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ATMOSPHERE
Earth as a System
http://www.teachersdomain.org/resou
rce/ess05.sci.ess.earthsys.hologlobe/
STRUCTURE OF THE
ATMOSPHERE
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Thermosphere
Mesosphere
Stratosphere
Troposphere – contains 75% of the mass of
the atmosphere and almost all of the
moisture and dust.
• http://www.teachersdomain.org/resource/
ess05.sci.ess.watcyc.vertical/
Troposphere
• Temperature decreases by 6.4oC every
1000m – lapse rate
• Solar radiation heats the air by conduction
• Contains most of atmospheric water,
vapour, cloud, dust
Stratosphere
• Steady increase in temp. caused by
increasing concentration of ozone O3 which
absorbs ultraviolet radiation.
• Winds light and increase with height
• Pressure falls and air is dry
Mesosphere
• Temperature falls rapidly as no water
vapour, cloud dust or ozone to absorb
incoming radiation
• Lowest temperature –90o C and strongest
winds
Thermosphere
• Temperatures rise rapidly with height – 500o
C due to increasing proportion of atomic
oxygen which absorbs incoming UV
radiation (like ozone)
Global Heat Budget
THE GLOBAL HEAT
BUDGET
• The atmosphere system involves
inputs and outputs.
• Incoming solar radiation is balanced
by outgoing terrestial energy from
the earth.
• The balance between input and output
is usually referred to as the Global
Heat Budget.
Input begins with solar energy
(insolation). Some insolation
is:
• Reflected by clouds and
scattered by gas particles
• Absorbed by water vapour,
dust and clouds.
Output is in the form of long wave
radiation emitted from the earth
–this balances the input of energy
from the sun:
• 94% of this radiation is absorbed
by water vapour and CO2 in the
atmosphere.
• 6% is radiated back into space.
LATITUDINAL VARIATIONS
• As well as a vertical transfer
of energy between earth and
space there is also a
horizontal transfer of energy
between high and low
latitudes.
• These energy variations are more
extreme between the tropics and
the poles. Such marked
contrasts are referred to as the
global temperature gradient and
are the result of a number of
factors:
• The curvature of the earth
• Due to the curvature of the
earth, the equator is closer to
the sun than the poles and as
a result, insolation at the
equator is more concentrated.
Atmosphere penetration
• The sun’s energy also passes
through a greater depth of
atmosphere at the poles
causing a lot of energy to be
diffused.
The Albedo effect
• Ice and snow reflect more
solar radiation back into space
making them cooler whereas
areas of dense vegetation
absorb radiation making them
warmer.
Seasonal Variations
• Seasonal variations in amount of radiation
received by the earth with latitude
• Sun ‘appears’ to be at Tropic of Cancer mid-June,
so northern hemisphere receives more insolation
• Mid-December the sun ‘appears’ to be at the
Tropic of Capricorn and so the southern
hemisphere receives the maximum insolation
ENERGY TRANSFER
• Such an imbalance in energy
receipt could theoretically result
in the lower latitudes becoming
warmer and the higher latitudes
becoming even colder. In reality
however, energy is transferred
from areas of surplus to areas of
deficit by atmospheric circulation
and by ocean currents.
ATMOSPHERIC
CIRCULATION
• The three cell model is a
useful tool in describing
atmospheric circulation and
energy transfer.
THE HADLEY CELL
• This extends from the
equator to about 30 N and S
of the equator. The intense
heating at the equator causes
the air to expand and become
lighter, producing an area of
low pressure.
• This warm, rising air contains
large amounts of moisture which
condenses to form cumulonimbus
clouds and heavy rainfall. The
rising air then spreads polewards
and sinks to the sub-tropics. The
sinking air produces high
pressure resulting in clear skies
• And little precipitation (these
areas correspond with the
desert areas of the world)
THE POLAR CELL
• Air over cold surfaces will
become cold, contract and
become heavy. It will therefore
sink and produce an area of high
pressure. The sinking air moves
towards lower latitudes where it
will expand and rise back up
creating a cell.
THE FERREL CELL
• The Ferrel Cell lies between
the Hadley Cell and the Polar
Cell. The HC and the PC are
thermally direct cells
(powered by temperature
differences).
• The Ferrel Cell is a thermally
indirect cell because it is
powered by the other two. The
FC transfers warm air from the
Hadley cell to the high latitudes
and transfers cold air form the
PC to the low latitudes for
warming.
ATMOSPHERIC CIRCULATION –
The Fuller Picture
• Atmospheric circulation is a
lot more complicated than is
suggested by the three cell
model. Recent research
questions the existence of
the Ferrel Cell.
In place of the Ferrel Cell it is now
argued that there are:
• Alternating patterns of high and
low pressure which travel at
relatively low levels.
• A series of high level, horizontal
wavelike motions called Rossby
waves.
• Rossby waves are very large, high
velocity belts of wind operating in
the upper atmosphere. They
have a distinct wave like motion
as they snake their way across
the globe. At their core are long,
narrow cylinders of very fast
flowing air called jet streams.
JET STREAMS
• Teachers' Domain: The Effect of Jet
Streams on Climate
GLOBAL WIND
CIRCULATION
• Whatever the actual workings
of energy transfer and
atmospheric circulation, broad
global patterns of winds and
pressure can be identified:
MODEL OF GLOBAL WIND
CIRCULATION
• The model of Global Wind
Circulation is more
complicated than it would
appear due to several factors:
• The earth’s tilt and
consequential seasonal
contrasts.
• In June in the Northern
Hemisphere, the earth’s axis
is tilted towards the sun and
the sun appears directly
overhead in the Tropic of
Cancer.
• In December, the earth’s axis
tilts away from the sun and
the sun appears to be
overhead at the Tropic of
Capricorn.
• The apparent movement of
the overhead sun is important
because it controls the belt
of maximum heating which
moves with the sun. This
called the thermal equator.
The distribution of land and sea.
• The wind pattern is more consistent
in the Southern Hemisphere
especially above latitude 30 degrees
south as there are virtually no land
masses to interrupt the winds and
heating and cooling properties of the
oceans means that a relatively
consistent wind pattern results.
• Over the Northern
Hemisphere, the large land
masses result in an altered
wind pattern due to the more
extreme temperature
differences experienced over
the continents in summer and
winter.
• Low pressure is dominant in
the summer due to the
intense heating of continental
interiors (winds therefore
spiral inwards anticlockwise
towards the centre of the low
pressure).
• High pressure dominates in
winter and winds blow
outwards in a clockwise
direction.
OCEAN CURRENTS
• Sea water has a high thermal
capacity, so the oceans are an
effective store of thermal
energy. In contrast with the
land, the seas warm to a greater
depth and also move and so
redistribute this energy.
• Upper ocean currents are generated
by prevailing winds blowing across the
surface of the ocean. These are
influenced by the rotation of the
earth and the distribution of the land
masses. The currents largely flow in
loops called gyres.
• http://www.teachersdomain.org/reso
urce/ess05.sci.ess.watcyc.gulfstream
/
• In addition to the surface
ocean currents of the world,
there is also and oceanic
conveyor belt , or deep ocean
circulation, that corresponds
to the atmosphere’s climate.
• Antarctica is important in this
pattern of movement, here vast
amounts of water freeze into ice, this
loss of fresh water causes the
remaining sea water to become more
saline and therefore more dense.
This denser water consequently sinks
and makes its way northwards
towards the equator where it is
warmed and returns southwards.
• Cold ocean currents flow from
the poles.
• Warm ocean currents flow from
the equatorial regions.
• http://www.teachersdomain.org/r
esource/ess05.sci.ess.watcyc.con
vey2/
• Below latitude 30 degrees,
the west coast of continents
have contact with cold ocean
currents e.g. Peru, and the
east coast of continents have
contact with warm ocean
currents e.g. Brazil current.
• Above 45 degrees, the
position is reversed – west
coast in contact with warm
currents e.g. NAD, and east
coast in contact with cold
current e.g. Labrador current.
• In the Pacific and Atlantic
oceans large loops (gyres)
appear which are associated
with cells of sub-tropical high
pressure.
Rainfall and Vegetation in Africa
• Rainfall
Vegetation
ITCZ
• Critical to our understanding
of the varying rainfall totals
and their seasonal distribution
in tropical Africa is the
seasonal movement of the
ITCZ.
•
The ITCZ is a belt of low
pressure produced by the
combination of equatorial heating
and the convergence of trade
winds, and migrates in response
to the changing location of the
thermal equator. (see hand out)
The Inter tropical Convergence Zone
• As the airflows converge at the
ITCZ, they rise and create a zone
of clouds and rainfall. Once the
air ascends it diverges and flows
polewards, descending over a
wide area centred around 30
degrees N and S. As it descends
it is warmed and results in dry,
cloudless conditions.
• The descending air at the
subtropics will be affected by
the air mass at the Earth’s
surface. The most important air
masses which affect Africa are
Tropical Continental and Tropical
Maritime.
• The Tropical Continental air
mass (Harmattan) is a hot and
dry air mass.
• The Tropical Maritime is a hot
and wet air mass.
• In July, the ITCZ has
reached its most northerly
extent and it pulls in hot,
moist tropical maritime air
bringing the Wet Season to
West Africa.
• By January, in response to the
changing position of the thermal
equator, the ITCZ has migrated
to the Tropic of Capricorn. Most
of Africa north of the equator
will experience its dry season at
this time.
• West Africa is also influenced
by Tropical continental air at
this time bringing dry, dusty
conditions.
ITCZ in Africa
• http://people.cas.sc.ed
u/carbone/modules/mo
ds4car/africaitcz/index.html
The ITCZ in Africa.url
http://www.srh.weather.gov/srh/jetstream/tropics/itcz.
htm
CLIMATIC CHANGE
• Causes of climatic change:
• Natural causes
• Man-made causes
Natural Causes
• Variations in solar energy –
sun spot activity occurring in
cycles.
• Milankovitch’s Cycle (wobble,
roll and stretch theory)
• Composition of the Earth’s
atmosphere – volcanic activity
can add dust particles into
atmosphere increasing the
absorption and scattering of
incoming solar radiation.
Man Made Causes
• Increased CO2 levels
• Deforestation – increases CO2
levels
• Flatulent cows - increased
population pressure has led to
increased food production – cows
produce a lot of methane gas.
• Deforestation, soil erosion etc
have increased the albedo
effect.
• Increased use of CFCs in
aerosols etc.
Consequences
• Predicted temp rise of 1.5 to
4.5 degrees C – this would
threaten wildlife, affect
agricultural areas, tropical
diseases would spread
• Sea levels would rise – many
low lying areas would be
flooded.
• Increase in extreme weather
conditions.
Solutions
• Reduce CO2 emissions
• Reduce use of nitrogen
fertilisers
• Less intensive livestock
production
• Ban CFCs