vapour in
the atmosphere measured in g/m3. Relative Humidity is defined as the actual vapour content
compared to the amount that the air could hold at a given temperature or pressure expressed
as a percentage.
(a) The Structure of the Atmosphere
The atmosphere is a pocket of odourless and colourless gas held to the earth by gravitational attraction,
the general accepted limit of the atmosphere by convention is 1000Km. Most of the atmosphere and
therefore the climate it controls is set within the upper 16km of the atmosphere from the Earth‘s
surface at the equator and 8km at the poles. Roughly half of the atmospheres mass lies just 6km from
sea level and 99% within 40km from sea level. Atmospheric pressure decreases rapidly with altitude
(you will get to observe this phenomena when you see how much a bag of crisps expands by at the top of
Mt. Etna during your second year trip), remember that mountaineers find it very hard to get a hot cup
of tea because water boils at 72 oc on Everest. Weather balloons and have been used to work this out
for pressure but for temperature recent satellite imaging shows a more complex change with altitude.
The change with altitude is used to divide the earth up into four layers.
The Troposphere: The bottom most of these is the Troposphere, temperatures here decrease by about
6.4 oC with every 1000m increase in altitude (environmental lapse rate) from a starting average
surface temperature of about 16 oC. This is because the atmosphere is warmed at the ground surface by
incoming solar radiation first heating the ground and this latent heat is then conducted to the overlying
atmosphere above, so the higher you are away from the ground surface the less the warming by conduction
and the colder it will be. The troposphere is an unstable layer containing most of the World‘s water vapour
and therefore clouds as well as dust and other particulate pollution. Wind speeds usually increase with
height and as mentioned earlier pressure decreases with height due to decreased gravity. At about 12km
from the Earth‘s surface the Tropopause is reached, this is the boundary that represents the
limit of the Earth‘s climate and weather systems. In the tropopause the temperatures remain the same
(about -60) despite any increase in height (this is how you know you have reached it), this phenomena is
termed an isothermal layer (simply meaning equal temperature). Jet aircraft cruise at approximately 9000m
just before the Tropopause is encountered.
The Stratosphere: The next layer is the Stratosphere, it is characterised by a steady increase in
temperature which is owed to the high concentration of atmospheric ozone (O3) which allow the
absorption of UV solar radiation from the sun but prevents some of this radiation being reflected back
to space causing warming. Winds are light in the lower layers of the stratosphere but increase with
height, pressure continues to fall and the atmosphere is much drier! The stratosphere like the two
layers above it act as a protective shield to meteorites that ‗burn up‘ before reaching its lower reaches.
At approximately 48km from the surface another isothermal layer (usually c- 25 oC) termed the
Stratopause is reached.
The Mesosphere: The next layer is the ‗middle layer‘ or Mesosphere, characterised by rapidly falling
temperatures once the upper limits of the Stratopause are left behind. The reason for the falling
temperatures is because there is no water vapour which has a warming effect and because there is no
cloud, dust or ozone to absorb incoming radiation and provide an insulating effect. The mesosphere
witnesses the lowest temperatures of up to -90oc and unimaginably strong 3000km/h winds! There is
another isothermal layer at 80km height, the top of the mesosphere known as the mesopause where
there is no change in temperature with altitude and the temperatures here can also be as cold as -90 oC.
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Thermosphere: The uppermost layer is known as the Thermosphere because it is a layer where the
temperatures rise very rapidly with height perhaps to reach 1500 oC. This is due to the increase in the
proportion of atmospheric oxygen (O2) which like ozone (O3) absorbs atmospheric UV radiation.
The Vertical Structure of the Atmosphere
(b) The composition of the Atmosphere
The atmosphere is composed of a mixture of gases mainly but it contains some liquids and even some
solids held nearer to the surface by gravity. The composition of the atmosphere is relatively constant
in its lower reaches, 10-15km or so where it can vary in its spatial occurrence over time and thus cause
fluctuations in temperature, pressure and humidity, thus affecting weather and climate!
The
atmosphere is in hot debate especially as scientists are currently trying to find out the extent to which
man‘s release of CO2 is linked to recent global warming (see climate change notes) observed in the geological
record as well as the issue of the hole in the ozone layer (not to be mixed up with global warming)
caused by the release of CFC‘s.
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Table showing data on composition of the atmosphere
Gas
Nitrogen
Permanent
Gases
Oxygen
Percentage by
Volume
78.09
20.95
Importance for Weather and Climate
0.2-4.0
0.03
Source of cloud formation. Reflects as well as
absorbs solar radiation keeping temperatures
constant. Provides majority of Earth‘s natural
‗greenhouse effect‘.
Absorbs long wavelength solar radiation and therefore
increases global temperatures i.e. adds to natural
greenhouse effect. Human activity releases carbon
dioxide (anthropogenic CO2) which is a major cause of
climate change.
0.00006
Absorbs incoming UV radiation
0.93
trace
No importance
trace
Absorbs/reflects atmospheric radiation e.g. volcanic
eruptions can cause cooling if dust is released into
upper atmosphere. Dust is import for cloud formation
as it forms condensation nuclei.
Affects levels of incoming solar radiation and is a
cause of acid rain.
Water
vapour
Variable
Gases
Carbon
Dioxide
No effect mainly passive
Ozone
Inert Gases
Non Gases
Pollutant
Gases
Argon
Helium,
Neon,
Krypton
Dust
Sulphur
dioxide,
nitrogen
oxide,
methane
trace
Other planetary
functions/source
Plant growth
Needed in respiration
Produced by
photosynthesis
Decreased by
deforestation
Essential for life. Can
be stored as ice/snow
Used by plants during
photosynthesis, is a
greenhouse gas that
causes warming. It is
increased by burning
fossil fuels and
deforestation
Shields animals and
plants from deadly suns
rays. Destroyed by
chlorofluorocarbons
(CFC‘s)
No use. Volcanic and
meteorite dust as well
as from soil erosion
No use the source is
industry, power
stations and car
exhausts
(c) Atmospheric heat budget (Energy in the Atmosphere)
The Earth‘s primary heat source is from the sun where it receives energy as incoming short-wavelength
radiation (insolation). It is this energy that controls our planet‘s climate and associate weather systems
which in turn control the amount of energy that the primary producers (plants) convert to stored energy
during photosynthesis. There are other sources of heat energy however, some comes from deep within
the planet‘s interior and is known as geothermal heat, this heat has originated from the Earth‘s
accretion 4.5 billion years ago as well as some of this geothermal energy owing its origin to radioactive
decay of unstable isotopes in the core. Other authors may class another source of heat being that
generated by urban settlements (but really this is just energy that ultimately has come from the sun and
has been stored chemically as fossil fuels or has come from geothermal sources).
Factors Controlling Solar Heating
There are four factors (astronomical factors) that control how much heating the Earth receives from
solar insolation. In summarising these factors we assume that no atmosphere surrounds the Earth as
the atmosphere can either absorb, reflect or scatter incoming solar radiation depending on the
proportion of many of its constituents (see table of composition above to see the effect of each component
on either absorption or reflection of energy) as it passes through the atmosphere.
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(i) The solar constant: is the amount of solar energy (insolation) received per unit area, per time on a
surface at right-angles to the suns beam at the edge of the Earth‘s atmosphere. Despite its name
(‗constant‘) it does vary slightly due to sunspot activity on the sun‘s surface, but this is unlikely to vary daily
or yearly weather but it may influence long-term global climate change.
(ii) The distance from the sun: The Earth‘s orbit
around the sun is not a perfect circle as you may have
drawn in science classes at school but it is in fact
more of an egg shape, this is what a geographer
describes as the eccentric orbit or eccentricity of
the orbit. This oval shaped orbit is enough to cause
6% variation in the solar constant as the sun appear
to be either closer or further away depending where
you are in the orbital cycle.
(iii) The altitude of the sun in the sky:
The equator receives more
energy than the poles as the suns energy strikes it head on and at times
exactly 90o (i.e. during the spring/vernal and the autumnal equinoxes – see
later notes.). In comparison in the higher latitudes for instance 60 oN and
60oS of the equator, the sun hits the surface of the earth at a lesser angle
(a more oblique angle) and therefore there are more atmospheres to travel
through and a greater surface area to heat up so the amount of heating in
these regions is less.
(iv) The length of night and day: The Earth is tilted on its axis at
23.5 o this controls the length of day and night (I hope you would
agree). If you are in a north of 66.5 o or in a region south of 66.5 o at
certain times of year these regions
receive no insolation at certain
times of the year.
Not all of the incoming solar radiation reaches the Earth‘s surface,
approximately half (45%) reaches the surface. So what happens to
the rest? Well a large amount of incoming insolation is absorbed by
ozone (O3), water vapour, carbon dioxide, ice particles or dust
particles which all reduce the amount that reaches the surface.
Thick cloud cover also plays a role as 10% may be reflected back to
space, similarly reflection back to space occurs on the surface itself for
example on snowfields 20% of the incoming radiation can be reflected.
The ratio between incoming radiation and the amount reflected back to
space expressed as a percentage is known as the Earth‘s albedo (not
to be mistaken with libido).
Both deforestation and overgrazing increase the Earth‘s albedo
causing less cloud formation and precipitation so desertification can
result! See below for some common factors controlling albedo.
Scattering of solar radiation occurs when gas molecules divert its
path and send it off in all directions, some will reach the surface and
this is called diffuse radiation, the scattering occurs at the blue end
of the EM spectrum and this is what causes the sky to appear blue.
As a result of absorption, reflection and scattering of the 45% of the
radiation that reaches the surface about 24% of incoming radiation
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reaches it directly and a further 21% will reach the surface by diffuse means. Once in contact with the
ground incoming radiation will heat the Earth‘s surface and the ground will radiate heat back towards
space where 94% of this energy will be absorbed (only 6% lost) by the Earth‘s greenhouse gases (water
vapour, methane and CO2). The outgoing terrestrial radiation is long wavelength or infra red radiation.
Factor
Cloud type: Thin Clouds
Thicker Stratus Clouds
Cumulo-Nimbus Clouds (Thunder Clouds)
Percentage
30-40
50-70
90
,<10
15
25
40
85
Surface Type: Oceans and Dark Soils
Coniferous Forests and Urban areas
Grasslands and deciduous forests
Light coloured deserts
Snowfields
Summary of what happens to incoming solar radiation
In order for the Earth to remain at a fairly constant temperature i.e. not to heat up or to cool down, a
state of balance or equilibrium must exist between inputs (incoming insolation) and outputs (outgoing
terrestrial radiation).
However there are significant spatial differences within the atmosphere,
although heat is lost throughout the entire atmosphere via terrestrial radiation, the heating of the
globe in the first place is unequal. It is in fact true that throughout the year the equator receives the
majority of solar insolation and from 40 oN to 35oS there is a net surplus of radiation i.e. inputs are
greater than outputs (positive heat balance) whereas at the poles there are less inputs than outputs i.e. a
net deficit of radiation (negative heat balance). This unbalanced heating of the Earth leads to some
fascinating consequences that drive the weather systems on the planet! The NET result of unequal
heating must be therefore the transfer of heat from one place to another as the Earth tries to spread
out this unequal heat. This is what drives the large and small scale atmospheric circulations (see
atmospheric circulation in later notes).
Key Terms:
Jet stream: a band of very strong winds, up to 250km/h, which occurs in certain locations in the upper atmosphere on
average 10, 000m. May be 100‘s of Km wide with a vertical thickness of 1-2000 m. They are the product of a large
temperature gradient between two air masses. There are two main locations: The Polar Front Jet where polar and
subtropical air meet over western Atlantic Ocean (c. 60o N and 60oS) and the Sub-Tropical Jet Front also westerly and
associated with the pole ward ends of the Hadley Cells (c. 25 oN and 25oS).
Airmass: a large body of air with similar temperature and humidity characteristics that it has acquired from where it has
originated over.
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The Earths Heat Budget
Horizontal Heat Transfers – 80% of heat is carried away from the tropics and is carried by winds
examples of which include the jet stream, hurricanes and depressions (see later notes on each of
these). The remaining 20% is carried by ocean currents pole wards.
Vertical Heat Transfers: Energy is transferred from the surface of the Earth vertically by radiation,
conduction and convection. Latent heat (heat expended when substances change state) also helps
transfer energy. For example evaporation of water from the ocean expends heat causing cooling
whereas condensation of water droplets which leads to cloud formation and precipitation releases heat
causing warming in the upper atmosphere. The vertical motion of air can transfer heat from areas of
high heat budget (such as the equator) by cooling of air as an air mass rises with altitude until it is
transferred horizontally by higher level flows of air transferring warm air to the poles.
Global factors affecting insolation and heating of the atmosphere
Factors that control the amount of insolation received at any point and hence the balance between incoming
and out coming radiation (heat budget), will vary considerably spatially (space) and temporally (time).
(a) Long Term Factors controlling insolation:
(i) Altitude: As discussed earlier the atmosphere is not warmed directly by the sun but by the
radiation of heat from the Earth‘s surface that can then be spread by conduction and convection.
Two things happen with altitude, there is a decreasing land area from which heat can be radiated, air is
under less pressure at altitude and the molecules in the air a therefore fewer and wider spaced per
unit area this mean air at height loses its ability y to retain heat, the phenomena known as
environmental lapse rate, 6.4oC/1000m. The opposite to this sometimes happens under high
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pressure (anticyclonic) conditions in the UK in the winter especially in the early morning where it
gets warmer with altitude, this is therefore opposite to the norm and is referred to as temperature
inversion.
(ii) Altitude of the sun: As the angle of the sun in the sky relative to the land surface decreases (or
comes more oblique) the amount of atmosphere the rays travel through and the amount of la nd area
being heated by the rays both increases causing more insolation to be lost through scattering,
absorption, reflection and radiation. Places at lower latitudes therefore have higher temperatures
than those at higher latitudes.
(iii) Proportion of land and sea: The sea is obviously more transparent than the land and is able to
absorb more heat energy to a depth of about 10m. Waves and currents can also then transfer this
heat to greater depths. The sea also has what is known as a greater heat capacity; allthis means is
that it takes more energy to raise the temperature (say an increase of one degree) of the sea than
that of the land. The specific heat capacity of water is roughly twice that of the land, therefore in
summer the ocean heats up more slowly than the continents on land but in winter the opposite is true
the continents loses its heat more quickly than the ocean, so oceans act as thermal reservoirs of
energy! This has some interesting implications, for instance have you noticed that coastal locations
always have lower annual temperature ranges (i.e. difference between highest and lowest temperatures)
compared to continental interiors which have larger temperature ranges i.e. warmer summers but colder
winters – this is known as continentality.
(iv) Prevailing Winds: The characteristics of a wind in terms of its temperature and humidity are
controlled by the type of area it passes over. A wind passing over a sea tends to be warmer in
winter but cooler in summer (see notes above on item 3 for explanation why) compared to a wind passing
over land. Winds passing over land tend to be drier and winds passing over oceans tend to pick up
more moisture.
(v) Ocean Currents: Ocean current are fundamental in the horizontal transfer of energy around the
globe. Warm ocean currents carry energy from the areas of excess solar heating at the equator
pole wards and hence give regions that they pass nearer the poles warmer maritime climates. Returning
cold currents carry cold water from the poles to the equator and hence have a cooling effect on
climates this whole phenomena is known as a the oceanic circulation or conveyor. In the British Isles
at approximately 58 oN we have an anomalously mild climate compared to other locations of similar
latitude. This is in part due to the prevailing winds and close proximity to the sea but more
importantly the ocean current that passes to the west of the British Isles bringing warmer water from
the equator known as the North Atlantic Drift (see diagram below)
A general model of oceanic circulation
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(b) Short Term Factors controlling insolation:
(i)
Seasonal Changes: at the spring (March 21st) and autumn (September 21st) equinoxes the sun is
directly over head at the equator and the equator will receive the maximum insolation
during these periods. But during the summer solstice (June 21 st) and the winter solstice the
sun is overhead at the tropics. The hemisphere receiving ‗summer‘ will receive the maximum
insolation.
(ii)
Length of day and night: Insolation can only reach the surface during daylight hours and
peaks at noon. There is no variation in day length at the equator and hence more constant
insolation is received. However, at the poles day lenght can vary greatly in winter at the
North Pole there would be continuous darkness but in summer the converse is true,
continuous 24h daylight! Amazing.
(iii)
Aspect: Refers to way in which slopes face. In the northern hemisphere the slopes that
face north and north east receive less solar heating as they are in shadow for most of the year
and are therefore cooler. – Remember corries form for this reason facing N/NE!
(iv)
Cloud Cover: Clouds act by reducing the amount of incoming solar radiation during the day
and therefore can lower daytime temperatures. They also act as insulation blankets during
the night therefore they keep the surface temperature high.
In a desert, daytime
temperatures would be high due to lack of cloud but nigh time temperatures could be very
low as there is little insulation prevention heat loss to space, deserts therefore show a large
diurnal range in temperature! I the tropical regions in the summer, temperatures can often
take a small dip due to the presence of the ITCZ and the associated higher cloud cover as
well as increased precipitation (see later notes of tropical continental climates).
(v)
Urbanisation: Alters the Earth‘s albedo and creates ‗head islands‘. Urban development
disrupts the climatic properties of the surface and the atmosphere. Thus in turn altering
the exchanges and budgets of heat, mass and momentum which control the climate of any
site. Land clearance, drainage and paving leads to a new microclimate on each site. The roof
level of a city (the urban canopy) affects the air near the surface but it also has downwind
effects away from a city. Buildings tend to cause greater air turbulence as well as cyclonic
wind action and uplift. Jets, vortices or gusts can be common between tall buildings e.g. Salford
quays Manchester. Others may be artificially sheltered. In rural areas the ground level
climate returns quickly but the area of the urban canopy layer takes much longer to recover
to a natural state. It can cause slower movements of weather fronts for instance due to
increased frictional drag.
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(d) General Atmospheric Circulation: The Tricellular Model
You should already be aware from earlier in the section that there is a surplus of energy at the equator
and deficit (shortage) at the poles and in the upper atmosphere. Because of the Earth‘s tilt of axis at
23.5o, in low latitudes solar radiation arrives almost at 90o to the surface and there is less atmosphere
for it to pass through so the surface is heated more intensely but in higher latitudes the solar radiation
arriving is oblique to the surface and there is more atmosphere to pass through and hence less heating
(see diagram p.20). Based on this unequal heating surely the equator would get hotter and hotter and
the poles cooler and cooler, this is not the case. We therefore think it is logical that this imbalance is
simply removed by a simple transfer known as a convection cell (Think about how heat is transferred in a
pan of beans, this is simple convection).
Modern advances in meteorology using satellites and radiosondes have given us a better picture of how this
works but still our understanding has not progressed too much more that a simple 3 cell
(Tricellular) model proposed in 1941 by Rossby. See below:
REMEMEBER Naming
Winds: Winds always
get their name from
the direction in which
they blow from! Not
where they blow to!!
Students often find this section quite hard but I would argue it is quite simple really, it works like this:
Explaining the Hadley Cell: The overhead sun causes intense solar radiation and therefore heating
(insolation) to heat the equatorial regions more than the poles. As hot air is less dense than cold air it
rises (as its particles are further apart and posses more energy – this is how a hot air balloon works). As
the air rises it cools, once it cools to the temperature of the surrounding air it stops rising (i.e. at the
tropical tropopause). It is in this region that an intense area of convectional uplift and subsequent
cumulonimbus cloud formation occurs. This is referred to as the Inter Tropical Convergence Zone
(ITCZ), in this area thunderstorms and rain are common. Once the air stops rising it begins to move
away from the equator and towards the poles. As the air cools further, becomes denser and the Coriolis
Force (the force caused by the Earth‘s rotation) diverts its flow (as a westerly flow) the air is forced to
slow down and subside (sink). This subsiding air forms the descending limb of the Hadley cell! The
descending limbs of the Hadley cell subside at about 30oN and 30oS of the equator to form a region of
sub-tropical anticyclones (high pressure areas) at the surface.
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At the tropics the pressure is HIGH because the air is sinking and the weight of the overlying air causes
high pressure. The result of this sinking air (preventing convectional uplift and therefore cloud
formation) in the tropics is clear skies and warm stable conditions. However, the converse is true at
the equator where the air is rising, removing weight of overlying air and hence causing equatorial low
pressure at the surface. The ITCZ occurs above the surface at the equator due to convectional uplift
and condensation of moisture forming huge cumulonimbus rain clouds and at the surface an area of
gentle variable winds known as the doldrums prevail! At the equator there is surface convergence of
winds because of the difference in pressure at the tropics compared to the equator, the easterlies (NE
Trades) are returning from 30oN and the westerlies are returning from 30oS (The SW Monsoon). The
reason the winds blow in this way is the same reason a bike tyre deflates when you get a puncture. Air
always moves from high (inside the tyre) to low pressure (the air surrounding tyre). Remember the words
of Mr Richardson “Winds always blow from high to low!”
Explaining the Polar Cell: The polar cell in the original Tricellular model was seen as a response of to
cold air sinking in the Polar regions and returning to lower latitudes as easterly winds.
Explaining the Ferrel Cell: The Ferrel cell was proposed because it was thought it was a response to
the movements of air set up by the other two cells. Some of the remaining air at the descending limb of
the Hadley cell that is not heading back to the equator is sent pole wards forming warm southwesterlies which pick up moisture as they pass over oceans. The warm south-westerlies meet cold arctic
air at the polar front (60 oN) and are forced to rise forming polar low pressure at 60oN and 60oS and
triggering the rising limb of the Ferrel and Polar cells respectively. The air at 60 oN and 60oS rises to a
lower altitude than that of the Hadley cell until it is the same temperature as the air around it (midlatitude Tropopause is reached), unstable conditions prevail here and produce heavy cyclonic rainfall
associated with the mid-latitude depressions.
Conclusion: The Tricellular model although basic goes to some extent to explain why where there is
descending air, the World‘s major hot deserts occur and where rising air occurs areas of intense
precipitation are common for instance at the equator and in the mid latitudes associated with low
pressure and depressions.
Coriolis force: effect of Earth‘s rotation on airflow. In N.
Hemisphere deflection of air to the right and in S.
Hemisphere deflection is to left.
Hence in Britain it
explains why air appraoching from tropics comes from a SW
direction instead of S.
Front: A boundary between a warm air mass and a cold air mass i.e. where to air
masses meet causing uplift, condensation, cloud formation and subsequent frontal
rainfall.
Geostropic Winds: a condition in the mid latitudes where winds blow parallel to
isobars because the pressure gradient and the Coriolis force are in balance.
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Tricellular Model: Cross Section of atmosphere showing winds, fronts and pressure caused by main
cells.
Relating
the
Atmospheric
Circulation Model to Features of
Earth’s climate
Equatorial low pressure is the
result of the rising limb of the
Hadley cell removing ‗weight‘ of air
from the surface. At this latitude a
variable calm winds known to sailors
as the Doldrums exist. In the
tropical latitudes between 3035oN /S calm warm conditions
occur due to the descending limb
of the Hadley cell causing high
pressure; these latitudes are
termed
the
horse
latitudes.
These winds were first recognised
by the Spanish sailors transporting
horses to the West Indies who often found themselves becalmed for weeks in wind less seas and were
forces to throw the horses over board in order to conserve food and water. Winds known as the Northeast trades blow from the high pressure in the tropics to the low pressure at the equator. They blow
from this NE direction instead of from N due to the effects of the Coriolis force. Some of the air in
the tropics does not return to the equator and blows from the south west to give the Warm SouthWesterlies, when these winds come in contact with cold polar air returning as easterlies they form the
polar front which causes the mid latitude depressions (low pressure systems) which dominate the UK
weather. At the Polar front the warmer south-westerly air is forced to rise above the cold polar air
which under-cuts it causing the rising limb of both the ferell and polar cells. The result is low pressure
at the surface here.
Geostrophic Winds
Winds generally always blow from high
pressure to low pressure down a pressure
gradient. Remember bike tyre analogy, if
you pop a bike tyre air moves from high
pressure in the tyre to low pressure outside
the tyre.
Variations in temperature and
altitude cause air pressure to change.
Normal winds are the result of this
movement down a pressure gradient from
higher to lower pressure. However, due to
the Coriolis acting against the pressure
gradient force in certain latitudes especially
mid latitudes the forces are balanced and a
high altitude wind is deflected at 90
degrees or parallel to the isobars (see diagram
right).
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4. The Climate of the British Isles
The British Isles is unique in its variable climate and its location on the edge of a continent trapped between
two seas and affected by the passage of 5 different airmasses! The UK climate is classes as a temperate
climate; this means there are very rarely extremes in its climate such as rainfall, droughts, winds and
temperature.
(a) Basic Climate Characterisics:
(i)
Temperature
Jan Average
July Average
These temp. maps for Jan and July point to some key influences on the factors controlling the climate.
Temperatures in July reach a peak in southern regions and generally decrease northwards. This can be
explained as there is a lower amount of insolation at higher latitudes. Also in July inland regions appear
to have higher temperatures than places nearer to the coasts as the cooler sea has less influence on
places inland.
This concept of warmer summer temperatures inland is known as continentality
and it is usually seen on larger land areas than in the British Isles but all the same it is still evident here
form these maps. It can also be seen that relief of the land also has an effect, the higher the altitude
the lower the temperatures both in summer and winter. Remember that the lapse rate is approximately
6.4 oC/100m ie. temperatures drop by this amount for every 100m height gained. British mountains are
not much higher than 1000m but in the highest peaks you might expect a temperature drop of 8oC at the
summit compared to in the valley. For instance the Southern Uplands have lower temperatures than the
more northerly central valley between Edinburgh and Glasgow. January temperatures are higher in the
areas bordering the Irish sea in the west as ocean currents and prevailing winds have a warming effect
(remember in winter the sea has a warming effect but in summer a cooling effect on the land). The
North Atlantic Drift brings warmer Gulf Stream waters to this western area of the B.I. The warming
effect is more greatly seen in winter than summer, so much so that even in the Scottish winter many
towns on the west coast such as Plockton enjoy warmer temperatures than they should really get for their
latitude. In Plockton itself Palm trees grow! There
is a north – south skewed rise in temperatures with Anglesey in west Wales being considerably warmer
than the Wash further north and to the eas
14
(ii)
Precipitation
The West
and
North
receive
the
most
precipitation and the South and particularly the East
of the country receive the least. The two most
important factors that control rainfall are direction
of the prevailing wind and altitude.
Relief or orogenic rainfall:
occur when moist
westerly air is forces to rise over mountains such as
the lake District, Snowdonia and the Pennines. Here
water vapour cools and condenses past its dew
point (where air becomes saturated) to form clouds
and rainfall.
For instance Keswick in the West
(Cumbria) receives 1500mm/year where Tynemouth
on the east coast at a similar latitude receives
almost a two thirds less at 660mm. This is known as
the rain shadow in the east as water is precipitated
in the mountains in the west and on the Pennines.
As the air sinks it warms and has less chance of
reaching its dew point as warmer air can hold more
water and therefore is less likely to result in rain.
Frontal Rainfall:
Britain is bombarded with frontal systems.
Fronts occur when a wedge or portion of warm air is forced to
rise and cool above a wedge of cooler air. This commonly occurs
when polar air undercuts warmer tropical air resulting in cooling,
condensation and cloud formation at a front (see more on fronts
later).
This is common in the winter when depressions
originating over the Atlantic hit the shores of Britain!
Convectional Rainfall: Some rainfall can attributed to intense
heating of the ground in summer months which leads to less dense
air rising. Rising encourages cooling past the dew point and
condensation occurs leading to cloud formation. In summer
months this usually occurs by mid afternoon and the result is
towering cumulo-nimbus thunder
storm clouds which yield sudden intense cloud bursts as soon
1
5
as water droplets are large enough to overcome the force of the updrafts caused by heating. This is
especially commoon over southern and eastern Britain.
(iii) Wind
The most common wind direction in England is from
the Southwest. But this varies day to day and
often northerly or north easterly winds are common
in winter. The strongest winds are found in the
North and West of the country as they face the
direction of the prevailing winds passing over the
Atlantic. Attitude also causes higher wind speeds
as there are fewer obstructions in the way to inhibit
wind flow.
Wind speeds generally increase with
height. The windiest places are mountain and hill
tops such as great Dun Fell, Cumbria where in a third
of the days a year wind speeds are classes as gale
force (73km/h for more than 10 mins).
(b) Air Masses affecting the British Isles
Air masses
Air masses are parcels of air that bring distinctive
weather features to the country. An air mass is a body
or 'mass' of air in which changes in temperature and
humidity are relatively slight. That is to say the air
making up the mass is very uniform in temperature and
humidity. An air mass is separated from an adjacent
body of air by a weather front. An air mass may cover
several millions of square kilometres and extend
vertically throughout the troposphere.
Types of Cloud
Cirrus - a tuft or filament (e.g. of hair)
Cumulus - a heap or pile
Stratus - a layer
Nimbus - rain bearing
There are now ten basic cloud types with names based on combinations of these words (the word 'alto',
meaning high but now used to denote medium-level cloud, is also used).
Clouds form when moist air is cooled to such an extent it becomes saturated. The main mechanism for
cooling air is to force it to rise. As air rises it expands - because the pressure decreases with height in
the atmosphere - and this causes it to cool. Eventually it may become saturated and the water vapour
then condenses into tiny water droplets, similar in size to those found in fog, and forms cloud. If the
16
temperature falls below about minus 20 °C, many of the cloud droplets will have frozen so that the cloud is
mainly composed of ice crystals. The ten main types of cloud can be separated into three broad categories
according to the height of their base above the ground: high clouds, medium clouds and low clouds.
High clouds are usually composed solely of ice crystals and have a base between 18,000 and 45,000 feet
(5,500 and 14,000 metres).
Cirrus - white filaments
Cirrocumulus - small rippled elements
Cirrostratus - transparent sheet, often with a halo
Medium clouds are usually composed of water droplets or a mixture of water droplets and ice crystals,
and have a base between 6,500 and 18,000 feet (2,000 and 5,500 metres).
Altocumulus - layered, rippled elements, generally white with some shading
Altostratus - thin layer, grey, allows sun to appear as if through ground glass
Nimbostratus - thick layer, low base, dark. Rain or snow falling from it may sometimes be heavy
Low clouds are usually composed of water droplets — though cumulonimbus clouds include ice crystals and have a base below 6,500 feet (2,000 metres).
Stratocumulus - layered, series of rounded rolls, generally white with some shading
Stratus - layered, uniform base, grey
Cumulus - individual cells, vertical rolls or towers, flat base
Cumulonimbus - large cauliflower-shaped towers, often 'anvil tops', sometimes giving
thunderstorms or showers of rain or snow
High pressure or anticyclone
In an anticyclone (also referred to as a 'high') the winds tend to be light and blow in a clockwise
direction. Also the air is descending, which inhibits the formation of cloud. The light winds and clear
skies can lead to overnight fog or frost. If an anticyclone persists over northern Europe in winter, then
much of the British Isles can be affected by very cold east winds from Siberia. However, in summer an
anticyclone in the vicinity of the British Isles often brings fine, warm weather.
Clouds
A classification of clouds was introduced by Luke Howard (1772-1864) who used Latin words to describe
their characteristics.
Cirrus - a tuft or filament (e.g. of hair)
Cumulus - a heap or pile
Stratus - a layer
Nimbus - rain bearing
There are now ten basic cloud types with names based on combinations of these words (the word 'alto',
meaning high but now used to denote medium-level cloud, is also used). Clouds form when moist air is
cooled to such an extent it becomes saturated. The main mechanism for cooling air is to force it to rise.
As air rises it expands - because the pressure decreases with height in the atmosphere - and this causes it
to cool. Eventually it may become saturated and the water vapour then condenses into tiny water droplets,
similar in size to those found in fog, and forms cloud. If the temperature falls below about minus 20
°C, many of the cloud droplets will have frozen so that the cloud is mainly composed of ice crystals.
17
The ten main types of cloud can be separated into three broad categories according to the height of
their base above the ground: high clouds, medium clouds and low clouds.
High clouds are usually composed solely of ice crystals and have a base between 18,000 and 45,000 feet
(5,500 and 14,000 metres).
Cirrus - white filaments
Cirrocumulus - small rippled elements
Cirrostratus - transparent sheet, often with a halo
Medium clouds are usually composed of water droplets or a mixture of water droplets and ice crystals,
and have a base between 6,500 and 18,000 feet (2,000 and 5,500 metres).
Altocumulus - layered, rippled elements, generally white with some shading
Altostratus - thin layer, grey, allows sun to appear as if through ground glass
Nimbostratus - thick layer, low base, dark. Rain or snow falling from it may sometimes be heavy
Low clouds are usually composed of water droplets — though cumulonimbus clouds include ice crystals and have a base below 6,500 feet (2,000 metres).
Stratocumulus - layered, series of rounded rolls, generally white with some shading
Stratus - layered, uniform base, grey
Cumulus - individual cells, vertical rolls or towers, flat base
Cumulonimbus - large cauliflower-shaped towers, often 'anvil tops', sometimes giving
thunderstorms or showers of rain or snow
Types of Cloud
Interpreting weather maps
Isobars - The lines shown on a weather map (synoptic map) are isobars - they join points of equal
atmospheric pressure. The pressure is measured by a barometer, with a correction then being made to
give the equivalent pressure at sea level. Meteorologists measure pressure in units of millibars (mb). In the
British Isles the average sea-level pressure is about 1013 mb, and it is rare for pressure to rise above
1050 mb or fall below 950 mb. Charts showing isobars are useful because they identify features such as
anticyclones and ridges (areas of high pressure) and depressions and troughs (areas of low pressure),
which are associated with particular kinds of weather. These features move in an essentially predictable
way
18
There are three important relationships between isobars and winds.
The closer the isobars, the stronger the wind.
The wind blows almost parallel to the isobars.
The direction of the wind is such that if you stand with your back to the wind in the northern
hemisphere, the pressure is lower on the left than on the right.
These make it possible to deduce the wind flow from the isobars.
Winds - The direction given for the wind refers to the direction from which it comes. For example, a
westerly wind is blowing from the west towards the east. In general, the weather is strongly influenced
by the wind direction, so information about the wind provides an indication of the type of weather likely
to be experienced. However, this approach is effective only if the wind is blowing from the same
direction for some time. A marked change in wind direction usually indicates a change in the weather.
Northerly winds tend to bring relatively cold air from polar regions to the British Isles. Similarly, southerly
winds tend to bring relatively warm air from the tropics. The characteristics of the air are also affected
by its approach to the British Isles. Air picks up moisture if it travels across the sea, but remains relatively
dry if it comes across the land.
Fronts - The boundary between two different types of air mass is called a front. In our latitudes a
front usually separates warm, moist air from the tropics and cold, relatively dry air from polar regions.
On a weather chart, the round (warm front) or pointed (cold front) symbols on the front, point in the
direction of the front's movement. Fronts move with the wind, so they usually travel from the west to
the east. At a front, the heavier cold air undercuts the less dense warm air, causing the warm air to rise
over the wedge of cold air. As the air rises it cools and condensation occurs, thus leading to the
formation of clouds. If the cloud becomes sufficiently thick, rain will form. Consequently, fronts tend to
be associated with cloud and rain. In winter, there can be sleet or snow if the temperature near the
ground is close to freezing
This means that as a cold front passes, the weather changes from being mild and overcast to being cold
and bright, possibly with showers (typical of cold polar air travelling over the sea). The passage of the
front is often marked by a narrow band of rain and a veer in the wind direction.
As the warm front approaches, there is thickening cloud and eventually it starts to rain. The belt of rain
extends 100-200 miles ahead of the front. Behind the front the rain usually becomes lighter, or ceases,
but it remains cloudy. As a warm front passes, the air changes from being fairly cold and cloudy to being
warm and overcast (typical of warm air from the tropics travelling over the sea). Also there is a
clockwise change in wind direction, and the wind is said to 'veer'.
19
Weather fronts: A weather front is simply the boundary between two air masses.
Cold front
Warm front
Occluded
front
This is the boundary between warm air and cold
air and is indicative of cold air replacing warm
air at a point on the Earth's surface.
On a synoptic chart a cold front appears blue.
The presence of a cold front means cold air is
advancing and pushing underneath warmer air.
This is because the cold air is 'heavier' or
denser, than the warmer air.
Cold air is thus replacing warm air at the
surface. The symbols on the front indicate the
direction the front is moving.
The passage of a cold front is normally marked at
the earth's surface by a rise of pressure, a fall
of temperature and dew point, and a veer of wind
(in the northern hemisphere). Rain occurs in
association with most cold fronts and may
extend some 100 to 200 km ahead of or behind
the front. Some cold fronts give only a shower
at the front, while others give no precipitation.
Thunder may occur at a cold front
This is the boundary between cold air and
warm air and is indicative of warm air
replacing cold air at a point on the Earth's
surface
On a synoptic chart a warm front appears red
The presence of a warm front means warm air
is advancing and rising up over cold air. This is
because the warm air is 'lighter' or less
dense, than the colder air. Warm air is thus
replacing cold air at the surface. The symbols
on the front indicate the direction the front
is moving.
As a warm front approaches, temperature and
dew-point within the cold air gradually rise and
pressure falls at an increasing rate.
Precipitation usually occurs within a wide belt
some 400 km in advance of the front. Passage
of the front is usually marked by a steadying
of the barometer, a jump in temperature and
dew point, a veer of wind (in the northern
hemisphere), and a cessation or near cessation
of precipitation.
1. Wind Speed
3. Precipitation
These are more
complex than cold or
warm fronts. An
occlusion is formed
when a cold front
catches up with a
warm front
When a cold front
catches up with a
warm front the warm
air in the warm
sector is forced up
from the surface.
On a synoptic chart
an occluded front
appears purple.
2.Cloud Cover
1.
Weather Station Entry
Synoptic Chart Symbols
20
21
22
(i) Infancy
Initially a warm air mass such as one
from the tropics, meets a cooler air
mass, such as one from the polar
regions. Depressions which affect the
UK normally originate over the Atlantic
Ocean
(ii) Maturity
The warm air rises up over the colder
air which is sinking. A warm sector
develops between the warm and cold
fronts. The mature stage of a
depression often occurs over the UK
(iii) Occlusion
The cold front travels at around 40
to 50 miles per hour, compared to the
warm front which travels at only 20
to 30 miles per hour. Therefore the
cold front eventually catches up with
the warm front. When this occurs an
occlusion is formed.
(iv) Death
Eventually the frontal system dies as all the warm air has been pushed up from the surface and all that
remains is cold air. The occlusion dies out as temperatures are similar on both sides. This stage normally
occurs over Europe or Scandinavia.
Weather changes associated with the passage of a depression
A depression is an area of low atmospheric pressure. It is represented on a weather map by a system of
closely drawn isobars with pressure decreasing towards the centre.
Depressions usually move rapidly from
west to east across the British Isles.
Winds move in an anticlockwise direction
around the centre of the depression.
These winds are usually quite strong, in
fact the closer the isobars are together
the stronger the winds will be.
A depression affecting the British Isles originates in the north Atlantic where two different air masses
meet to form a front. The two air masses involved are:
polar maritime air (Pm) - air from the northwest Atlantic, which is cold, dense and moist
23
tropical maritime air (Tm) - air from the southwest, which is warmer, less dense and also moist
24
These two bodies of air move towards each other, with the warmer, less dense Tm air from the south
rising above the colder, more dense Pm air from the north. The rising air twists due to the rotational effect
of the Earth's spin. This twisting vortex causes a wave or kink to be produced in the front forming
out
in
the
Atlantic,
which
increases
in
size
to
become
a depression.
This means that as a cold front passes, the weather changes from being mild and overcast to being cold
and bright, possibly with showers (typical of cold polar air travelling over the sea). The passage of the
front is often marked by a narrow band of rain and a veer in the wind direction.
As the warm front approaches, there is thickening cloud and eventually it starts to rain. The belt of rain
extends 100-200 miles ahead of the front. Behind the front the rain usually becomes lighter, or ceases,
but it remains cloudy. As a warm front passes, the air changes from being fairly cold and cloudy to being
warm and overcast (typical of warm air from the tropics travelling over the sea). Also there is a
clockwise change in wind direction, and the wind is said to 'veer'.
(d) Origin and Nature of Anticyclones:
Associated weather conditions in summer and winter
An anticyclone is an area of relatively high atmospheric pressure. It is represented on a weather map by a
system of widely spaced isobars with pressure increasing towards the centre.
Anticyclones move slowly and remain
stationary over an area for several
days or weeks (blocking anticyclone).
Warm dry anticyclonic conditioins summer.
In an anticyclone (also referred to as a 'high') the winds tend to be light and blow in a clockwise
direction. Also the air is descending, which inhibits the formation of cloud. The light winds and clear
skies can lead to overnight fog or frost.
25
If an anticyclone persists over northern Europe in winter, then much of the British Isles can be
affected by very cold east winds from Siberia. However, in summer an anticyclone in the vicinity of the
British Isles often brings fine, warm weather.
Summer
In summer, anticyclones mean:
hot daytime temperatures - over 25 C.
cooler night time temperatures - may
not fall below 15 C.
clear skies by day and night generally
hazy sunshine may exist in some areas
early morning mists/fogs will rapidly
disperse
heavy dew on ground in morning
east coast of Britain may have sea fogs or
advection fog caused by on-shore winds
thunderstorms may be created due to
convectional uplift.
Pressure: High to subsiding air.
Wind Direction: Clockwise, blowing outwards from the centre of high
pressures.
Wind Speed: Calm or gentle winds due to gentle pressure grsdients.
Relative Humidity: Low as descending air is warming and encourages evaporation rather than condensation.
Cloud: Often cloudless - although heat of day can produce thermals leading to cumulo-nimbus clouds.
Precipitation: Usually dry due to sinking air, apart from mist and dew in early mornings (radiation cooling) and the
risk of a thunderstorm (convectional uplift) after a few days of high pressure.
Temperature: Very warm/hot during day and cool at night due to absence of cloud, intense insolation and radiation.
Advection Fog
Forms when warm air passes over or meets cold air, to give rapid cooling. This type of fog forms when
the air is over saturated with water droplets. It is common on the north east coast of Scotland and
northern England and the IOM. Salt from the sea acts as a nucleus for fog condensation. Oceans usually
retain their heat for longer than land so when the warmer moist air over the sea comes in contact with
colder dry air blowing from the land condensation and fog formation prevails. This also works if the moist
warm air is blowing off the sea! These fogs can often be persistent and recurring over several days.
Winter
In winter, anticyclones result in:
cold daytime temperatures - below
freezing to a maximum of 5 C.
very cold night time temperatures below freezing, with frosts.
clear skies by day and night generally.
Low level cloud may linger, and radiation
fogs may remain in low-lying areas as a
result of temperature inversion.
high levels of atmospheric pollution in
urban areas, caused by a combination of
subsiding air and lack of wind.
Temperature Inversion:
Is where temperature decrease with altitude is much less or in extreme
cases temperature actually increases with altitude. Temperature inversions happen when high pressure
dominates and can form in various ways:
(i)
Radiate cooling of air near ground at
(iv)
Radiative heating of the upper
night
Atmosphere
(ii)
(iii)
Advective cooling – warm air over
cold air or cold surface
Warm airmass undercutting cold air
mass at a front
26
e) Storm events and responses to them – The ‘Great Storm’ of 1987
With winds gusting at up to 100mph, there was massive
devastation across the country and 18 people were killed.
About 15 million trees were blown down. Many fell on to roads
and railways, causing major transport delays. Others took
down electricity and telephone lines, leaving thousands of homes
without power for more than 24 hours.
This is the Hengist passenger ferry that was
washed up on to the shores of the The
Warren in Folkestone during the great Storm.
Buildings were damaged by winds or falling trees. Numerous
small boats were wrecked or blown away, with one ship at
Dover being blown over and a Channel ferry the Hengist was
blown ashore near Folkestone. While the storm took a human
toll, claiming 18 lives in England, it is thought many more may have been hurt if the storm had hit during
the day.many parts of the UK in the middle of October 1987.
The storm gathers
Four or five days before the storm struck, forecasters predicted severe weather was on the way. As
they got closer, however, weather prediction models started to give a less clear picture. Instead of
stormy weather over a considerable part of the UK, the models suggested severe weather would pass to
the south of England - only skimming the south coast.
During the afternoon of 15 October, winds were very light over most parts of the UK and there was
little to suggest what was to come. However, over the Bay of
Biscay, a depression was developing. The first gale warnings for
sea areas in the English Channel were issued at 6.30 a.m. on 15
October and were followed, four hours later, by warnings of
severe gales.
At 12 p.m. (midday) on 15 October, the depression that
originated in the Bay of Biscay was centred near 46° N, 9° W
and its depth was 970 mb. By 6 p.m., it had moved north-east to
about 47° N, 6° W, and deepened to 964 mb.
At 10.35 p.m. winds of Force 10 were forecast. By midnight, the
depression was over the western English Channel, and its central
pressure was 953 mb. At 1.35 a.m. on 16 October, warnings of
Force 11 were issued. The depression moved rapidly north-east,
filling a little as it went, reaching the Humber estuary at about
5.30 am, by which time its central pressure was 959 mb.
Dramatic increases in temperature were associated with the
passage of the storm's warm front.
Warning the public
During the evening of 15 October, radio and TV forecasts mentioned strong winds but indicated heavy
rain would be the main feature, rather than strong wind. By the time most people went to bed,
exceptionally strong winds hadn't been mentioned in national radio and TV weather broadcasts. Warnings
of severe weather had been issued, however, to various agencies and emergency authorities, including
the London Fire Brigade. Perhaps the most important warning was issued by the Met Office to the Ministry
of Defence at 0135 UTC, 16 October. It warned that the anticipated consequences of the storm were
such that civil authorities might need to call on assistance from the military.
In south-east England, where the greatest damage occurred, gusts of 70 knots or more were recorded
continually for three or four consecutive hours. During this time, the wind veered from southerly to
south-westerly. To the north-west of this region, there were two maxima in gust speeds, separated by a
period of lower wind speeds. During the first period, the wind direction was southerly. During the
27
latter, it was south-westerly. Damage patterns in south-east England suggested that whirlwinds
accompanied the storm. Local variations in the nature and extent of destruction were considerable.
How the storm measured up
Comparisons of the October 1987 storm with previous severe storms were inevitable. Even the oldest
residents of the worst affected areas couldn't recall winds so strong, or destruction on so great a scale.
The highest wind speed reported was an estimated 119 knots (61 m/s) in a gust soon after midnight
at Quimper coastguard station on the coast of Brittany (48° 02' N 4° 44' W).
The highest measured wind speed was a gust of 117 knots (60 m/s) at 12.30 am at Pointe du Roc
(48° 51' N, 1° 37' W) near Granville, Normandy.
The strongest gust over the UK was 100 knots at Shoreham on the Sussex coast at 3.10 am, and
gusts of more than 90 knots were recorded at several other coastal locations.
Even well inland, gusts exceeded 80 knots. The London Weather Centre recorded 82 knots at 2.50
am, and 86 knots was recorded at Gatwick Airport at 4.30 am (the authorities closed the airport).
A hurricane or not?
TV weather presenter Michael Fish will long be remembered for telling viewers there would be no
hurricane on the evening before the storm struck. He was unlucky, however, as he was talking about a
‗different storm system‘ over the western part of the North Atlantic Ocean that day. This storm, he said,
would not reach the British Isles — and it didn't. It was the rapidly
deepening depression from the Bay of Biscay which struck. This storm
wasn't officially a hurricane as it did not originate in the tropics — but
it was certainly exceptional. In the Beaufort scale of wind force,
Hurricane Force (Force 12) is defined as a wind of 64 knots or more,
sustained over a period of at least 10 minutes. Gusts, which are
comparatively short-lived (but cause a lot of destruction) are not taken
into account. By this definition, Hurricane Force winds occurred locally
but were not widespread. The highest hourly-mean speed recorded in
the UK was 75 knots, at the Royal Sovereign Lighthouse. Winds reached
Force 11 (56–63 knots) in many coastal regions of south-east England.
Inland, however, their strength was considerably less. At the London Weather Centre, for example,
the mean wind speed did not exceed 44 knots (Force 9). At Gatwick Airport, it never exceeded 34
knots (Force 8).
The powerful winds experienced in the south of England during this storm are deemed a once in 200
year event — meaning they were so unusually strong you could only expect this to happen every two
centuries. This storm was compared with one in 1703, also known as a 'great storm', and this could be
justified. The storm of 1987 was remarkable for its ferocity, and affected much the same area of the
UK as its 1703 counterpart.
Northern Scotland is much closer to the main storm tracks of the Atlantic than south-east England.
Storms as severe as October 1987 can be expected there far more frequently than once in 200 years.
Over the Hebrides, Orkney and Shetland, winds as strong as those which blew across south-east
England in October 1987 can be expected once every 30 to 40 years. The 1987 storm was also
remarkable for the temperature changes that accompanied it. In a five-hour period, increases of more
than 6 °C per hour were recorded at many places south of a line from Dorset to Norfolk.
The aftermath
Media reports accused the Met Office of failing to forecast the storm correctly. Repeatedly, they
returned to the statement by Michael Fish that there would be no hurricane — which there hadn't been.
It did not matter that the Met Office forecasters had, for several days before the storm, been
28
warning of severe weather!
The Met Office had performed no worse than any other European
forecasters when faced with this exceptional weather event.
However, good was to come of this situation. Based on the findings of an internal Met Office enquiry,
scrutinised by two independent assessors, various improvements were made. For example, observational
coverage of the atmosphere over the ocean to the south and west of the UK was improved by increasing
the quality and quantity of observations from ships, aircraft, buoys and satellites, while refinements
were made to the computer models used in forecasting. Some argue that the reason that Fish failed to
spot the approach of the storm was due to a reduction in funding and subsequent removal of a weather
boat out in the Bay of Biscay! So you could argue that he was allowed to become a scape-goat!
5. The Climate of the Wet/Dry Savannas
(a) Description of the climate characteristics of the Tropical Continental Climate
The tropical continental type of climate in West Africa occurs mainly in those areas which are situated
between equatorial and the hot desert climate types. For part of the year these areas lie under the
influence of the dry trade winds, but for the rest of the year they are invaded by the belt of
convectional rains. Consequently there is an alternation of wet and dry seasons. Because the tropical
continental is essentially a transitional climate between tropical rain forests and the hot deserts, variations
occur with increasing latitude from the equator. The length of the dry season and unreliability of
precipitation increases pole wards. The main characteristic features of the tropical continental
climate are:
Temperatures are high throughout the year, although
annual and daily ranges tend to be larger than the
equatorial type but not as high as the hot desert climate.
At the equatorial/rain forest margins temperature ranges
from 22 to 28°C over a year and at the hot desert margins
temperature ranges from 18 to 34°C over a year.
The rainfall is highly seasonal in its distribution. The
bulk of the rain falls during the summer months (May, June,
July & August), and the rest of the year is very dry (i.e. the
winter months). With increasing distance from the equator,
the rainfall decreases in amount, and the dry season
becomes longer and more severe. At the equatorial/rain
forest margins precipitation is over 1000mm a year with
only 1 dry month and at the hot desert margins
precipitation is less than 500mm a year, with 9 or 10 dry
months.
Both relative humidity and the amount of cloud cover
vary with the season, being generally high during the rains and
much lower during the dry season.
The wind patterns also change with the seasons. In
West Africa, for example, warm, moist winds blow from the
south in the summer months and warm, dry winds blow form
the north in the winter season.
Climate Data
Atar, Mauritania
29
J
F
M
A
M
J
J
A
S
O
N
D
30
Temp oC
21
23
25
29
32
35
34
34
34
31
25
21
Rainfall
(mm)
3
0
0
0
0
3
8
31
28
3
3
0
Description of the Atar climate graph
Average annual temperature: 29°C
Average annual range: 21 to 35 = 14°C
Highest temperatures: summer (June, July, August, Sept)
Total Rainfall: 79mm/year
Dry season (< 50mm): 12 months
Wet season (> 50mm): 0 months
Maximum rainfall: single peak in August
The hot desert climate is experienced in West Africa to the north of about 18°N. In this region the
rainfall is very light, everywhere averaging less than 250mm a year. It is also highly irregular in its
occurrence. Away from the coast temperatures are more extreme than in any other part of West
Africa, with particularly large daily (diurnal) ranges being experienced.
Calabar, Nigeria
J
F
M
A
M
J
J
A
S
O
N
D
Temp oC
27
27
28
27
27
27
25
25
26
26
27
27
Rainfall
(mm)
43
76
152
213
312
406
450
406
427
310
191
43
31
Description of the Calabar climate graph
Average annual temperature: 27°C
Average annual range: 25 to 28 = 3°C
Highest temperatures: winter (Nov, Dec, Jan, Feb, March, April)
Total Rainfall: 3029mm/year
Dry season (< 50mm): 2 months (winter)
Wet season (> 50mm): 10 months (summer)
Maximum rainfall: double peak in June and September
Kano, Nigeria
J
F
M
A
M
J
J
A
S
O
N
D
Temp oC
26
27
30
34
33
30
28
26
25
28
26
26
Rainfall
(mm)
0
5
10
15
75
125
220
265
150
25
0
0
32
Description of the Kano climate graph
Average annual temperature: 28°C
Average annual range: 25 to 34 = 9°C
Highest temperatures: at end of dry season (April & May)
Total Rainfall: 1040mm/year
Dry season (< 50mm): 7 months in the winter (Oct to April)
Wet season (> 50mm): 5 months in the summer (May to Sept)
Maximum rainfall: single peak in August (wet season)
(b) Explanation of the Tropical Continental Climate of West Africa:
‗The role of subtropical anticyclones and the inter-tropical convergence zone (ITCZ)‘
1. Temperature
In West Africa temperatures are high throughout the year due to the sun being overhead for many
months. The angle of the sun affects the amount of atmosphere the sun's radiation has to pass through.
This then determines the amount of radiation that reaches the earth's surface. When the sun is
directly overhead and radiation travels through least amount of atmosphere enroute to the earth's surface
temperatures are higher. However, there is a short cooler season (in comparison with the equatorial
climate) in tropical continental areas during the summer months due to two factors. Firstly, this is the
time of maximum rainfall and the increased cloud cover reduces incoming solar radiation. Secondly,
during the summer months the sun is not directly overhead in the northern hemisphere (it is overhead
further north at the Tropic of Cancer in the Northern Hemisphere, June 21st). This increases the amount
of atmosphere that the sun's radiation has to travel through reducing the amount of heating and also
increases the land area being heated as the insolation is spread over a larger area.
2. Wind
The seasonal wind patterns in West Africa (warm, moist winds blow from the south in the summer
months and warm, dry winds blow form the north in the winter season) are influenced by the tropical
continental (Tc) and tropical maritime (Tm) air masses.
33
3. Rainfall
Rainfall is the most important element in the climate of West Africa. The amount and seasonal distribution
of rainfall in West Africa is largely determined by fluctuations in the position of two important air masses
and their associated wind systems.
The tropical continental air mass (cT or Tc) originates over the Sahara Desert, and consequently is warm
and dry. Associated with the Tc air mass are easterly or north-easterly winds, which in West Africa are
known as the Harmattan, which have a drying influence on the areas over which they pass.
The tropical maritime air mass (mT or Tm) originates over the Atlantic Ocean to the south of the
equator, and consequently is warm and moist. Associated with the Tm air mass are moisture-laden winds
called the Southwest Monsoon.
Wet Season in Summer Months
The migration polewards of the ITCZ during the summer months brings rainfall to this area
of West Africa giving it a wet season (e.g. Kano in Nigeria). Rainfall results from the moist
unstable Tm air brought in by the Southwest Monsoon winds from the Atlantic Ocean.
Areas at the poleward limit of ITCZ movement are only briefly affected (i.e. latitude
20°N), thus they have only a brief wet season and low annual rainfall totals (e.g. Atar in
Mauritania).
Nearer the equator the wet season lasts whilst the ITCZ is poleward, and the area is under
the influence of the Tm air mass brought in by the Southwest Monsoon winds. Maximum
rainfall occurs with the passage polewards of the ITCZ and on its return, thus giving a
double maxima in some areas (e.g. Calabar in Nigeria).
34
The diagram shows what is happening in cross-sectional view.
The winter months are the dry season in West Africa. During the winter the ITCZ retreats southward,
and in January is situated just to the north of the Gulf of Guinea coast at latitude 5°N. As a result, the
influence of the Southwest Monsoon winds is restricted to that part of West Africa which lies to the
south of latitude 5°N. The remainder of West Africa in January lies under the drying influence of the
Harmattan, and consequently receives very little rainfall at that time of year.
Dry Season in Winter Months
The migration equator wards of the
ITCZ during the winter months
brings the hot, dry desert
Harmattan winds across this area
of West Africa giving it a dry
season (e.g. Kano in Nigeria).
Rainfall is restricted to the far
south of the area and results
from the moist unstable Tm air
brought in by the Southwest
Monsoon winds from the Atlantic
Ocean (e.g. Calabar in Nigeria).
Areas way to the north have the
longest dry season due to the
drying influence of the Harmattan
most of the year and due to the
fact that the moist Southwest
Monsoon winds are pushed back way
to the south (e.g. Atar in
Mauritania)..
The diagram shows what is happening in cross-sectional view
So, the alternating wet and dry seasons in West Africa are directly related to the positions of the Tc
and Tm air masses. When the Tm air mass moves over the area (in the summer months) it is hot and wet,
35
with winds blowing from the south, and when the Tc air mass moves over the region (in the winter
months) it is hot and dry, with the winds blowing from the north. But what causes these two air masses
to move over the region at different times?
Surface winds always blow from areas of high pressure to areas of low pressure. As you can see from
the surface pressure map below a band of high pressure can be seen along the line of the Tropic of
Capricorn (23.5 S) and another just north of the Tropic of Cancer (23.5 N), these pressure systems are
the subtropical highs. In between these two areas of high pressure is an area of low pressure occurring
roughly along the line of the equator, hence generally termed equatorial low pressure. The arrows on the
pressure map indicate the relative movement of the surface winds, and over West Africa that means the
winds blow in towards the equator. The winds blowing from the high pressure in the north are part of
the Tropical continental air mass (Tc) and the winds blowing from the south are part of the Tropical
maritime air mass (Tm).
The low pressure
area
where
these two air
masses (Tc &
Tm) meet
is
known as the
Inter-Tropical
Convergence
Zone
(ITCZ).
The
pressure
map below shows
where the ITCZ is
located.
The location
of the ITCZ
is linked to
intense
heating
by
the overhead
sun. Over the
equatorial
regions the
sun
is
directly
overhead for
most of the
year
and
heats
this
area up more
than
any
other.
This intense heating of the land surface causes the air directly above to be warmed up, causing it to
eventually rise. Rising air forms a low pressure area beneath it. This is the low pressure that the surface
winds flow into from north and south. However, as you can see the position of the ITCZ is not
36
stationary, but fluctuates slowly throughout the year, following with a lag of a month or two, the
apparent movement of the overhead sun. This can be seen in the pressure map below. In this map for
July the ITCZ is much further south than in the January pressure map above.
So the seasonal wind
patterns,
variations
in temperature and the
alternate wet and dry
seasons of the tropical
continental climate are
all caused by
the
influence of the Tc
and Tm air masses.
These two air masses
move
over west
Africa due to the way
air moves from areas
of high pressure to
areas of low pressure.
In the winter the low
pressure is situated
over the equator or south of the equator, causing the dry Tc winds from the Sahara to influence West
Africa. In the summer months the low pressure area is situated further north (about 20°N) towards to
Tropic of Cancer, allowing the wet Tm winds from the Atlantic to influence West Africa. The area of low
pressure where the winds converge form the surrounding high pressure system is known as the Intertropical convergence zone (ITCZ). The ITCZ is associated with the heating from the overhead sun. Intense
heating from the overhead sun heats the ground up causing convectional uplift to occur. This uplift causes
a low pressure area to develop at the ITCZ and follows the movement of the sun through the year. So
finally why does the sun seem to move through the year?
It is the tilt of the Earth (23.5°) on its axis which causes the position of the overhead sun to move
during the year. As the Earth circles the sun for one half of the year the northern hemisphere is tilted
towards the sun giving it its summer months. For the other half of the year the northern hemisphere is
tilted away from the sun giving it its winter. The net effect is that the overhead sun seems to move
from the Tropic of Cancer to the Tropic of Capricorn over a year. See the diagram below. This causes
the pressure belts to move which in turn creates transitional climates with seasonal rainfall patterns
such as the tropical continental climate of West Africa.
All of the discussion above led us finally to set the tropical continental climate into a global context. The
various climates found on the Earth are all related to the global atmospheric circulation system. Below is
much simplified diagram to show you how the whole system works!
37
6. Tropical Revolving Storms (Hurricanes)
1. Origin of Hurricane Hazard
The ingredients for a hurricane include a pre-existing weather disturbance, warm tropical oceans
(>26oC), moisture, and relatively light winds aloft. If the right conditions persist long enough, they can
combine to produce the violent winds, incredible waves, torrential rains, and floods we associate with
this phenomenon.
A hurricane is a type of tropical cyclone, which is a generic
term for a low pressure system that generally forms in the
tropics 5-15oN or S as in this location the effect of the
descending limb of the Hadley Cell causing sub-tropical high
pressure is weaker, therefore encouraging evaporation.
The cyclone is accompanied by thunderstorms and, in the
Northern Hemisphere, a counter-clockwise (cyclonic)
circulation of winds near the earth's surface (and anticyclonic outflow in the upper atmosphere). They range
from 200 to 600km in diameter and can cover an area
of 1,300,000 km², and last for a few weeks. They occur
on the Western side of ocean basins and track westwards
until they hit landfall where their energy is dissipated. It
is the effect of the Coriolis Force that causes them to
begin to spiral in a cyclonic direction.
In terms of their potential for destruction, hurricanes are the world's most violent storms. The amount
of energy produced by a typical hurricane in just a single day is enough to supply all of the USA's electrical
needs for 6 months!
38
Tropical cyclones develop through a range of weather systems:
Tropical Disturbance
This is the initial mass of thunderstorms which has only a slight wind circulation. Although many
tropical disturbances occur each year, only a few develop into true huricanes.
Tropical Depression
An organized system of clouds and thunderstorms with a defined surface circulation and
maximum sustained winds of between 23 and 38 mph.
Tropical Storm
An organized system of strong thunderstorms with a defined surface circulation and maximum
sustained winds of 39-73 mph.
Tropical Cyclone (Hurricane)
An intense tropical weather system of strong thunderstorms with a well-defined surface
circulation and maximum sustained winds of 74 mph (118 km/h) or higher.
When the winds from these storms reach 39 mph (34 kts), the cyclones are given names. Years ago, an
international committee developed names for Atlantic cyclones. In 1979 a six year rotating list of
Atlantic storm names was adopted — alternating between male and female hurricane names. Storm
names are used to facilitate geographic referencing, for warning services, for legal issues, and to reduce
confusion when two or more tropical cyclones occur at the same time. Through a vote of the World
Meteorological Organization Region IV Subcommittee, Atlantic cyclone names are retired usually when
hurricanes result in substantial damage or death or for other special circumstances. The names assigned
for the period between 2003 and 2009 are shown below.
Names for Atlantic Basin Tropical Cyclones
2003
Ana
Bill
Claudette
Danny
Erika
Fabian
Grace
Henri
Isabel
Juan
Kate
Larry
Mindy
Nicholas
Odette
Peter
Rose
Sam
Teresa
Victor
Wanda
2004
Alex
Bonnie
Charley
Danielle
Earl
Frances
Gaston
Hermine
Ivan
Jeanne
Karl Lisa
Matthew
Nicole
Otto
Paula
Richard
Shary
Tomas
Virginie
Walter
2005
Arlene
Bret
Cindy
Dennis
Emily
Franklin
Gert
Harvey
Irene
Jose
Katrina
Lee
Maria
Nate
Ophelia
Philippe
Rita
Stan
Tammy
Vince
Wilma
2006
Alberto
Beryl
Chris
Debby
Ernesto
Florence
Gordon
Helene
Isaac
Joyce
Kirk
Leslie
Michael
Nadine
Oscar
Patty
Rafael
Sandy
Tony
Valerie
William
2007
*Allison
Barry
Chantal
Dean
Erin
Felix
Gabrielle
Humberto
Iris
Jerry
Karen
Lorenzo
Michelle
Noel
Olga
Pablo
Rebekah
Sebastien
Tanya Van
Wendy
2008
Arthur
Bertha
Cristobal
Dolly
Edouard
Fay
Gustav
Hanna
Ike
Josephine
Kyle
Laura
Marco
Nana
Omar
Paloma
Rene
Sally
Teddy
Vicky
Wilfred
2009
Ana
Bill
Claudette
Danny
Erika
Fred
Kate
Larry
Mindy
Nicholas
Odette
Peter
Rose
Sam
Teresa
Victor
Wanda
For every year, there is a pre-approved list of names for tropical storms and hurricanes. These lists
have been generated by the National Hurricane Center since 1953. At first, the lists consisted of only
female names; however, since 1979, the lists alternate between male and female.
39
Hurricanes are named alphabetically from the list in chronological order. Thus the first tropical storm or
hurricane of the year has a name that begins with "A" and the second is given the name that begins with
"B." The lists contain names that begin from A to W, but exclude names that begin with a "Q" or "U."
There are six lists that continue to rotate. The lists only change when there is a hurricane that is so
devastating, the name is retired and another name replaces it.
If we're unlucky enough to deplete the year's supply of names we won't, contrary to popular opinion, simply
start using names from next year's list. In that case, the National Hurricane Center will turn to the Greek
alphabet and we'll have Hurricanes Alpha, Beta, Gamma, Delta, etc.
2. Hurricane Distribution
The map below shows the main zones of tropical cyclone formation and their local names. Worldwide,
their spatial distribution is not even. Hurricanes are generated between 5° and 20° either side of the
equator. They are the end product of a range of weather systems that can develop in the tropics. All
involve into areas of low pressure into which warm air is drawn.
Tropical cyclones do not occur all year round, but in distinctive seasons when the conditions necessary
for their formation occur. The season of tropical cyclone occurence is related to the movement of the
ITCZ with the overhead sun between the Tropics. Tropical cyclones occur during the summer season in
each hemisphere.
Certain factors seem important in the formation of tropical cyclone.
A location over seas with surface temperatures in excess of 26°C. This provides the initial heat
energy, the moisture to power intense condensation and convection, and a friction-free surface
to allow the continuous supply of warm, moist air.
A location at least 5° N/S of the equator. This allows for sufficient spin from the Earth's
rotation to trigger the vicious spiral in the centre of the hurricane.
A location on the western side of the oceans where descending air from the subtropical high is
weaker, allowing large scale upward convection to occur.
The presence of upper air high pressure. This ensures that air is sucked into the hurricane
system, causing a rapid uplift, huge volumes of condensation and massive clouds.
40
3. Scale of Hurricanes
Hurricanes are categorized according to the strength of their winds using the Saffir-Simpson
Hurricane Scale. A Category 1 storm has the lowest wind speeds, while a Category 5 hurricane has the
strongest. These are relative terms, because lower category storms can sometimes inflict greater
damage than higher category storms, depending on where they strike and the particular hazards they
bring. In fact, tropical storms can also produce significant damage and loss of life, mainly due to
flooding.
Scale
Wind Speed
Pressure
Storm Surge
1
118 - 153 kph
>980 mb
1.2 - 1.6 m
2
154 - 177 kph
965 - 979 mb
1.7 - 2.5 m
3
178 - 209 kph
945 - 964 mb
2.6 - 3.8 m
4
210 - 249 kph
920 - 944 mb
3.9 - 5.5 m
5
250 + kph
<920 mb
> 5.5 m
Damage Potential
Minimal: Damage to vegetation and poorly anchored
mobile homes. Some low-lying coasts flooded. Solid
buildings and structures unlikely to be damaged.
Moderate: Trees stripped of foliage and some
trees blown down. Major damage to mobile homes.
Damage to some roofing materials. Coastal roads
and escape routes flooded 2-4 hours before cyclone
centre arrives. Piers
damaged and small
unprotected craft torn loose. Some evacuation of
coastal areas is necessary.
Extensive: Foliage stripped from trees and many
blown down. Great damage to roofing materials,
doors and windows. Some small buildings structurally
damaged. Large structures may be damaged by
floating debris. Serious coastal flooding and escape
routes cut off 3-5 hours before cyclone centre
arrives.
Evacuation
of coastal residents for
several blocks inland may be necessary.
Extreme: Trees and signs all blown down. Extensive
damage to roofing, doors and windows. Many roofs
of smaller buildings ripped off and mobile homes
destroyed. Extensive damage to lower floors of
buildings near the coast. Evacuation of areas
within 500m of coast may be necessary and lowlying areas up to 10km inland. Major erosion of
beaches.
Catastrophic: Complete roof failure of many
residential and industrial buildings. Major damage
to lower floors of all structures lower than 3m above
sea level. Evacuation of all residential areas on low
ground within 16-24km of coast likely.
4. Effects of Hurricanes
Hurricanes have major impacts both on people and on the physical environment. These effects can be
split up into the following:
Physical Environment
High winds which destroy trees.
Tidal surge which causes flooding of low land near coast and eco-system damage in oceans.
Hugh rainfall causes flooding and landslides which remove vegetation & reduce slope angles.
Built Environment
Loss of communications (roads, elevated highways, railways, bridges, electricity lines).
Loss of homes
Loss of industrial buildings/facilities
41
Human Environment
Death and injury
Destruction of homes causing homelessness, refugees.
Loss of factories/industry causing a loss of livlihood and unemployment.
Loss of communications hindering rescue/emergency services and rebuilding/rehabilitation.
5. Factors Affecting Damage by Hurricanes
The effect of a hurricane on a community depends on a number of factors such such as physical factors,
economic factors and political factors:
Physical Factors
The intensity of the hurricane (measured from 1 to 5 on the Saffir Simpson Hurricane scale)
will be a major influence.
The hurricane intensity will be modified by the distance to the path of the hurricane, known as
the storm corridor. Destruction is significantly higher along the storm corridor.
There is also a relationship between distance from the sea and the amount of damage because
the hurricane dies as it moves inland.
Whether a settlement lies on the right or left of a hurricane's path can influence the
destruction caused. The hurricane's travel speed (perhaps 50 kph) is therefore added to the
windspeeds on the right of its path but subtracted from those on its left. This can result in a 96
kph difference in windspeed depending on which side of the storm centre you lie.
The travel speed of a hurricane also determines how long the hurricane takes to leave a
location. Hurricanes usually nudge their vicious wind circulations along at a leisurely 6-50 kph.
Slower moving hurricane systems can cause more damage because the destructive winds take
longer to move on.
High relief will exaggerate already high hurricane rainfall levels. Flooding, arising from these
high rainfalls, is often a major component of hurricane deaths and damage. Landslides are equally
dangerous in areas of high relief. On the other hand low relief will make a region more vunerable
to storm surges. Coastal flooding can be the main killer in a hurricane. The storm surge and
subsequent flooding is the greatest hazard to the people of Bangladesh since much of the
country is low-lying delta and floodplain, which is densely populated agricultural land. Most of the
coastal area is below 3m, where large numbers of people live on unstable sandbanks in the deltas.
An illustration of how physical factors can make the damage caused by a hurricane worse can be seen in
the case of Hurricane Mitch in 1998. Hurricanes, therefore, represent a mixture of hazards. Once the
hurricane strikes, damage, death and destruction may depend more on economic and political factors
than anything else!
Economic Factors
Economically the patterns of death and damage are related to the stage of development of the affected
nation. Poorer countries suffer because building codes, warning systems, defences, emergency service
and communications infrastructure may be inadequate, resulting in high death tolls. Wealthier countries
stand a better chance of evacuating people in time but have more to lose in simple material terms , so
therefore suffer greater economic losses!
Event
Cyclone Gorky (Bangladesh)
Hurricane Andrew (USA)
Date
May 1991
August
1992
Windspeed
232 kph
Death Toll
131,000
Damage ($US)
1.7 billion
264 kph
60
20 billion
42
The death tolls illustrate the huge differences between the vulnerability of American and Bangladeshi
citizens but the damage totals can be misleading. The raw figures suggest that the USA suffered more
damage because of the higher costs incurred but this is not true. The USA suffered a higher monetary
cost because the buildings, contents and infrastructure had a higher monetary value.
However, money is not a reliable measure since the loss of a home has a big impact on the family
whatever it cost. Indeed the loss of an American home worth $150,000 (and covered by insurance) may
be less significant than a tin and wood shack on the Ganges delta that represents years of irreplaceable
(and uninsured) savings.
An illustration of how economic factors can make the damage caused by a hurricane worse can be seen in
the case of Hurricane Mitch in 1998.
Political Factors
Political factors influence the underlying causes of poverty and vulnerability, but it is not simply national
politics and priorities which are to blame. International relationships are also responsible as shown in the
1988 Hurricane Gilbert in Jamaica. Prior to Hurricane Gilbert in 1988, Jamaica was already in debt partly as a result of previous hurricane damage. The high interest repayments on the debts saw the
Jamaican Government attempting to improve their economy by cutting public spending and reducing
inflation (by raising interest rates) to lower prices to encourage people to spend and make industry
more competitive. The increased interest rates reduced profits in the construction industry and
houses were built cheaply and shoddily. Cutbacks in health budgets reduced nutritional levels in a
country where more than 30% of the population live in poverty, more than 50% of women of childbearing
age are anaemic and 50,000 children under five are malnourished.
The combination of declining building standards and decreasing healthcare served to increase
Jamaica's vulnerability. Hurricane Gilbert came along and devastated the island in 1988, causing huge losses
to Jamaica's economy, estimated at some US $7 billion. This further increased Jamaica's debt, so the
Government is now looking at the possibility of mining peat from Jamaica's coastal wetlands to provide a
cheap fuel source. This will help the balance of payments and, economically, makes sense. Unfortunately,
it would also remove the first line of defence against hurricane surges. To pay for repairs from the
last hurricane it seems Jamaica has to increase its vulnerability to the next - a very vicious "vicious circle".
If the burden of Third World debt could be reduced, LEDC's could increase their "disaster resistance" by
focusing investment on development schemes aimed to improve the welfare of the rural poor.
The politics of war can also have an effect. In the case of Nicaragua in 1988 the long guerilla war and
US sanctions increased the impact of hurricanes, reducing the country's ability to cope. An illustration
of how political factors can make the damage caused by a hurricane worse can be seen in the case of
Hurricane Mitch in 1998.
6. Hurricane Management
Although hurricanes are neither the biggest nor the most violent storms experienced on the surface of
the Earth, they combine these two characteristics to become amongst the most destructive. During the
peak hurricane season in the northern hemisphere (1 June- 1 November) hurricanes pose a major
threat to human life, agriculture and assets both at sea and on land. The area of the Atlantic at
greatest risk from damage caused by hurricanes is between 10 and 30 degrees latitude. However, degraded
storms may travel back across the Atlantic and cause serious wind damage or flooding in Europe.
Since hurricanes have such a dramatic impact on human life, it is not surprising that people have invested
time and money in trying to predict their development, path and intensity. Prior to 1898, when the first
warning system was established, hurricanes arrived on land unannounced, resulting in enormous
43
damage and the tragic loss of life. Since then, methods of tracking and prediction have improved
enormously, notably with the introduction of air reconnaissance flights in 1944, the use of radar
technology and satellite imagery in 1960, and the introduction of computer-assisted modelling
techniques.
Today, one of the most elaborate and comprehensive hurricane warning services is funded by the US
government and located in Miami, Florida. At the National Hurricane Centre five hurricane specialists
are employed to monitor the path and development of hurricanes in the Atlantic and Eastern Pacific
Oceans. The specialists work shifts during the peak hurricane season to ensure 24 hour coverage of any
storm activity.
Methods of hurricane tracking and prediction
Satellite imagery forms the main source of information used by the hurricane specialists. Geostationary
satellites (predominantly GOES-7 and METEOSAT 3 and 4) supply images to the Centre at half hour
intervals. These are initially interpreted by satellite analysis teams, including the Satellite Analysis Branch
(SAB) located at the National Meteorological Center (NMC) in Washington, and the Tropical Satellite
Analysis Forecasting (TSAF) group located in Miami. The teams analyse images in the visible and infrared wavebands of the electromagnetic spectrum to produce an estimate of each individual storm's location
and intensity. Any revolving cloud activity located between 0° and 140° W is also brought to the
attention of the specialist on duty, as such systems often represent embryo tropical cyclones.
The independent analysis teams may produce different estimates of the location of the storm centre,
especially if the centre is poorly defined.
The hurricane specialist therefore has to co-ordinate the teams' information sets with a wide variety of
surface data in order to produce the most accurate estimate for the location of the storm.
This is particularly important because the computer models rely heavily on the accuracy of the location
estimate to produce reliable forecasts.
Satellite image used by the National Hurricane Centre
to monitor Atlantic hurricanes
The sources of surface information include measurements of
precipitation, wind and pressure characteristics available
from land and sea-based permanent recording centres, and
naval and commercial shipping in the immediate vicinity of
the storm. The availability of such measurements depends
heavily on the location and intensity of the storm,
particularly if it presents a threat to shipping. If the
surface data sources are limited and the storm is likely to
present a threat to land (increasing the need for reliable track
predictions) the specialist on duty may authorise a
reconnaissance flight through the centre of the storm.
The role of reconnaissance flights
In a reconnaissance flight a WC 130 aircraft equipped to make accurate measurements of wind speed
and direction, temperature, dew point and pressure is flown at 24,000 feet (approximately 7.3 km)
through the storm several times. This procedure, though safe, requires a team of six highly qualified
experts and a great deal of initial capital input. In addition it has been estimated that each hour of a
reconnaissance flight costs around $2,500 US dollars, and an estimated 10-12 flying hours are required
per mission. For this reason, only when storms present a major threat to US assets or Caribbean
44
countries, justifying the benefit of accurate information on the nature and location of a storm, can a
reconnaissance flight be' authorised. The higher number of reconnaissance flights flown through
Atlantic storms is one of the reasons why they are more accurately predicted than Pacific storms.
The role of computer technology
All the available data on a particular storm are entered on to a computer and can be compiled as a single
image. The specialist can manipulate the image by enhancing the scale and definition, or adding colour,
degrees of latitude and longitude, and coastlines.
Computer generated maps allow a storm's path to be tracked and predicted
Using all the data an estimate of the
initial position of the storm (or
position at the time of the final satellite
image) and the actual position of the
storm (or location at the time of
broadcast) is made.
These data are then entered into a
database and transmitted to the
National Meteorological Center (NMC) in
Washington. Here meteorological data
from stations all over the world are
compiled and the information is used to
run climate simulation models on the
powerful
'Cray'
computer.
Approximately
12
models
are
currently used including statistical,
dynamical and statistical-dynamical
models.
Statistical models such as CLIPER are based on the idea that the track of a storm under a given set of
meteorological conditions is repeatable. Thus by comparing the location of the storm and the
surrounding atmospheric conditions with the actual tracks of storm activity recorded over the last 50
years an estimate of the forecast track is provided. The reliability of this model is not only affected by
statistical limitations but also by the presumed accuracy of storm activity records in the past. However,
statistical models do have the advantage of requiring only limited computer power to run, and producing
results within seconds.
Dynamical models are based on far more detailed and complex ideas about the climate of the Earth.
These mathematical models assume the Earth is a rotating sphere surrounded by a gas (representing the
atmosphere). Data are collected from all over the world and used to predict global atmospheric
conditions. To predict the development and track of tropical cyclones a spiralling vortex is inserted into a
model of predicted atmospheric conditions - rather like a spinning cork in a basin. Dynamical models are
considerably superior to statistical models but they too have their limitations. For example, the process of
data collection takes approximately 2 hours, and a further hour is required to run the model.
The predictions are therefore not received by the specialists in time for the production of the
'immediate' package and have to be used in the following shift forecast, by which time the information is
comparatively out of date. As with the statistical models, the forecast is only as accurate as the data
input, and a regular compatible supply of data is required from Third as well as First World countries.
The accuracy of the forecast therefore varies on a daily basis and a simple check of the reliability of
the forecast can be made by comparing the predicted atmospheric conditions with the actual
atmospheric conditions of the previous day.
45
Dynamical-statistical models are the most complex and potentially the most accurate models,
incorporating the principles of both the above types of model. However they are still in the early stages
of development.
The human element
All the separate model forecasts described above are collated on a single map. However, as a result of the
differing natures of the models and their limitations, the forecasts produced by each are often
different, and sometimes contradictory. The role of the specialist is to translate all the different objective
forecasts into a single subjective forecast. To do this he or she uses statistics which indicate the reliability
of each model in previous predictions, coupled with personal experience to weight each particular model.
The track and intensity forecast issued by the previous watch and the storm's proximity to land
must also be taken into consideration before predictions can be finalised.
When a tropical cyclone is threatening land the specialist must decide which section of the
coastline to place under watch or warning. This is done in conjunction with the predicted track forecast
and the local authorities who can provide advice about suitable breaks in the watch or warning zones.
The specialist must weigh up the advantages of over-preparing the population against the disadvantages
caused by under-preparing them. It may seem obvious that the specialist should err on the side of
caution - after all lives and consider- able property damage are at stake. However, the considerable
personal inconvenience and financial cost of evacuation must be taken into account!
For example, it has been estimated that the financial cost of evacuating a 500 km stretch of the US
coastline is approximately US$50 million (measured in terms of lost business and tourism, coupled with
the expenses of property protection and evacuation procedures). It is also vital to prevent repeated
unnecessary warnings lulling the population into a false sense of security and generating complacency.
The hurricane specialist must also define each storm and classify the intensity based on the SaffirSimpson scale. Although satellite images can be used to estimate the intensity of the storm using the
shape and patterns of cloud formation, and the various surface recording stations provide accurate records
of wind speed at a particular place and time, the conditions within a storm are, spatially and temporally,
highly variable. This means that the probability of actually measuring the highest wind speed of a particular
storm is remote. The difference therefore, between a 'very strong tropical storm' and a
'weak' hurricane is not only nominal but subjective, depending on the data available and the specialist on
duty. Whilst this has no significant direct impact on the public it does affect statistical records and hence
the statistical models.
Hurricane prediction: a success story?
Methods of hurricane prediction and tracking are constantly updated and improved. An important part of
the specialists' work is the detailed collection of data and model verification (checking the forecast
accuracy) for each individual storm. Verifications of some of the models are beginning to indicate that
they are producing superior forecasts to the subjective analyses. There is therefore a realistic hope of
producing accurate 72 hour forecasts in the near future. (Due to the chaotic nature of the climatic
system accurate forecasts beyond 72 hours are unlikely)
In terms of saving life the hurricane prediction service is an undoubted success. Since 1898 the death
toll has been continually and dramatically reduced due to the increased accuracy and efficiency of the
warning systems, and better education and communication facilities. The material damage caused by
hurricane activity, however, has dramatically increased in this period. This is predominantly because of
increased wealth and the concentration of material goods and people in zones at risk from hurricane
activity. A 1992 survey pointed out that 80-90% of the US coastal population had never experienced a
46
category 3 hurricane. This bred a false sense of security as the population did not comprehend, or ignored,
the risk of hurricane activity and took inadequate precautions and unnecessary risks.
The enormous damage caused by Hurricane Andrew in 1992 was largely a result of this complacency. As
the population density in coastal regions at risk from hurricane activity increases and the evacuation
systems are put under stress, there is a danger that in the future, death tolls from hurricanes may
again rise. Perhaps the challenge for the hurricane prediction service is to co-ordinate the increased
accuracy of prediction techniques with education programmes and improved safety and protection measures
to ensure the continued protection of life and property.
Hurricane Mitch, Central America (LDC‟s) 1998
On 22nd October 1998 a tropical storm formed in the Atlantic. Within 4 days the storm had grown to a
category 5 hurricane (Saffir Simpson scale) gusting at over 200 mph (320 kph). It remained a category
5 hurricane for 33 hours then the windspeeds began to fall as Mitch drifted towards Honduras. But wind
was not the problem with this monster. Mitch made landfall on the Honduras coast on 30th October.
Normally when a hurricane hits land it begins to die. The warm, moist oceanic air which drives the
hurricane's energy is replaced by dry continental air. Lack of moisture means lack of condensation - so
no more release of latent heat to drive the hurricane. Unfortunately a whole host of background causes
had reduced the vulnerability of this area, and so when the trigger event of Hurricane Mitch all hell
broke loose. A disaster waiting to happen!
Background Causes
1. Economic
This region has debts of $4 billion owing to the west. Every day Nicaragua and Honduras spend a
total of $2 million on repayments to creditors
Poverty has increased in both absolute and relative terms. Many towns had no storm drains.
2. Political
In a recent survey measuring perceptions of corruption among private business leaders in 89
countries around the word, Honduras was ranked third.
The natural reservoir of Laguna de Pescado on a tributary of the River Choluteca was formed
some years ago after a landslip blocked the river. The authorities never got round to removing it.
Communities were allowed to build on river banks and steep, unstable hill slopes.
3. Social
Tropical rainforest is disappearing from the Caribbean coast at a rate of 80,000 ha a year,
caused mostly by farmers burning trees to create farmland.
Towns grew rapidly as people migrated from rural areas for jobs.
Many thriving towns were situated on fertile farming area near the west coast of Honduras and
Nicaragua which was formed from volcanic ash. These volcanic soils are easily washed away.
4. Environmental
Many towns were situated in narrow, steep-sided mountain valleys.
Soils were saturated by weeks of wet weather.
47
Trigger Causes
48
Hurricane Mitch hit Honduras on 29th October 1998; it was a Storm 5 category hurricane. It
was the fourth fiercest in the Caribbean this century.
When the storm reached Central America it stalled for 2 days over the mountains of central
Honduras.
The mountains forced air to rise to 2,000m, cool and condense, then dump huge amounts of
moisture picked up from the sea.
Over 2 days about 40 cubic km of water fell on Honduras, and neighbouring areas of Nicaragua
and El Salvador.
Hurricane Katrina (MDC), 2005: Video Notes
Hurricane Pam: One year before Katrina Pam hits starting doomsday scenario planning for the next
major hurricane. New Orleans identified as being at most risk!
Baton Rouge, Louisiana July 2004:
New Orleans Levees damaged by floods
61 000 dead
380 000 injured and sick
½ million people homeless and ½ million buildings damaged
1 million people evacuated
Washington takes charge of relief effort.
responsibility.
Local, state and Federal Government now their
Occurrence:
The date, place and time of landfall were predicted
by the National Hurricane Center Miami. However, it
was still one of the most deadly hurricanes of modern
times!
August 24th 2005 Tropical thunderstorms in Atlantic
central Caribbean 38mph winds classified as a
tropical storm.
In Miami the national hurricane
centre predicts within 36 hours hurricane conditions
will affect south Florida.
Hurricane Katrina
eventually ranked as the 6th strongest hurricane of
all time. "Hurricane Katrina in 2005 was the largest
natural disaster in the history of the United States.
Preliminary damage estimates were well in excess of
$100 billion, eclipsing many times the damage
wrought by Hurricane Andrew in 1992.
Map of Central New Orleans
49
Thursday Aug 25th 2005
National Hurricane Center spot a mass of Tropical thunderstorms in the Atlantic with a counter
clockwise rotation – 33mph – Tropical storm
Miami – National hurricane centre – 36 hours hurricane conditions predicted for S. Florida. 74mph – a
category 1.
06:30 hits Florida Shore: $460 million damage, 14 dead, High winds – slow moving 8mph (weak but slow ½
the speed of a normal hurricane)
Lead time given in order to prepare response: Truck loads of material gathered – bottled water pop
tarts, flash lights. Warlmart – Bentonville, Arkanas set up emergency response team.
Advisory estimated within 36 hours
Shelters, feeding units in the west and gulf coast set up by red cross. Every response starts from the
bottom up. FEMA (Federal Emergency Management Association) becomes involved.
Friday 26th Aug
National Guard deployed
Oil Companies evacuated offshore
11:30 –Katrina Strengthens to a Cat 2 soon to be a 3 within 24hrs
Target of Katrina Florida Pan Handle of Louisiana (New Orleans).
New Orleans is built below sea level, crecsent shaped, gulf of
mexico 100 miles away, Mississippi runs through it ,Lake
Pontchartrain to the north. Some areas 6ft lower such as the 9th Ward, protected by earthen levees
and flood walls. Some walls and levees are sinking and in need of desperate repair, usually mainteained by
the US army corps.
5pm NW of Florida Quays  Target west of Florida pan handle, New Orleans will be hit within 72hours
People don’t fear hurricanes here as it is a common occurrence.
50
11pm Buras Louisiana 72 miles south of New Orleans is predicted to be place of landfall and will later be
proved to be accurate.
Saturday 27th Aug
FEMA wants to arrange distribution points with federal government. Katrina loses energy overland then
re-energise over the Gulf of Mexico, conditions greater than 26oc encourage this!
Category 3 - 115mph
Propels storms surge landwards. EVACUATION starts when storms is 30 hours away, mayor warns low
lying areas to evacuate!
FEMA was downgraded after 911 and taken out of the whitehouse and put into office of security. Many
blame this reason later for its slow response.
12ft waves – New Orleans flood gates close including those on the Industrial Canal , 17th St Canal and
the London Canal. They are only protected by walls 13-18ft
high.
Personal responses begin, people secure properties, board up
windows and stockpile food and water.
Saturday night in the French Quarter the bars are full – the
ultimate fatalistic approach to the hazard.
Sunday 28th Aug
worst case scenario – Katrina becomes a Cat 4. Cat 5 by 7am hit shore within 24 hours 125 miles wide.
Superdome a 70 000 seat stadium home of the New Orleans Saints football team acts as a refuge centre.
At 8 am Superdome takes in people. Mandatory evacuation still not ordered yet!
9.20 Bush Calls governor Blanco to discuss plan for evacuation.
only 20 hours to go! – Too late!
Mandatory evacuation ordered now with
6pm a curfew is imposed on people in the city of New Orleans.
10‘s of thousands not moving. Reguests to bring food and water to Superdome to last 5 days. State
insists on no drugs, guns or alcohol. 2.5 millions of bottles of water arrive one million MRE‘s (Meals Ready to Eat)
also arrive. 1 million now evacuated. Wait for buses to the super dome takes hours.
Winds now 200mph - Traffic on roads out snarls up heading inland due to high volumes of traffic. Gas
stations close and run out of fuel. Grocery stores run out of supplies. Some people decide to stay in
homes and stick it out!
Thousands of people have no cars to move out, approx 20% of the population. These same people don‘t
have money either as the storm hit at end of the month and their benefit cheques had already run out! Its
the poorest often the black population that will suffer the most in this type of disaster! Therefore they
have to be moved by bus, it a slow process. The poverty level in New Orleans is 23% double that of
national average. Murder rate in many wards is also higher than average. The deprived 9th Ward is typical
of this poverty and is 4ft below sea level.
President Bush still on vacation now declares a state of national emergency. State Search and rescue
teams (262 people) delayed! Due to FEMA bureaucracy in Washington DC Headquarters.
51
Mon 29th Aug
Katrina cat 3 or 4 weakens and leading edge hits towns on gulf coast.
4am winds drive a 14-17 ft storm surge inland.
5:10 Electricity lost including Superdome – only using back up generator. 100 000 without power
6:10 Katrina 60 miles east of New Orleans. Storms surge pushes up Mississippi River at the „funnel‟
where the intercoastal waterway meets the industrial canal to the east of this is the lower 9th ward and
the St Bernard Parish. Superdome roof damaged as winds rip away 12ft sections. Levees overtopped
Phone system down across city.
7:45 Lower 9th Ward Levees erode along the industrial canal! New Orleans east also 12 ft above sea
level. Levees on lake Ponchatrain also burst!
10:00 am Katrina NE at Bay St Louis, Waiverly, Port Cristiane and Gulf port on the state boarder
(lousiana/mississippi).
Biloa, Mississippi
Mobile Alabama –10 ft under water
Jackson Barrocks floridaPeople move to higher floors. FEMA has no emergency equipment as it us an
agency that relies on private contractors. This slows down the emergency response. General Honore
leads Millitary response. US military and Private contractors involved in relief effort (FEMA has no
actual equipment and relies on third parties such as these). TV phone radio and Satellite out, local
network also out.
Lake Ponchatrain overtopped by storm surge that overtops levees
London avenue and 17th St Canals full with water, London avenue
canal west side now fails and 17th St Canal east side fails and
covers the Western Parish now covered with 6-9ft of water.
Lower 9th ward is now a lake!
Entertainer Fats Domino now rescued.
1pm media report downtown ―Orleans missed the bullet but took a deadly punch‖. This was the message
Washington DC was receiving  how wrong! The Grand Casino on Gulf Port was moved 150 yards.
Thursday 1st Sept
80 percent of New Orleans damaged and under water
200 000 homes destroyed
249 police officers left their posts!
Many criticisms now voiced! No system in place early enough.
20 000 people in superdome, toilets back up and they are left with no food, no water, no medicine
1 million evacuated. FEMA did not know where the shelter points were and dis not knbow who needed
the help!
Wide spread looting in all areas – humanitarian effort delayed as order restored first. Bureaucratic
process also stops rescue teams from beginning work for 2 days even though they are ready to go as
paper work needed to be signed.
52
The armed corps tried to block uo the levees that failed with sandbags, however this failed. The pumps
also failed to drain the city.
Reports of mass murder and gang rapes turn out to be incorrect and over exaggerated. Media also over
reposted some statistics. Still why is there no state control? Governor of Louisana, Kathleen Blanco
does not want to appear politically weak!
Local responses include a local bus company that takes 6 buses full of supplies to the disaster zone.
Generators don‘t arrive that were requested therefore can‘t pump out the city therefore delaying
relief effort. As the generators were not working the Superdome sewage could not be pumped out
causing a further medical disaster and spread of disease. The levees that were designed to save the city
actually keep the water in. Michael Brown FEMA director quits after Hurricane Katrina as he failed to
recognise Louisiana state as a problem area.
After the event President Bush admits that Federal, State and Local Governments were not prepared
and he takes full responsibility for the shortcomings.
The broken levees were repaired by engineers and the flood water in the streets of New Orleans took
several months to drain away. The broken levees and consequent flooding were largely responsible for most
of the deaths in New Orleans. One of the first challenges in the aftermath of the flooding was to repair
the broken levees. Vast quantities of materials, such as sandbags, were airlifted in by the army and air
force and the levees were eventually repaired and strengthened. The reopening of New Orleans was
delayed due to the landfall of Hurricane Rita.
Although the USA is one of the wealthiest developed countries in the world, it highlighted that when a
disaster is large enough, even very developed countries struggle to cope.
Hurricane Rita hits just a few weeks after Katrina and this time the government at all levels is
much better prepared.
Hurricane Rita was the fourth-most intense Atlantic hurricane ever recorded and the most intense
tropical cyclone ever observed in the Gulf of Mexico. Rita caused $11.3 billion in damage on the U.S. Gulf
Coast in September 2005.[1] Rita was the seventeenth named storm, tenth hurricane, fifth major hurricane,
and third Category 5 hurricane of the historic 2005 Atlantic hurricane season.
Rita made landfall on September 24 between Sabine Pass, Texas, and Johnsons Bayou, Louisiana, as a
Category 3 hurricane on the Saffir-Simpson Hurricane Scale. It continued on through parts of
southeast Texas. The storm surge caused extensive damage along the Louisiana and extreme
southeastern Texas coasts and destroyed some coastal communities. The storm killed seven people directly;
many others died in evacuations and from indirect effects.
Hurricane Katrina tracked over the Gulf of Mexico and hit New Orleans, a coastal city with huge areas
Summary of Impacts:
1,500 deaths in the states of Louisiana, Mississippi and Florida.
Costs of about $300 billion.
Thousands of homes and businesses destroyed.
Criminal gangs roamed the streets, looting homes and businesses and committing other crimes.
Thousands of jobs lost and millions of dollars in lost tax incomes.
Agricultural production was damaged by tornadoes and flooding. Cotton and sugar-cane crops
were flattened.
53
Three million people were left without electricity for over a week.
Tourism centres were badly affected.
A significant part of the USA oil refining capacity was disrupted after the storm due to flooded
refineries and broken pipelines, and several oil rigs in the Gulf were damaged.
Major highways were disrupted and some major road bridges were destroyed.
Many people have moved to live in other parts of the USA and many may never return to their
original homes.
54
7. Global Climate Change
‗The issue of climate change is one that we ignore at our own peril. There may still be
disputes about exactly how much we're contributing to the warming of the Earth's
atmosphere and how much is naturally occurring, but what we can be scientifically certain
of is that our continued use of fossil fuels is pushing us to a point of no return. And unless
we free ourselves from a dependence on these fossil fuels and chart a new course on
energy in this country, we are condemning future generations to global catastrophe.‘
Barack Obama 2009.
Description of Climate Change since the Last Ice Age
The global climate has varied over geological time
considerably.
There has been colder periods
known as glacial and warmer periods known as
interglacials (see diagram right).
The climate of Britain has varied greatly over
the last 20 000 years. There has been a
gradual warming since the end of the last
Pleistocene Ice Age. The Pleistocene was an
epoch of cool glacial and warmer interglacial
periods which began about 2 million years ago
and ended in the British Isles about 11 500
years ago.
Temperatures continued to rise
after the localised glacial re-advancement and
cooler conditions of the Younger Dryas (13 000-11 500 yrs BP.) and warmed to reach the climatic
optimum 6 000 years ago in the Atlantic Period and since then a gradual cooling leading to the "Little Ice
Age" (not a real Ice Age by the way) between 1500 & 1700 years BP. In the last 150 years, however,
there has been a rapid warming associated with the human enhanced greenhouse effect.
The climate graph below shows how climate in Britain has changed over the last 15 000 years since the
last Ice Age.
The current view held by the majority of climatologists is we are currently in a warmer period of time
known as an interglacial. One of the key questions climatologists hope to be able to answer is will there
55
be a new glacial period soon (geologically speaking), or will temperatures continue to rise in the
near future?
The phenomena of observed warming over the last 150 years referred to as recent global warming
appears to be linked to anthropogenic (man-made) increases in carbon dioxide levels associated with the
burning of so called fossil fuels, namely: coal, oil and natural gas which release carbon dioxide that has
been stored and subsequently locked away in the underlying strata for up to 350 million years. Indeed
evidence obtained by a team of Northern American Scientists calling themselves the Greenland Ice
Sheet Project 2 (GISP2) and their European counterparts the Greenland Ice Core Project (GRIP)
reveal temperature does indeed appear to increase when atmospheric CO 2 is high. Ice cores of up to
3km in depth taken from the Greenland Ice sheet have allowed trapped bubbles of air to give scientists
a proxy (estimation) of the climate: Firstly, the composition of stable isotopes of oxygen have been
analysed using mass spectrometry and the ratio of O16 to O18 calculated and compared to that of
Standard Mean Ocean Water (SMOW), which is the water from the deep oceans which is uniform in
composition to which all other ocean water samples are compared. You can think of it as comparing the
sample to a kind of ‗chemical zero‘ so it can be determined weather a sample is anomalously enriched or
deficient in heavier O18. If the air sample from the ice contains a high ratio of O16 to O18 then this
indicates a period of cooler conditions (See later in notes for how this works). CO 2 concentrations have
also been measured from air bubbles in parts per billion using mass spectrometry and concentrations of
high CO2 appear to match that of higher temperatures indicating that the two are linked (see graph
below)
Temperature calculated from ice core data plotted with C O2 Concentration for the last 150 thousand years
.
The graph below (although complicated looking) shows the melt years of the Greenland Ice Sheet over
the last 15 000 years compiled by the GISP2.
The dotted red line shows individual melting events
indicating a warmer climate and the black line is a mean or average showing the frequency of melting events.
You should notice that the melting events correspond to the periods of warmer temperatures indicated
in the table and graph we used in lessons. This is the evidence that supports the current theory of
temperature change.
Graph Shows GISP2 Ice Melt Periods over the last 15 000 years (Holocene)
56
If you want to read more about how climate has changed over the last 15 000 years please try
this link, it is a fascinating website! http://muller.lbl.gov/pages/IceAgeBook/history_of_climate.html
(a) Evidence for Climate Change
1. Historical Records
Historical records have been used to reconstruct climates dating back several thousands of years.
Historical data can be grouped into three major categories. First, there are observations of weather
phenomena, for example the frequency and timing of frosts, or the occurrence of snowfall.
Meteorological data, in the form of daily weather reports, are available for the British Isles from
1873 onward. Secondly, there are records of weather-dependent environmental phenomena, termed
parameteorological phenomena, such as droughts and floods. Finally, there are phenological records of
weather-dependent biological phenomena, such as the flowering of trees, or the migration of birds.
Major sources of historical palaeoclimate information include: ancient inscriptions; annals and chronicles;
government records; estate records; maritime and commercial records; diaries and correspondence;
scientific or quasi-scientific writings; and fragmented early instrumental records.
Much of this
historical evidence is fragmental or incomplete and therefore does not give us an entire archive of past
climate. In recent history a variety of events have been evidenced through historical records such as:
records of vineyards in Southern Britain dating back to 1600 years BP when climate was warmer than
today.
Exeter University school of Archaeology and Geography has identified 7 Romano-British
vineyards 4 in Northamptonshire (then the Nene Valley) and one in Cambridgeshire, Lincolnshire and
Buckinghamshire respectively. Northamptonshire would have had a slightly warmer climate and was in
the lower end of the precipitation range meaning less fungal infections making grape growing conditions
favourable. Frost fairs were also held on the River Thames in Tudor times (1400-1600AD) indicating
colder climatic conditions, although the Old London bridge constricted the Thames‘ flow and may be in
part to blame for the easier onset of freezing than today. Indeed in 1683 the Thames froze for 2
months to a thickness of 11 inches. The last frost fair was held in 1814 and since then the Thames has
never completely frozen.
2. Ice Cores
As snow and ice accumulates on ice caps and sheets, it lays down a record of the environmental
conditions at the time of its formation. Information concerning these conditions can be extracted from ice
and snow that has survived the summer melt by physical and chemical means. Palaeoclimate
information during the Ice Age (last 130,000 years) has been obtained from ice cores by three main
57
approaches. These involve the analysis of: a) the (isotopic)
composition of the water in the ice (see below); b)
dissolved and particulate matter in the ice; and c) the
physical characteristics of the firn and ice, and of air
bubbles trapped in the ice, such as carbon dioxide and
methane concentrations in air bubbles trapped in the ice.
Carbon dioxide concentrations correspond with other
indicators; carbon dioxide values were low during colder
periods and higher during warmer phases. The majority of
research has taken place on the Greenland ice sheet by the
European Greenland Ice Core Project (GRIP) and by their
North American counterparts the Greenland Ice Sheet
Project 2 (GISP2).
The Russians have also drilled
boreholes into the ice and the most famous is Vostok in
Greenland. These two teams have retrieved ice core samples
totalling just over 3km in length i.e. 3km deep into the ice.
Oxygen isotopes taken by these teams of scientists have
been used to deduce the past climate over
the last 420 000 years. Ice core records go back no further than this, but evidence recorded in deep
marine sediments do (see later notes).
3. Dendrochronology
The study of the relationships between annual tree growth and climate is called dendrochronology. Trees
record climatic conditions through growth rates. Each year, a tree produces a growth ring made up of
two bands: a band reflecting rapid spring growth when the cells are larger, and a narrower band of
growth during the cooler autumn or winter. The width of the tree ring indicates the conditions that
prevailed during its growth cycle, a wider ring indicating a warmer period. The change in ring width from
year to year is more significant than the actual width because bigger growth rings tend to be produced
during the early years of growth, irrespective of the weather. It is possible to match and overlap
samples from different sources, for example from living trees (e.g. Bristlecone Pines), trees preserved
for 10, 000 years in river terraces in Europe and from beams from older houses, all extend the dating
further back in time.
NB. Bristlecone Pines in California which have been living for the past 5,000 years, give a very acurate
measure of the climate.
58
However dendrochronology is fraught with complications and limitations. For instance there are other
factors apart from temperature which affect tree growth, for instance soil type, rainfall, human
activity, light, carbon dioxide concentration and disease.
4. Pollen Analysis (and movement of vegetation belts)
Many plant species have particular climatic requirements which
influence their geographical distribution. Pollen grains can be
used to determine the vegetation changes and by implication, the
changes in climatic conditions. The first plants to colonise land as
climate warms up after an ice age are low tundra plants, mosses
and heather. Once these become established trees like birch,
pine and willow will start to grow. Eventually when conditions are
56
much milder trees like oak and elm flourish. So by boring into peat bogs (which preserve the ancient
pollen grains) pollen from different plants gives an indication of the climate at that time.
5. Oxygen Isotope Analysis
As long ago as the 1950's a significant breakthrough in knowledge about past climate change came from
the analysis of tiny fragments of calcium carbonate (shell material) that constantly accumulate on parts of
the deep ocean floor. These are the shells of single-celled marine organisms called formanifers which make
up elements of planktonic life in the surface waters. At the time of their formation, the shells lock up key
information about oxygen isotopes present in the ocean surface water. It was discovered by Emiliani in
1954 that the ratio of the heavy isotope of oxygen (O18) to the lighter one (O16) can be interpreted to
give an estimate of sea surface temperature. When ice sheets grew during colder glacial times, the
evaporation of water from the oceans was reduced. Because of this reduction the lighter and more readily
evaporated O16 was preferentially evaporated into the air, leaving the oceans relatively enriched in the
heavier O18 isotope. This means that the shells of foraminifers growing during glacial times were relatively
enriched in O18, whereas the oxygen locked in the air bubbles in the growing ice sheet was relatively
enriched in the lighter O16 isotope.
Data from the Vostok and GISP2 cores for the last 120 000 years showing peaks of 018 (relatively
warm) and troughs of O18 on the graph (relatively cool). Notice the trend in greater O18 isotopes in the
ice accumulating in the ice sheet over the last 15 000 years. This suggests a warmer interglacial period.
57
6. Deep Marine Sediments
Deep marine sediments also record changes in the past climate as silts and lime muds are deposited over
time. Within these muddy layers are trapped microfossils of foraminifera (forams) and diatoms which
have CaC03 (calcium carbonate) shells made up from the oxygen held in the ocean water at the time of
their formation. Therefore, oxygen isotope ratios can be calculated and shells enriched in O18 would
indicate cooler conditions. The main advantage of deep sea sediments as a record of climate change is
they date much further back in time than the the 420 000 year record obtained from the ice sheets.
Deep marine sediments also provide evidence of past climatic temperature deduced from the most
abundant species of single celled foriminifera (Forams) and Diatoms in a sample which can be recognised by
their test (exoskeleton) morphology (shape). This is determination of species based on shape is
termed morphospecies, it is estimated that there are over 4000 such species living in the benthic
environment (deep marine).
Studies of present day marine environments by marine biologists and
micropalaeontologists have highlighted the common species of diatoms and forams that occur in waters
of certain temperatures. By using the present is the key to the past principle climate scientists
researching the in the field of micropalaeontology can use this modern day species distribution to infer
the temperatures found in past deposits based on the microfossils found in a sample. That is to say
different shaped foraminifera prefer different environments.
Diatoms
Foraminifera
7. Glacial and Post Glacial landscapes and Deposits
Glacial Landscapes such as those found in North Wales, the Lake District and Scotland in particular
could not have been caused by present day climatic conditions so therefore indicate the role of ice. Cwn
Idwall and the Nant Ffrancon as well as the tills at Aberogwen indicate glacial erosion and
glacial/fluviouglacial deposition respectively. Periglacial overprinting after the ice diminished has left
its effects plain on the landscape such as patterned ground in the NE Cairngormes and the Tors on
Dartmoor.
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8. Radio Carbon Dating
Modern methods of dating materials, such as carbon-14 dating, allow specific dates to be added to the
sequence of temperature changes identified from pollen and dendrochronology. All vegetation fix (take
into their structure) carbon dioxide from the atmosphere via photosynthesis and store it as
carbohydrate (CH2O) such as starch. C14, found in bone/wood from prehistoric organic remains is
unstable whereas C12 and C13 are stable. C14 has a half Life of 5,730 years +/- 40 years i.e. Half of C14 present in a sample will decay during this time period. If a scientist compares the amount of C14
isotopes in a prehistoric sample with the amount of C14 in a sample from a growing plant today the age
can be deduced. After ten half lives there is not much C14 left in the sample therefore this method is
only accurate to 50,000 years BP.
9. Insect Analysis and Coleoptera (Beetles)
These are insects with the largest known number of species.
Among the beetles (Coleoptera) alone there are about 350
000 different species- more than all flowering plants
combined – so, not surprisingly many are adapted highly to
specific ecological niches. For example many species have
fastidious preferences for warmth or cold. Insect taxonomy
is based on the characteristics of their exo-skeletons.
Skeletons are often well preserved in sediments, but
become disaggregated into their component parts (heads,
thorax etc.).
To date the main contribution has come from fossil Coleoptera, in particular in the reconstruction of the
late Pleistocene palaeoclimates in mid-high latitude regions such as northwest Europe. Their thermal
likes and dislikes coupled with their ability to migrate rapidly, make Coleoptera the ideal indicators of
past temperatures. Studies at a variety of European sites have allowed Russell Coope (1975;Coope and
Lemdhal, 1995) to estimate average July temperature changes before 15 000 to about 10 500 Cal.yr
BP.At the beginning of this period temperate assemblages were replaced by arctic ones, and at
Glanllynnau in North Wales –at least an amazingly rapid 1oC per decade. Beetle assemblages have now
been used to provide evidence of both winter and summer temperatures using the mutual climatic range
method (Aktinson et al., 1987). In the Holocene (after 11 500 Cal. year BP) the use of Coleoptera is
much more problematic.
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(c) Recent Global Warming - its Causes and Effects
Causes:
The Earth has warmed up by about 0.6°C in the last 100 years. During this period, man-made emissions
of greenhouse gases have increased (eg. carbon dioxide concentration in the atmosphere has risen to
370 parts per million (ppm) from 270 ppm), largely as a result of the burning of fossil fuels and
deforestation. In the last 20 years, concern has grown that these two phenomena are, at least in part,
associated with each other. That is to say, global warming is now considered most probably to be due to
the enhanced greenhouse effect. Other greenhouse gases released by mans activities may be also
playing a role in the onset of anthropogenic (man-made) global warming.
For instance rapid
development in LDC‟s is driving demand for cheap meat production which is firstly causing
deforestation to allow low grade pasture for grazing. Secondly cheap meat production releases methane
into the atmosphere which is a greenhouse gas that causes warming.
(i) Effects on the UK
Introduction: Most critical of the risks associated with global warming is an increase in frequency and
intensity of extreme weather such as hot spells, drought and storms. Accompanying a projected rise in
average surface temperature of between 0.9 and 2.4°C by 2050 will be the increased occurrence of
hot, dry summers, particularly in the southeast. Mild wet winters are expected to occur more often by
the middle of the 21st century, especially in the northwest, but the chance of extreme winter freezing
should diminish.
Higher temperatures may reduce the water-holding capacity of soils and increase the likelihood of soil
moisture deficits, particularly if precipitation does not increase as well. These changes would have a
major effect on the types of crops, trees or other vegetation that the soils can support. The stability
of building foundations and other structures, especially in central, eastern and southern England, where
clay soils with a large shrink-swell potential are abundant, would be affected if summers became drier
and winters wetter.
Any sustained rise in mean surface temperature exceeding 1°C, with the associated extreme weather
events and soil water deficits, would have marked effects on the UK flora and fauna. There may be
significant movements of species northwards and to higher elevations. Predicted rates of climate
change may be too great for many species, particularly trees, to adapt genetically. Many native species
and communities would be adversely affected and may be lost to the UK, especially endangered species
which occur in isolated damp, coastal or cool habitats. It is likely that there would be an increased invasion
and spread of alien weeds, pests, diseases and viruses, some of which may be potentially harmful.
Increased numbers of foreign species of invertebrates, birds and mammals may out-compete native species.
Climate changes are likely to have a substantial effect on agriculture in the UK. In general, higher
temperatures would decrease the yields of cereal crops (such as wheat) although the yield of crops
such as potatoes and sugar beet would tend to increase. However, pests such as the Colorado beetle on
potatoes and rhizomania on sugar beet, currently thought to be limited by temperature, could become more
prevalent in the future. The length of the growing season for grasses and trees would increase by about
15 days per degree Celsius rise in average surface temperature, an increase that could improve the
viability of crops such as maize and sunflower, which are currently grown more in warmer climates.
Increases in Eustatic sea level (Global), and the frequency and magnitude of storms, storm surges and
waves would lead to an enhanced frequency of coastal flooding. A number of low-lying areas are particularly
vulnerable to sea level rise, including the coasts of East Anglia, Lancashire, Lincolnshire and Essex, the
Thames estuary, parts of the North Wales coast, the Clyde/Forth estuaries and the Belfast Lough.
Flooding would result in short-term disruption to transport, manufacturing and housing, and
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long-term damage to engineering structures such as coastal power stations, rail and road systems. In
addition, long-term damage to agricultural land and groundwater supplies, which provide about 30% of
the water supply in the UK, would occur in some areas due to salt water infiltration.
Water resources would generally benefit from wetter winters, but warmer summers with longer growing
seasons and increased evaporation would lead to greater pressures on water resources, especially in the
southeast of the UK. Increased rainfall variability, even in a wetter climate, could lead to more droughts in
any region in the UK. Higher temperatures would lead to increased demand for water and higher peak
demands, requiring increased investment in water resources and infrastructure. An increase in temperature
would increase demand for irrigation, and abstraction from agriculture would compete with abstractions
for piped water supply by other users.
Higher temperatures would have a pronounced effect on energy demand. Space heating needs would
decrease substantially but increased demand for air conditioning may entail greater electricity use.
Repeated annual droughts could adversely affect certain manufacturing industries requiring large
amounts of process water, such as paper-making, brewing and food industries, as well as power
generation and the chemical industry.
Sensitivity to weather and climate change is high for all forms of transport. Snow and ice present a very
difficult weather related problem for the transport sector. A reduction in the frequency, severity and
duration of winter freeze in the British Isles would be likely under conditions associated with global
warming and could be beneficial. However, any increase in the frequency of severe gale episodes could
increase disruption to all transport sectors.
The insurance industry would be immediately affected by a shift in the risk of damaging weather events
arising from climate change in the British Isles. If the risk of flooding increases due to sea level rise,
this would expose the financial sector to the greatest potential losses.
UK tourism has an international dimension which is sensitive to any change in climate which alters the
competitive balance of holiday destinations worldwide. If any changes to warmer, drier summer
conditions occur, this could stimulate an overall increase in tourism in the UK. However, any significant
increase in rainfall, wind speed or cloud cover could offset some of the general advantages expected
from higher temperatures. The British Ski industry would be an example of tourism that would be adversely
affected by rising temperatures.
Interestingly a rise in temperatures means that many of the UK peat bogs that formed just after the
last Pleistocene glaciation are shrinking as they dry out. This has negative knock on effects for species
diversity as well as allowing carbon dioxide that has been trapped for many years in organic matter to be
liberated as the peat bogs dry out which in effect reduces the NET carbon dioxide stores available on
land. Durham University is currently leading cutting edge research into this phenomena by studying the
peat bogs of Upper Teesdale near Middleton in Teesdale. This is similar to the process that happens in
the oceans. As temperatures rise more CO 2 is lost form the oceans which in turn causes positive
feedback and even more warming of the system.
In light of the recent political attention given to this issue and the associated media coverage ‗climate
change‘ looks set to be a key issue for scientists to resolve for decades to come. Scientists know much
about the causes of climate change but are still undecided upon the likely outcome in terms of global
temperatures the planet will experience in the next 100 years. Equally scientists are divided in terms of
the most appropriate action to take, if any at all to curb the recent warning. In fact there still remains a
huge division in the scientific community over this issue where some regard the recent warming as just a
mere warmer period caused by the earths natural cycles of warmer and cooler periods while others regard
the recent warming a result of human activity and believe the temperatures experienced in the coming
years are likely to be higher than those ever before experienced by man.
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(ii) International Effects
The Intergovernmental Panel on Climate Change (IPCC) report presents a stark warning of the possible
effects resulting from accelerated anthropogenic climate change. Changes that are evidenced now and
predicted future changes are listed below:
Melting Polar Ice Caps
The scientific community are now in agreement that recent global warming has been responsible for a
rapid and large scale shrinking of polar ice. In fact actual rates of ice break and loss are much
greater than previously expected a decade ago and temperature have risen twice as fast in the
Arctic than anywhere else. Ice is being lost at a rapid rate on both main ice sheets, namely
Antarctica (the biggest ice mass) and the Greenland Ice Cap in the Arctic. For example, the largest
single block of ice in the Arctic, the Ward Hunt Ice Shelf, had been around for 3,000 years before it
started cracking in the year 2000. Within two years it had split all the way through and is now breaking
into pieces. The situation appears to be similarly chronic in the Southern Ocean, for instance,
during the 31st January 2002 to the 3rd July 2002 a section of the Larsen Ice Shelf in Antarctica
broke up. The Larsen B sector collapsed and broke up, 3,250 km² of ice 220 m thick disintegrated,
meaning an ice shelf covering an area comparable in size to the US state of Rhode Island disappeared
in a single season. Larsen B was stable for up to 12,000 years, essentially the entire Holocene period
since the last glacial period.
The Arctic has lost 1.7 million km2 of ice since 1980 shrinking to an area of 6.1 million km2 in
2005. This is a rate of 9% loss in the Arctic per decade. This has lead to some members of the
scientific community to forecast a dire worst case scenario of a total loss in arctic sea ice by 2050
(i.e. in your life time!!!) and a more unlikely date as early as 2013. Between 1980-2001, thirty of the
world‘s glaciers had thinned significantly by 6m.
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Location of Larsen B
The net result of ice break up of
land ice like this is sea levels rise
(as more water is added into the
system), remember this would not be
the case for melting sea ice as a
specific volume of sea ice already
displaces its volume of water as it is
floating in the sea, therefore
melting sea ice has no effect on
eustatic (global) sea level,
As well as rising sea levels the lack
of polar ice reduces the cooling
effect the ‗great ice sheets‘ have on
the oceans so the oceans become
warmer!
Also ice reflects more
radiation back to space as it has a high
albedo, the lack of ice therefore
allows the Earth to absorb more
heat.
Both of these factors mean that the
oceans are expanding.
The most
obvious affect of this is that eustatic (global) sea levels will rise (see notes below).
The melting of the World‘s cold environments goes beyond ice sheets, for instance as mentioned in
the last section work by Durham University suggests that drying out of peat bogs is releasing more
C02 globally as the peat shrinks. Similarly to peat, permafrost (permanently frozen ground) around
the edges of the ice caps in periglacial regions is melting at an unprecedented rate. This thawing of
the permafrost whole sale in places like Siberia is allowing trapped methane (CH4) to be liberated
into the atmosphere. Methane is also a greenhouse gas which has the NET result of causing further
warming which may well cause more melting of the permafrost accelerating this process further.
One effect of melting huge volumes of shelf
ice and adding therefore enormous volumes of
fresh water is that the North Atlantic Drift,
part of the oceanic thermohaline circulation
system might shut down due to desalination of
the oceans. It is the North Atlantic Current
that gives us as well as the USA our warm
climate for our latitude. If the circulation
shuts down then the effect may well be a
short lived (on a geological scale) cooler period
where ice sheets actually advance again as
they did at the end of the last Pleistocene
glaciation. In the Allerod at 15, 000 years ago the temperatures rose as the ice sheets receded and
finally disappeared meaning much of the ice cap held on North America melted and drained into the
Atlantic causing desalination and led to the shutdown of the North Atlantic Drift which plunged
parts of the Northern Hemisphere back into a short lived cold period where ice actually re-advanced
in the Younger Dryas and lasted for 1000 years.
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Rising Sea Levels
Both increased water from melting glaciers and land-based ice sheets as well as thermal expansion of
water due to increased heating are leading to higher eustatic sea levels. According to the IPCC global
sea levels rose at an average rate of 1.8mm/year between 1961 and 2003. The rate being faster from
1993-2003 at about 3.1 mm/year, with the total 20 th century rise being about 17cm. The aptly named
Stern Report predicts sea levels will rise from 28-43cm by 2080 although some authorities predict that
complete melting of polar ice may occur and increase sea levels by 4-6m.
Rising sea levels when coupled with the more unpredictable stormy weather a warmer global climate is likely
to bring would have really adverse affects on low-lying areas of the world. More coastal erosion and
coastal flooding would occur as well as contamination of underground water sources For instance much
of the Netherlands and Bangladesh would be adversely affected by coastal flooding. As well as island
nations like the Maldives; over half of that nation's populated islands lie less than 6 feet above sea level.
Even major cities like Shanghai and Lagos would face similar problems, as they also lie just six feet above
present water levels. The problem of rising sea levels will even pose a problem closer to home here in
the UK. Low-lying estuaries such as the Thames estuary and East Anglia could be badly affected by
coastal flooding. London would pose a huge risk and damage to the national and global economy could
result if large scale floods occur in the future. For instance in 2007 the Thames Barrage was nearly
overtopped by a higher than expected storm surge. If this coincides with a high spring tide in the future
many experts fear overtopping could be likely.
Rising seas would severely impact the United States as well. Scientists project as much as a 3-foot sealevel rise by 2100. According to a 2001 U.S. Environmental Protection Agency study, this increase would
inundate some 22,400 square miles of land along the Atlantic and Gulf coasts of the United States,
primarily in Louisiana, Texas, Florida and North Carolina. This would therefore seal the fate of New
Orleans as uneconomic to redevelop.
Food Shortages
A warmer Arctic will also affect weather patterns and thus food production around the world. Although
the planet is generally getting warmer and therefore more water available, some areas are expecting
less rainfall and some areas will expect to receive more. i.e. it won‘t be an even pattern of precipitation
spread across the globe.
Countries that are less developed and therefore less able to cope with food shortages are expected to
be hit worse by the effects of changing climate and shifting weather patterns. For instance much of
Africa, the Middle East and India are expecting considerably lower cereal yields as the result of lower
rainfall in these areas. Whereas places like Bangladesh may expect to see higher rainfall during the
Monsoon and more extreme flooding.
Wheat farming in Kansas, for example, would be profoundly affected by the loss of ice cover in the
Arctic. According to a NASA Goddard Institute of Space Studies computer model, Kansas would be 4
degrees warmer in the winter without Arctic ice, which normally creates cold air masses that frequently
slide southward into the United States. Warmer winters are bad news for wheat farmers, who need
freezing temperatures to grow winter wheat. And in summer, warmer days would rob Kansas soil of 10
percent of its moisture, drying out valuable cropland.
Health
By 2080 290 million more people may well be exposed to an increased risk of Malaria, especially in China
and central Asia. As areas these areas become wetter and will receive a higher proportion of rainfall it
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would be logical to expect an increase in Malaria spreading mosquitoes that live in swampy areas. Other
water-borne diseases may well increase in such areas especially as the climate becomes milder.
Health will also be adversely affected by increased malnutrition in Africa especially, as rains and crops
failure becomes more common.
Ecological Damage/Extreme Weather Events
As the polar regions continue to warm habitats may well become lost in both Antarctica and the Arctic.
For instance habitat for Whales and dolphins may diminish as the food chain breaks down. Other
species of sub-arctic flora and fauna may also be in tundra areas. This will threaten the way of life of
native people like the Inuit.
Forest and tundra ecosystems are important features of the Arctic environment. In Alaska, substantial
changes in patterns of forest disturbance, including insect outbreaks, blow down, and fire, have been
observed in both the boreal and southeast coastal forest. Rising temperatures have allowed spruce bark
beetles to reproduce at twice their normal rate. A sustained outbreak of the beetles on the Kenai Peninsula
has caused over 2.3 million acres of tree mortality, the largest loss from a single outbreak recorded in
North America. Outbreaks of other defoliating insects in the boreal forest, such as spruce budworm,
coneworm, and larch sawfly, also have increased sharply in the past decade.
Climate warming and insect infestations make forests more susceptible to forest fire. Since 1970, the
acreage subjected to fire has increased steadily from 2.5 million to more than 7 million acres per year. A
single fire in 1996 burned 37,000 acres of forest and peat, causing $80 million in direct losses and
destroying 450 structures, including 200 homes. As many as 200,000 Alaskan residents may now be at
risk from such fires, with the number increasing as outlying suburban development continues to expand.
The increase in forest fires also harms local wildlife, such as caribou
Extreme weather events like the great UK storm of 1987 and Hurricanes such as Mitch and Katrina may
become more common. As well as these weather events it is probable that droughts and heat waves will
also become more common for certain regions.
Severe Water Shortages
Reduced rainfall coupled with the salination of coastal water sources by sea water flooding and saltwater
incursion of aquifers will result in less water availability for drinking, irrigation and industry, This may
well lead to future water wars in the Middle East in places like Israel as well as those living in India. These
places will be the worst affected. It is expected that 3 billion people could suffer water stress by 2080.
Mass migration / War and Tension
Changing climate and weather patterns in places like Africa especially may result in mass migration of
people across borders from one country to another. For instance if rains fail in the wet season causing
drought, then crop failure and famine may well result causing migration of refugees to places of refuge
and food supply. In such harsh conditions this may lead to civil war and unrest.
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(iii) Effects on the Wet/Dry Savanna (Tropical Continental Climate in W. Africa)
Climate change could have far reaching effects on the global climate as temperatures continue to rise
and as the amount of water circulating in the atmosphere increases in its proportion. These effects
could be especially felt in the sensitive Savanna regions which experience a tropical continental climate,
one such place being Kano, Nigeria (12oN).
Savanna grasslands represent unique sensitive ecosystems which are characterised by mainly tall
elephant grass which is broken by scattered isolated trees and shrubs. Many scientists fear that this
ecosystem could be taken over by woody trees and shrubs which are likely to colonise through
vegetation succession as the Sahel experiences greater precipitation over the next 50 years.
The savannas in their current state are both ecologically unique and economically vital for the survival of
communities who live in these areas. Colorado State University published research in the scientific
Journal Nature (2005) that rainfall is the most important controlling factor on savanna development.
From this work came two classifications:
Stable savannas - are those that receive less than 650mm of rainfall per year and subsequently
allows tree growth to be restricted and therefore grasses can co-exist.
Unstable Savannas – receive more than 650mm rainfall per year. The amount of trees in such
savannas is not controlled by the amount of rainfall alone at present, but by the regulating
effect of fires and grazing by wild animals which clear grasses and encourage tree growth.
Trees in these regions such as the Baobab Tree have become adapted to fires (pyrophytic
adaptations such as thick fire resistant bark) and survive preferentially over grasses.
Kano currently has an average rainfall of 1040mm so therefore the savanna is already classed as
unstable and increased rainfall can only act to further encourage tress and result in the demise of the
grassland habitat! Complete loss of this sensitive ecosystem is likely in this region in the next 50 years
as the average precipitation is set to increase in the majority of climate change models.
The balance between trees and grasslands influences vital characteristics of the ecosystem such as
livestock production as well as water balance and as a result drinking water supplies. Changes to the
grasslands that cause a reduction in the species diversity would be fundamentally detrimental to local
indigenous tribes who have adopted a sustainable way of life over hundreds of years. As well as
threatening the viability of indigenous populations, climate change may well damage local tourism in
countries such as Nigeria that rely on it heavily for revenue which funds investment and development.
Climate change may well threaten and decrease the species diversity and ecology of game reserves that
so many tourists come especially to witness. Although more trees would mean more elephants generally,
an invasion of trees would cut down on the number of other large mammals in the savanna which is bad
news for safari based tourism.
Diagram below is for illustration only to show increased rainfall would produce a savanna more characteristic of the
equatorial latitudes i.e. parkland „closed savanna‟:
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It is very important to understand what drives the savannas in order to help manage them as climate
conditions change. Two schools of thought exist when trying to explain their origin, firstly rainfall is
important in their development and secondly disturbances such as fire and grazing help regulate them.
Initially savannas were classified as a climate type under Koppen‟s early classification (as rainfall is
controlled by climate) in the early 1900‘s as other controlling factors such as disturbances were poorly
understood. It is most likely that both of these factors are responsible for regulating the savanna
depending on the point at which they occur in the season (i.e. wet/dry). There is considerable dispute about
the future of much of North Africa under climate change, underscoring the difficulties in assessing
one of the most complex mechanisms on the planet.
However, climate change might have some beneficial effects too for the tropical continental regions of
West Africa. Rising temperatures in the Sahara desert could actually be beneficial, reducing drought in
the Sahel region immediately south of it. Reindert Haarsma and colleagues, of the Royal Netherlands
Meteorological Institute were the first to consider the roles of both land and sea-surface
temperatures.
The Haarsma computer model suggests that if emissions of greenhouse gases are not reduced, higher
temperatures over the Sahara would cause 25% to 50% extra daily rainfall in the Sahel by 2080
during the months from July to September. The Sahara desert heats up faster than the oceans,
creating lower atmospheric pressure above the sands. This in turn leads to more moisture moving in from
the Atlantic to the Sahel. Also warmer air has a higher capacity to hold moisture allowing greater
precipitation to the south of the Sahara Desert.
Additional rainfall would allow greater agricultural yields, growth of a greater range of crops and
create a longer growing season as it is likely that the wet season may lengthen. Evidence from the
Journal Biosciences (2002) confirms notions of a wetter climate and suggests that further north in the
dry desert climatic zone, parts of the Sahara Desert are showing signs of ‗greening‘ due to increased
rainfall (1982-2002). This has made some in the scientific community predict a return to the Sahara
Desert being a lush green savanna again as it was some 12,000 years ago!
However on the downside climate change could disrupt the pattern of seasonal rains that is brought
about by the movement of the ITCZ north over Kano. It is the associated movement of the ICTZ seasonally
in the summer months that brings Kano the SW Monsoon and therefore its wet season. If the ITCZ
gets ‗stuck‘ too far south or fails to move as far north as 12 oN early enough in the wet season then the
rains of the Monsoon may not come and areas such as Kano may experience crop failures. Even if
there is more rain per year on average for this region, it is likely that rains may become more unreliable
and the above effects more likely.
Increased rainfall and increased likelihood of frequent extreme weather events such as prolonged
monsoon rainstorms would increase the likelihood of flooding in the region. Flash floods apart from
obvious short term effects such as damage to homes, business and people could also result in long term
effects such as soil erosion and actually lead to subsequent reduction in agricultural yields in places
least equipped to deal with such losses.
A wetter climate in this part of West Africa may also mean an increase in mosquitoes and therefore an
increase in malaria cases. Malaria is already endemic in Nigeria according to the WHO where the
mortality rate in children under five 729 per million. Other water borne diseases may become more
common as temperatures rise and the climate becomes wetter
The other side of the argument goes however, that global warming may have the opposite effect on this
region and cause drier conditions for West Africa as well as much of the continent as a whole and
desertification may result. Some predictions estimate a 50% drop in yields from rain fed agriculture
by 2020.
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Whichever prediction, if any comes to be the correct outcome for Tropical Continental West Africa in
the future, it is important to note that there will be regional disparities in terms of magnitude of the
effects suffered. For instance more marginal areas to the north of the region may suffer greater
hardship from increasingly unreliable rainfall patterns and shortened growing season and a more
unpredictable climate. Although not technically in the tropical continental region, places like Sudan
would undoubtedly suffer significantly from the effects of lower rainfall and famines such as those
experienced in the 1980‘s. Famines in the Sahel region would lead to mass migration of people to
neighbouring countries and humanitarian disasters would therefore be common place.
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(iv) Responses to climate change
Stern Report: The Stern report, the Treasury‘s comprehensive analysis of the economics of climate
change, estimates that not taking action could cost from 5 to 20 per cent of global GDP every year,
now and in the future. In comparison, reducing emissions to avoid the worst impacts of climate change
could cost around one per cent of global GDP each year.
Other Findings:
The report identifies that there is still time to act to avoid the worst impacts of climate change.
Climate Change would have serious impacts on growth and development.
The costs of stabalising the climate are high but are manageable, but waiting and doing nothing
will have much greater costs in the future.
Action on climate change is required across all countries and it need not cap the aspirations of
economic growth in the richest or poorest countries.
A range of options exists to cut emissions but strong deliberate policy is required to encourage
their take up.
Climate change demands an international response based on a shared understanding of common
goals and therefore agreement on frameworks for action.
Future key international frameworks should include: emissions trading, technology co-operation,
action to reduce deforestation.
International:
The global nature of the threat of climate change means that it must be tackled through international
co-operation, common policies and unified actions e.g. burning fossil fuels in China will have an impact on
the opposite side of the world so therefore we must take a global perspective. In reality these
intentions are have been difficult to realise!
The Kyoto Agreement (Protocol)
Is an agreement or rule implemented by the United Nations Framework Convention on Climate Change
with the intention of combating global warming. The aim is to stabilise greenhouse gas emissions to
such a level that further damage to the world‘s environmental system is halted.
Recognising that developed countries are principally responsible for the current high levels of GHG
emissions in the atmosphere as a result of more than 150 years of industrial activity, the Protocol places a
heavier burden on developed nations under the principle of ―common but differentiated responsibilities.‖
There are 5 main principal concepts to the agreement:
Reduce greenhouse gases by committing Annex I countries to legally binding emissions limits;
Implementation to meet objectives i.e. prepare policies and measures to reduce greenhouse
gases, increase absorption of gases and use all other mechanisms available to reduce levels such
as emissions trading / credits;
Minimise risks to developing countries by establishing a climate change adaptation fund that
richer states contribute to in order to help developing states over come future challanges.
Account / review / report integrity of the Protocol;
Aid compliance of countries to the protocol by establishing a compliance committee to police it.
The agreement was first signed 11th December 1997 in Kyoto Japan. 187 states have signed and
ratified the agreement to date. Under the protocol 37 industrialised countries (Annex I) have committed
themselves to reduce the levels of the 4 most common greenhouse gases (Carbon Dioxide
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(CO2), Methane (CH4), Nitrous Oxide (NO), Sulpher hexafluoride (SF6) CFC‘s are controlled under a
different agreement called Montreal Agreement) The Annex I counties committed to reduce their
collective greenhouse gas emissions by 5.2% of 1990 levels. However this target does not include
aviation or shipping. All other member countries have also pledged to reduce their emissions.
The EU (European Union) and its member states ratified the agreement in May 2002 and Russia
November 2004 clearing the way for the treaty to become legally binding by 16 February 2005.
The
USA is the most notable nation that is non-party to the protocol (under the past Bush administration) even
though the USA accounts for 36% of greenhouse gases at 1990 levels! The USA uses the non- inclusion
of China and India as an excuse to opt out of the agreement.
The Kyoto Protocol is generally seen as an important first step towards a truly global emission
reduction regime that will stabilize GHG emissions, and provides the essential architecture for any
future international agreement on climate change.
By the end of the first commitment period of the Kyoto Protocol in 2012, a new international
framework needs to have been negotiated and ratified that can deliver the stringent emission reductions
the Intergovernmental Panel on Climate Change (IPCC) has clearly indicated are needed.
Unfortunately the UK is not alone in not being able to reach its target of reducing emissions by 20%
of 1990 levels by 2010 and has revised its time scale to a more realistic cut of emissions by 6 0% by
2050. Emissions in the UK are actually on an upward trend due to an increase from the energy sector.
Carbon Credits
What emerged from the discussions held during the construction of the Kyoto Protocol is that all
nations release CO2, this CO2 must therefore be absorbed via tree planting or other process that can
absorb it such as sequestration. Or secondly a country could just cut its CO2 emissions in the first
place. If that country produces more CO2 than it can absorb within that country, it must purchase an
‗absorption ability‘ from another nation that has not produced as much CO2 as it can potentially absorb.
The absorption ability purchased is a Carbon Credit and it is equal to one tonne of CO2 called a CO2 e (CO2
equivalent). A nation might have a shortfall of 500,000 CO2 credits as it produces an excess of CO2
compared to that it absorbs. In this scenario a heavily polluting nation must then buy credits from another
nation that has CO2 absorbing ability for instance through the planting of trees that would fix and soak
up excess CO2. The cost per credit can be anywhere between $10-40. This therefore makes an economy
that relies heavily on carbon uneconomic and this financially discourages this type of behaviour. The
planting of trees as long as they are not later cut and burned reduces CO2 in the atmosphere as does
encouraging ploughing that discourages CO2 release during harvesting. Forests can be left to stand and
weeds and hedge rows can be encouraged between fields. Fuel consumption can be cut and power
generation can be made more efficient in order to reduce CO2 usage. This increase in stores and
decrease in emissions means that a nation will need to buy less (or could even sell its credits if it has a
surplus) carbon credits making it a stronger economy. This is what is referred to as a low carbon
economy and illustrates how leading the way in low carbon initiatives could be highly profitable for a
nation. The money used to purchase credits will ultimately be returned to developing new low carbon
/ energy efficient technologies. For instance New Zealand has already funded some of its new wind
farms from the profit it has made from carbon trading. Ireland has purchased 95% of its credits in
contrast from overseas in order to offset its heavy reliance on industry based around fossils fuels.
Critics to the scheme point out that the richest nations will just simply pay as you go to pollute in order
to fuel industry and emissions may not actually be cut at all as economies continue to grow. Other problems
exist whereby some industries suffer more than others, for instance aviation is becoming an increasingly
uncompetitive business and the downfall of BA can in part be attributed to the fact that is
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discouraged from expanding operations. This is also true for Virgin airlines who wanted to expand into
Australia but could not do so as carbon trading credits made it uneconomic to do so.
Other problems exist as in the UK for instance the CO2 emissions are lower than they should be as most
of the consumable goods such as Appliances, Clothes and Textiles as well as the majority of things we
buy on the high street are no longer made locally in the UK but are outsourced to be made cheaply in less
developed countries such as China. As china is the factory for the World it therefore incurs huge
amounts of carbon emissions.
In the future the result of building a low carbon economy whereby the cost of a product is also
controlled by its CO2 footprint may mean that instead of an increasingly globalised world economy that
we have seen over recent decades, we may well see a return to reliance on local economies where trading
occurs locally.
Carbon Capture
To prevent the carbon dioxide
building up in the atmosphere, we
can catch the CO 2, and store it. As
we would need to store thousands
of millions of tons of CO2, we
cannot just build millions of
containers, but must use natural
storage facilities. Some of the best
natural containers are old oil and
gas fields, such as those in the North
Sea.
The diagram on the left shows a
conceptual plan for CCS, involving 2 of
the common fossil fuels, methane gas
(also called natural gas) and coal.
Methane gas is produced from offshore gas fields, and is brought onshore by pipeline. Using existing oilrefinery technology, the gas is 'reformed' into hydrogen and CO 2. The CO 2 is then separated by a newlydesigned membrane, and sent offshore, using a corrosion-resistant pipeline. The CO2 goes to an oilfield.
The CO2 is stored in the oilfield, several km below sea level, instead of being vented into the atmosphere
from the power station
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Post-combustion capture involves removing the dilute CO2 from flue gases after hydrocarbon
combustion. It can be typically built in to existing industrial plants and power stations (known as retrofitting) without significant modifications to the original plant. This is the type of technology favoured by
the UK Government in its competition for state support.
There are several methods that can be used to capture the CO2. The most common method is passing
the CO2 through a solvent and adsorbing it and amine solvents are typically used. A change in
temperature and/or pressure will then release the CO2. Another process in development is calcium cycle
capture where quicklime is used to capture the CO2 to produce limestone, which can then be heated to
drive off the CO2 and quicklime which can then be recycled. All of these require additional energy input
to drive off the CO2 from the solvent - this typically results in extra energy costs of 20-30% compared
to plants with no capture. New solvents are under development to reduce these penalties to 10%.
Other post-combustion possibilities, currently being researched, include cryogenically solidifying the
CO2 from the flue gases, or removing CO2 with an adsorbent solid, or by passing CO2 through a membrane.
Pros:
Feasible to retrofit to current industrial plants and power stations.
Existing technology - 60 years experience with amine solvents - but needs 10x scale-up.
Currently in use to capture CO2 for soft drinks industry.
Cons:
High running costs – absorber and degraded solvents replacement.
Limited large scale operating experience.
Energy Production and the role of Renewables
Globally energy production accounts for 60% of emissions and the remainder is from private and other
industries. In the UK however, 80% of our energy comes from non-renewable fossil fuels that release
huge amounts of CO2. This problem can be dealt with by:
Reducing the emissions from power stations before they are released;
Using alternative sources of renewable energy
Reducing demand for energy by using less in industry, homes and transport.
Work needs to be done to adopt renewable energy resources. For instance in the UK mechanisms have
been put in place to reduce our dependency on fossil fuels by 10% by 2013 by adopting renemables such
as:
Wind Energy
Geothermal Power
Ground Source Heat Pumps
Waste fired power stations using animal and flood waste
Biodiesel
Solar energy
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Tidal energy
National:
EU emissions trading System UK low carbon transition plan
Emissions trading allows the government to regulate the total amount of emissions produced in the country
by setting an overall cap for the scheme. They then allow individual industries and companies the
flexibility to decide how and where the emissions reductions will be achieved.
Participating
companies are allocated allowances but can emit in excess of their allowance by purchasing additional
allowances from the market that other industries have sold back because they have may well met their
target. The environmental well being of this is not compromised as the total amount of allowances
remains fixed (think of the total amount of money is fixed in a game of monopoly). The EU climate
change programme attempts to address the need to reduce atmospheric CO2 by means of the EU
Greenhouse Gas Trading Scheme. Members can either make the savings in their own country or buy
these emissions reductions from other countries.
Carbon Trust
The Carbon Trust is an independent company set up in 2001 by Government in response to the threat of
climate change, to accelerate the move to a low carbon economy by working with organisations to reduce
carbon emissions and develop commercial low carbon technologies.
Aims: The Carbon Trust's mission is to accelerate the move to a low carbon economy now and develop
commercial low carbon technologies for the future.
They cut carbon emissions by providing business and the public sector with expert advice, finance and
certification to help them reduce their carbon footprint and to stimulate demand for low carbon
products and services. The trust claims to have saved over 17 million tonnes of carbon, delivering costs
savings of over £1billion.
The trust aims in future to cut carbon emissions by developing new low carbon technologies. They do
this through project funding and management, investment and collaboration and by identifying market
barriers and practical ways to overcome them. The work on commercialising new technologies will save over
20 million tonnes of carbon a year by 2050.
Advertising and Awareness Campaigns
There have been many TV and radio campaigns as well as poster campaigns to raise awareness of climate
change. E.g. act on CO2.
-Act on CO2 Website
Provides information for individuals and business on how to reduce C02 emissions.
Local:
Car Sharing Schemes / Cycle to work schemes / Walking Buses
Many car sharing schemes have been set up where people share a lift into work or share a journey so there
are less cars on the daily commute that only contain one passenger. To take this further the government
has proposed incentive schemes whereby one lane on the motorways (such as the hard shoulder) can be
only used by those lift sharing. There are also many local ride to work schemes whereby the
government subsidises cycle equipment by offering individuals who buy bikes tax breaks on
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their salary as was as a small monthly deduction from their wage. This is often set up for free by local
bike stores and there are many different types of scheme.
Home Energy Monitor
Online facilities are now offered by energy companies that allow homeowners to monitor their energy
consumption and carbon emissions in order to save both money and reduce CO2 emissions.
Some energy companies also provide energy monitors that can be plugged in at home that monitor energy
consumption and help raise customer awareness of energy wastage.
School Culture of Energy Saving
The education is doing its bit to increase awareness of the climate change issue and reduce the amount
of energy wasted and hence amount of unnecessary CO2 released. This can happen as early as primary
school such as Pendle Primary School, Clitheroe where students as young as five are elected as light
monitor and are responsible for turning the lights off in classrooms at break time.
Free energy saving light bulbs
Many local councils have government grants made available that they have used to buy energy saving
light bulbs which can be collected for free from local police stations. Energy companies such British Gas
have also provided their customers with energy saving bulbs. Obviously these companies are pressured into
doing this as they have a corporate responsibility to encourage a sustainable future.
Grants for improving energy efficiency
Local councils have grants available that local residents can apply for to help fund/part fund energy
saving improvements such as cavity wall and loft insulation. Blackburn Council for instance has money
available from central government that helps people on certain benefits improve the energy efficiency
of their homes. The grants can be between £2700 -£4000 and can be used to upgrade heating systems.
Householders in Blackburn can also apply for grants of up to £2,500 per property towards the cost of
installing a certified renewable technology product by a certified installer. Grants are also available in
the Blackburn with Darwen area for the installation of technologies in public sector buildings, not-forprofit organisations and charitable bodies. They can apply for between 30% and 50% of the installation
costs of approved technologies. There is a maximum of £1 million in grant funds available per site.
In Lancashire all residents can get detailed advice on energy efficiency saving solutions from the
Lancashire Energy Advice Centre (Blackburn Town Hall)
Recycling Message
Schemes that encourage recycling such as reduce, re-use, recycle cuts the waste of plastics, metals and
paper therefore saving emissions in manufacturing new materials.
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8. Climate on a local scale: Urban Climates
Climatic regions by definition are large areas characterised by similar climatic conditions that persist
over time, in reality they are not entirely homogeneous or constant. In order to understand climate on a
local scale geographers refer to the term microclimate which involves the study of the climate on a
much smaller scale. For instance you could study the differences in microclimate between a large deep
valley compared to a high altitude mountain range or compare the microclimate of a large conurbation
with that of its ‗more rural‘ hinterland.
Urban climates are arguably the most interesting microclimates as human interference has effects on
the climate on a local scale that then has an impact on the lifestyle / quality of life for the population
living in such cities.
(a) Temperature:
“Why are urban areas hotter than rural areas?”
E.g. Manchester can be 2oC hotter than surrounding rural
areas, why?
Temperature in cities is primarily controlled by:
Atmospheric Composition
Air pollution in cities makes light transmission leaving
the city surface (while rebounding back to space)
significantly less than nearby rural areas which have lower levels of pollution e.g. in Detroit
(manufacturing region), USA there is 9% less transmission of reflected radiation back to space
increasing to 25% less transmission of light on a calm day.
Daytime heating of the boundary layer occurs because aerosols (pollution) absorb solar radiation during
the day but this does not have as much effect up to the mean roof level (urban canopy layer). Incoming
solar radiation (short wave) is actually reduced by pollution but is counterbalanced (offset) by the lower
albedo (i.e. surfaces are darker and tend to absorb heat, therefore causing warming). Also cities have a
feature referred to as urban canyons (shape and arrangement of buildings) which effectively increases
the surface area of the area being heated as cities have a greater surface area of material that can be
potentially heated compared to those in the rural.
Urban Surfaces
The nature of the urban surface
controls how well it heats up:
-Character – i.e. type of
surface, some surfaces heat up
better than others and also
have higher heat capacities
leading
to
hotter
city
temperatures;
-Density of urban surface – i.e.
total surface area of structures
as well as building geometry
(shape) and arrangement.
City centres have relatively high heat absorption and therefore high temperatures; however at street
level the readings can be confusing and are often lower than expected due to shading from tall buildings.
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The geometry of urban canyons is important as it effectively increases the surface area by trapping
multiple reflected short wave radiation and also reduces reflection from the surface back to space as
there is a restricted “sky view” again due to building geometry.
Anthropogenic Heat Production
Traffic (cars and other vehicles), industry, buildings and even people release heat. Amazingly these
causes release similar levels of heat energy than that of incoming solar radiation in winter!
In the year 2000 the Boston-Washington DC Megapolis (great city) had an estimated 56 million
residents inhabiting a land area of 32, 000km 2 - which produced enough anthropogenic heat to account
for an equivalent of 50% of the winter radiation and 15% of the total summer radiation!
In the Arctic regions anthropogenic heat provides enough heat to provide a positive heat balance otherwise
conditions in such arctic urban areas would be much cooler.
Urban Heat Island Effect
The NET effect of urban thermal processes (human activity) is to make urban temperatures
considerably greater than those in surrounding rural areas.
The greatest effect occurs in the mid latitudes while under the influence of clear and calm conditions
(anticyclones) which prevents cloud formation and cloud cover.
The result is rural areas become
disproportionately cooler and make the effect appear more apparent.
 Factors contributing to urban heat islands:
1) Thermal heat capacity of urban structures is high. Canyon geometry – dominates the canopy
layer by heating from conduction and convection from buildings loosing heat and traffic;
2) By day there is absorption of short-wavelength radiation by pollution;
3) Less wind in urban areas due to more shelter from urban canyons therefore less heat
dissipation;
4) Less moisture in local atmosphere in cities due to there being less vegetation and quicker run-off
meaning there is less heat needed for evaporation of this moisture present so the remaining heat
not used up by the evaporation process has the effect of increasing temperatures.
The result of the above is that average urban temperatures can be 5-6 oC warmer than rural
areas and 6-8 oC warmer in the early hours of the morning during calm nights as the city radiates
the heat it absorbed during the day. It is heat loss from buildings that is by far the greatest
factor in controlling urban heat islands.
Urban heat islands show the greatest increase in mean temperatures in the largest cities that have
undergone huge population growth, for instance Osaka, Japan has a high population density which many
attribute as a cause for a 2.6 oC temperature rise over the last 100 years as well as it having very tall
buildings. Similarly many North American cities show a similar temperature rise and show the greatest
temperature difference between rural and urban environments – up to 12 oC for American cities with a
population over 1 million people. European cities show a much smaller temperature difference as
buildings here are generally lower and have shallower urban canyons. It is also now recognised that
urban population density has a greater effect on the city temperatures compared to simply city
population size i.e. higher population densities such as Tokyo (12 million population) would have a
greater temperature rise than a similar sized city with a lower population density such as Illinois (13
million population) for example.
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Case Study: London 1930-1960 average temperatures
City Centre 11oC
Suburbs 10.3 oC
Countryside 9.6 oC
Calculations suggest that London‘s domestic fuel use in
the 1950‘s increased temperatures by 0.6 oC on
average in winter. The regional wind speed needs to be
low and the city sheltered by topography for a heat
island to operate effectively. The heat island might be
so great that it may generate its own inward spiralling
wind systems at the surface.
Consequences of heat islands:
Large Cities tend to suffer badly during heat wave conditions when tarmac, paved surfaces and
bricks heat up and retain heat at night time making the effects of heat wave conditions worse.
For instance in 1987 Athens suffered tragic consequences through this process and hundreds
died through heat stress and dehydration.
Snow tends to lie for less time in city centres and near major roads in town centres but may lie
for longer in city centre parks where the surface materials are grass and heat up less quickly. This
might have possible ramifications for the continuation of transport development.
As discussed earlier heat islands might have a positive effect of actually making it possible to
inhabit inhospitable arctic areas in winter.
(b) Precipitation
Temperatures are generally higher in cities and therefore the air in cities can hold more moistures than
that in cooler rural areas and relative humidity levels are subsequently 6% lower in cities. Usually there is
less vegetation cover in cities and less surface water stores meaning lower evapotranspiration rates.
In terms of cloud cover cities often experience thicker cloud cover which is more frequent than that
experienced in rural areas. This is mainly because of convection currents are deflected upwards above
cities causing condensation and cooling as they rise above the urban boundary layer (area affected by
urban surface). Also cities contain more sooty particulate matter form factories and exhaust fumes
which form cloud forming nuclei in the atmosphere above cities. Excess cloud cover helps to explain
why on average large cities are 5-15% wetter based on their average rainfall totals.
In addition 30-60km downwind of a large city such areas receive on average 1/3 more monthly
precipitation than areas upwind of large cities. This can be explained by urban heat islands causing
increased air moisture content as temperatures are warmer and because evaporation of water from gardens
and cooling towers of power stations allows more moisture to be carried by the prevailing winds leaving the
city. It takes time for moisture to condense to sufficient size to fall as rain, so therefore rain is heavier
downwind of cities.
(Left) Photochemical Smog over Athens
Cities also suffer from a 400% increased probability of hail
storms resulting from intense convectional uplift from
rapidly warming man made surfaces such as tarmac and
concrete. Cities also are 25% more likely from suffering
thunderstorms in the summer for the same reason e.g.
London, in the Northern Suburbs thunderstorms are more
likely as rising thermals are encouraged by ridges of high
ground. Cities are also more likely to suffer from thicker
and more frequent fogs as firstly they release more
pollutants from industry and
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transport resulting in more particulate matter in the atmosphere. This particulate matter also
encourages fog formation as it allows condensation nuclii for water droplets, this linked with a greater
chance of calm conditions (due to urban canyons providing shelter) makes fogs a persistant problem
especially under anticyclonic conditions when air is still preventing fogs and smogs from being blown
away (e.g. Athens, Mexico city and Manchester pre 1970‘s due to burning vast amounts of coal).
Modern smog does not usually come from coal but from vehicular and industrial emissions that are acted
on in the atmosphere by sunlight to form secondary pollutants that also combine with the primary emissions
to form a form of man-made low level ozone called photochemical smog (e.g. Athens).
Case Study: Los Angeles
The smog that occurs is a result of a combination of a number of factors. The various forms of pollution
from vehicles (8 million in LA), industry and power stations become trapped in the lower atmosphere
due to the occurrence of a temperature inversion. This is a phenomenon which occurs during the summer
months prevents mixing of the upper and lower atmosphere trapping the pollutants. The pollution
consists of nitrogen oxides, ozone, sulphur dioxide, hydrocarbons and various other gases, brush fires
can add even more pollution to the atmosphere. The pollution exacerbates breathing problems such as
asthma and causes a huge increase in the
number of breathing associated admissions to
casualty and may even result death in very
sensitive or unwell people. City dwellers often
become upset by the high level of pollution due
to the risk to health that it poses, however
most people are also unwilling to give up their
car to help reduce pollution! The response of
the city government is to impose restrictions on
emissions by industry and cars, but many of the
large companies fear impact on their profits and
therefore prevent any effective cuts from
being made. Overall it seems as though the
political will to make a difference is not there.
(c) Air Quality
The quality of air in urban areas invariably is poorer than that of rural areas. Although in the UK air
quality has improved since the decline of the manufacturing industry in the 1970‘s as well as the
requirement for vehicles to be fitted with catalytic convertors. This said cities still have on average 7
times more dust in the atmosphere than those rural surrounding areas. The main factors contributing
to such particulate solid matter and gaseous pollutants is combustion (burning) of fossil fuels in power
stations and by private / public transport. Cities tend to have 200 times more sulphur dioxide (SO 2), 10
times more nitrogen dioxide (NO 2), 10 times more hydrocarbons and twice as much carbon dioxide (CO 2),
These anthropogenic pollutants increase likelihood of cloud cover and precipitation as well as increasing
the chance of photochemical smogs. All of the above absorb and retain more heat from incoming solar
radiation but conversely reduce the sunlight levels in cities.
Sulphur dioxide and nitrogen dioxide which are both considerably higher in urban areas cause acid rain
and can pollute areas downwind of major industrial cities and even major industrial countries (can have a
huge effect down prevailing wind e.g. UK acid rain blows over Scandinavia and causes acid rain over the
continent).
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There are primary pollutants that usually come from burning fossil fuels e.g. carbon monoxide from
incomplete combustion from car exhausts and secondary pollutants that combine with other molecules in
the atmosphere and can be acted on by sunlight to form new more poisonous substances such as the
formation of photochemical smog.
The main atmospheric pollutants are:
Sulphur Oxides (SOx): especially SO2 is released from burning fossil fuels.
Nitrogen Oxides (NOx): especially NO2 is released from high temperature combustion and can
be recognised as the factor causing the brown haze in the plume downwind of cities.
Carbon Monoxide (CO): is a poisonous gas released by incomplete combustion of hydrocarbons
or even wood with the main contributor being vehicle exhausts.
Carbon Dioxide (CO2): Odourless and colourless greenhouse gas emitted from combustion.
Volatile Organic Compounds (VOC‟s): Released from hydrocarbons and some solvents such as
paints.
Particulate Matter: is a measure of smoke and dust content of the atmosphere measures in
PM10 which is a particle 10 µm (microns) in diameter. Smaller than this will enter nasal cavity and
ultra fine particles less than 2.5 µm will enter the bronchial tubes in the lungs and cause respiratory
problems.
Unfortunately the highest levels of particulate matter occur
in developing countries where legislation on emissions is not
very strict such as India and China.
These are also the
manufacturing centres for the World.
To compound
problems in such LDC‘s they are also the countries that are
witnessing the most rapid population growth and already have
the highest populations.
This means that an increasing
proportion of the world‘s population are at risk form
particulate matter induced health problems (respiratory
problems) which will reduce life expectancy in these developing
countres and it is the poorest in the developing world who
suffer the costs of cheaply manufactured products (that
the developed World demands) the most.
(d) Winds
Urban structures have a significant effect on the microclimate in terms of wind patterns and wind
speeds.
(a) Air flowing in narrow streets: Causes air pollution as
turbulent eddies pick up dust and particulate matter reducing air
quality.
(b) Tall buildings: may create a downwash effect in the lee
(sheltered side) so that emissions from chimneys high up at
urban canopy level can‘t escape and become trapped at ground level.
Obviously the amount of pollution
will depend on the meteorological
conditions at the time which
control either air turbulence or
subsidence.
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(c) Air flowing narrow streets – Venturi Effect: is the effect of higher wind speed caused by
narrowing of streets creating ―wind tunnels‖.
Building also cause deflections to wind which causes circulations about tall buildings.
These are a collective
set of unpleasant effects that tightly packed city architecture suffers regularly from. Manchester‘s most
recent Spinningfields retail and leisure development just off Deansgate has been plagued by these effects
so much so that the purpose built pavement cafes here are very unpleasant to use indeed unless you like dust
or worse in your Mocha!
Developers must therefore try to reduce the
effects of spiralling air dynamics if they want
shoppers to linger for longer and therefore
spend money. If developers can‘t build the
structures any smaller they sometimes try to
build the building on a pedestal of one or two
stories so it is the pedestal that suffers the
unpleasant winds and not the entrances to
shopping malls that often suffer the worst
effects of wind tunnelling and increased wind
speed.
Manchester Spinning Fields Development – A Venturi Nightmare!