Download Physical Environments Atmosphere Revision

Lornshill Academy
Geography Department
Higher Revision
Physical Environments - Atmosphere
Physical Environments
Atmosphere
Global heat budget
The earth’s energy comes from solar radiation,
this incoming heat energy is balanced by the
amount of heat escaping back into space. This
balance is called the Earth’s Heat Budget.
Incoming solar heat, insolation, from the sun is
absorbed and reflected meaning not all the
heat reaches the earth’s surface. 26% of the
energy is reflected back into space by the
atmosphere and 18% of the heat is absorbed
by the atmosphere due to dust particles
retaining the heat.
This leaves 56% which travels to the earth’s surface. Not all of this is absorbed. 6% is
reflected from the earth’s surface and this makes up the earth’s albedo. This means only
50% reaches the earth’s surface and is balanced out by the long wave radiation escaping back
into the atmosphere.
Latitudal Variation
As well as variations in the amount of energy
received from the Sun across the Atmosphere.
There is also variations in the amount of energy
received from the Sun across the latitudes of
the Planet. Between approx. 35° N and 35° S
there is a surplus of energy because insolation
exceeds outgoing radiation.
The curvature of the Earth means the same
amount of insolation is spread over a much great surface area the further north or south
from the equator. – The Sun’s heat is more concentrated nearer to the Equator. The angle
the insolation has less atmosphere to travel through near to the equator and so less energy is
lost by reflection and absorption. Due to the different land covering more reflection
(albedo) takes place near the Poles (ice / snow covered) than near the Equator (tropical
forests). This means there is a surplus of energy at the equator and a deficit at the poles.
Energy is therefore transferred from areas of surplus to areas of deficit in two ways:
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By atmospheric circulation
By oceanic currents.
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Atmospheric Circulation
We have established that some parts of the globe receive more heat from the sun than
others. It is this difference in temperature that leads to the formation of winds.
Global winds account for 80% of heat transfer
Rising air = low air pressure
Sinking air = high air pressure
The Hadley Cell
1. At the Hadley Cell intense heating at the equator causes warm air to rise into the
upper atmosphere creating low pressure areas called the Doldrums.
2. The rising air is forced north and south due to the coriolis force where it cools and
sinks at 30o North and 30o South creating a high pressure area called the Horse
Latitudes.
3. On reaching the earth’s surface the Hadley cell is completed as some of the air is
returned back to the equator via the north-east and south-east trade winds where the
cycle is repeated. Thermally direct cell, powered by warm air rising at equator.
The Polar Cell
4. At the Polar Cell, cold, dense air sinks creating a high pressure area. Some air moves
north and south to lower latitudes via the Polar Easterlies where it expands as it
moves into more space and is warmed by the earth’s surface. The air eventually meets
warmer air brought via the North and South Westerlies at 60o North and 60o South
where upon it rises and creating a low pressure area.
Also a Thermally direct cell powered by cold air sinking at the Poles.
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The Ferrel Cell
5. The final cell involved is the Ferrel Cell. It obtains its energy from the Hadley and
Polar Cells. This cell feeds warm air to high latitudes and transfers cold air back to
the sub-tropics for warming. This cell is Thermally indirect and is powered by the
other two cells.
Coriolis Force
Air is deflected by the spinning of the earth (CORIOLIS FORCE), which means that in the
Northern Hemisphere winds are deflected to the right, and in the Southern Hemisphere to
the left.
Rossby Waves
These are large belts of fast moving winds traversing the globe at high altitudes of between
10,000 and 12,000 metres. The pattern is wavelike due to the influence of temperature and
pressure differences between land and sea areas.
Jet Streams
There are streams of very fast moving air known as jet streams within these waves. These
occur due to differences in temperature between the air masses. These waves and jet
streams contribute greatly to the movement of energy throughout the world.
Ocean Currents
You should be able to describe and explain the general pattern of ocean currents on a world
map.
The currents form a pattern of large loops. Each loop is called a GYRE!
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Land Masses
If there were no landmasses, the ocean circulation would be largely controlled by surface
wind system, with 3 closed loops in each hemisphere. The distribution of the major land
masses break down this pattern and only in the Pacific and Atlantic Oceans, where there is
sufficient room, do we see the development of loops or gyres which are controlled by
pressure cells. When currents meet land masses their waters tend to diverge and flow away
in opposite directions.
Rotation of the Earth
The coriolis force deflects winds and currents to the right in the Northern Hemisphere and
to the left in the Southern Hemisphere. The currents therefore move clockwise in the
Northern Hemisphere and anticlockwise in the Southern Hemisphere.
Winds
Prevailing winds tend to move waters in the same direction. In the Tropics, the Trade Winds
push them westward towards the equator and in the higher latitudes the Westerlies drive
them in the opposite direction.
Temperature difference and convection
The water at the equator is warmer and less dense compared to polar waters where it is
colder and dense. Due to this convection currents are set up resulting in the poleward flow
of warm light water. The warm water will eventually cool, become dense and sink. The cold
water will eventually warm, become less dense and return to the equator. This cycle is
repeated over and over. This process is essential for maintaining the energy balance and
bringing up vital nutrients from the depths in Polar Regions which are then redistributed to
lower latitudes.
Case study Atlantic Ocean
In the Northern Atlantic a series of currents combine to circulate water in a clockwise
direction called a Gyre.
At the equator water warmed by the overhead sun starts to move in a north westerly direction
towards the Caribbean by the North equatorial current, transferring warm seas to higher
latitudes. The current then hits the North American continent and is deflected in a north
easterly direction towards Europe forming the warm Gulf Stream current. Again higher
latitudes receive warmer water. Cooler water from the Arctic & Northern Canada flow south
and mix with the gulf stream cooling it, which is then sent southwards to Africa as the gulf
stream is deflected by Europe. This cooler current flows towards Africa as the Canaries
current, which is then deflected by Africa towards the equator. This spreads cooler water to
the equatorial region, where it is then heated up by the sun.
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The prevailing North Easterly Trade & Westerly winds, from falling air at the sub tropics, help
to circulate the currents. The Coriolis Effect, whereby the earth spins from the west to east
also aids the clockwise circulation of the currents. The combined effect of these two factors
help to circulate the warm and cold water around the globe.
Density differences due to differential heating result in chilled water sinking to the ocean
floor and returning equatorwards. Cold dense polar water sinks, then spreads towards the
equator where it pushes up the less dense warmer water which moves off towards the polar
areas
Intertropical Convergence Zone
Air Masses
Air masses are large volumes of air with relatively uniform characteristics. Air masses
originate where the surface geography is constant such as; over oceans, deserts, large plains
and ice covered areas. As air slowly moves over these areas it acquires uniform temperature
and humidity characteristics.
The ITCZ is where the North Easterly trade
winds meet the South Easterly trade winds.
The air at the equator is being heated by the
overhead sun and therefore rises, the ITCZ can
therefore also be described as a band of low
pressure. Due to the earth spinning on a tilted
axis the position of the overhead sun migrates
northwards, when we have our summer solstice
(longest day) and then migrates southwards,
our winter solstice (shortest day). This means
the ITCZ is also going to travel north and south
throughout the year. This means the heavy rain
showers on the West coast of Africa are very
seasonal.
Origin, Nature and Characteristics of Air Masses
Tropical Maritime
Origin - Atlantic Ocean in Tropical Latitudes.
Nature (what it is like before it moves) - Warm, moist and unstable (water picked up through
evaporation over Atlantic).
Weather Characteristics - Brings warm, humid and rainy weather.
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Tropical Continental
Origin- Sahara Desert (large land mass in tropical latitudes).
Nature (what it is like before it moves) - Warm, dry and stable (no water picked up through
evaporation due to being over land).
Weather Characteristics - Brings dry and warm weather.
The ITCZ moves to follow the Thermal Equator. This moves due to the tilt of the Earth (23½
°), moving north towards June / July and then south. This means it passes over some places
(near the actual equator) twice. When the ITCZ moves North the Tropical maritime air mass
follows influencing the weather. When the ITCZ moves South the Tropical continental air
mass has more influence.
The Inter-tropical Convergence Zone (ITCZ) is where the North-East Trade Winds (Tropical
Continental cT air mass) and the South-West Trade Winds (Tropical Maritime mT air mass)
meet. At this point an area of uplift is created. This causes instability and periods of heavy
rainfall.
Variations
The Rainfall patterns vary for a number or reasons:
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The further North the less the peak rainfall due to the lessening impact of the ITCZ.
The less annual rainfall due to the increase influence of the Tropical continental air
mass.
The earliest peak rainfall is in Lagos as the ITCZ passes this first.
Lagos has two peaks as the ITCZ passes overhead twice.
Further South the larger annual rainfall due to the influence of the Tropical maritime
air mass.
July
In July, the sun moves to the Tropic of Cancer and
drags the ITCZ with it. The result is that moist mT
air penetrates quite far north.
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January
In January, the sun moves to the Tropic of
Capricorn and drags the ITCZ with it. The
result is that dry cT air from the Sahara almost
extends to the coast.
Farmers in North Africa rely on the ITCZ
(especially this) and the mT to reach them in
July. Over the last few decades it has been
noted that the ITCZ is not travelling as far
north as it used to, causing obvious disastrous
consequences.
ITCZ's effect on the region's climate
The ITCZ has varying effects on climates.
Gao
Gao, with around 200 mm of rainfall per year, is a
hot desert climate, with only a limited amount of
precipitation in summer as the ITCZ migrates north.
Gao’s climate is influenced by the hot, dry cT air for
most of the year and as can be seen in the graph
below, it therefore has fewer days of rain and very
low total annual precipitation.
This is because it is to the north of the ITCZ for
most of the year.
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Abidjan
Abidjan, with around 1700 mm of rainfall per year,
is a tropical rainforest climate. As the graph below
displays, it has a twin-peak regime with a major peak
in June and a smaller peak in October/November.
It is on the Gulf of Guinea coast and is therefore
influenced by hot, humid mT air for most of the
year. This results in a higher total annual
precipitation and a greater number of rain days.
The twin precipitation peaks happen because the
ITCZ moves north in the early part of the year,
bringing rainfall and then south later in the year, again bringing rainfall.
Bobo-Dioulasso
Bobo-Dioulasso has a total annual precipitation of
around 1000 mm and has a clear wet season/dry
season regime.
As the graph above shows, it receives more rain days
and heavy summer precipitation from June until
August when the ITCZ is furthest north. This brings
rainfall to the area as the mT air mass is dominant.
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