Higher Geography Physical Environments Atmosphere
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Higher Geography
Physical Environments
Atmosphere
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Contents
Atmosphere Introduction page 2
The Atmospheric Map
page 3
Energy Exchanges Between the Atmosphere and the Earth
page 4
Differences in Heating the Earth’s Surface (insolation) with Latitude
page 5
Redistribution of Energy: An Introduction page 6
Atmospheric Circulation page 7
Ocean Currents page 11
Air Masses & Air Streams: An Introduction
page 14
Air Streams page 15
ITCZ Page 16
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Atmosphere Introduction
The atmosphere is a layer of transparent, odourless gasses that surround the Earth, and is kept
in place by gravity. It acts as a filter on incoming energy from the sun (solar radiation) and plays a
part in redistributing the sun’s energy over the surface of our planet. It is responsible for our
weather and climate, and, (when converted by photosynthesis in green plants) supports all forms
of life.
Various gasses combine to form he atmposphere. Although
nitrogen and oxygen together make up 99% of the atmosphere
by volume, changes in the relatively small amounts of cardon
dioxide and ozone are causing great concern among scientists
as are the increasing amounts of pollutants sulpher dioxide ,
nitrogen oxide, and methane. The atmosphere also contains
water vapour, dust, (both play an important role) and inert
gasses.
Gas % by
volume Importance for weather & climate
Other functions / source
Permanent Gasses
Nitrogen 78.9 Mainly Passive Needed for plant growth
Oxygen 20.95
Produced by photosynthesis: reduced by deforestation
Variable Gases
Water Vapour 0.20 – 4.00
Source of cloud formation and precipitation, reflects / absorbs incoming radiation. Provides majority of natural ‘greenhouse effect’.
Essential for life on earth. Can be stored as snow / ice.
Carbon Dioxide 0.03
Absorbs long-wave radiation from space and so contributes to ‘greenhouse effect’. Its increase due to human activity is a cause of global warming.
Used by plants for photosynthesis; increased by burning fossil fuels and deforestation.
Ozone 0.00006 Absorbs harmful ultra-violet radiation
Reduced / destroyed by chlorofluocarbons (CFCs)
Inert Gasses
Argon 0.93
Helium, neon, krypton
Trace
Gasses:
Dust Trace
Absorbs/reflects incoming radiation. Forms condensation nuclei necessary for cloud formation.
Volcanic dust, meteoritic dust,, soil erosion by wind.
Pollutants:
Nitrogen - 78.084%
Oxygen - 20.95%
Argon - 0.934%
Carbon Dioxide - 0.036%
Neon - 0.0018%
Helium - 0.0005%
Methane - 0.00017%
Hydrogen - 0.00005%
Nitrous Oxide - 0.00003%
Ozone - 0.000004%
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The Atmospheric Map
The gases in the atmosphere become thinner as you move further away from the Earth’s
surface, and so the atmosphere (weight of air) decreases rapidly with height (altitude). Most
of our weather and climate is concentrated within 16km of the Earth’s surface at the equator
and 8km at the Poles. 50% of air (atmospheric gases) is within 6km above sea level.
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Energy Exchanges Between the Atmosphere and the Earth
The Earth and the atmosphere can be viewed as a closed system, dependent on continuing
inputs of energy from the sun (called solar radiation). There are also some very small energy
inputs from the Earth’s interior (geothermal energy and the tides (tidal energy).
The sun energy is sent in the form of short wave solar radiation (insolation), yet only half
passes right through the atmosphere, reaching and heating the Earth’s surface.
The diagram below shows the energy transfers and exchanges which are responsible. The other
50% are ‘lost’ in two ways – reflection and absorption.
1. Reflection
by clouds – 21%
by gas and dust – 5%
by the earth’s surface – 6%
Total reflected 32%
This is called the Earth’s albedo.
Note that the Earth’s surface reflection is uneven. Light surfaces (ice caps,
deserts etc reflect more that dark surfaces)
2. Absorption
by cloud cover – 3%
by water vapour, gas or dust – 15%
Total absorbed 18%
Total reflected (32% + total absorbed (18%) =50 %
Leaving 50% to be absorbed by the Earth’s surface. When solar radiation reaches the
surface (called solar insolation) it is transformed into heat and then emitted as slower
long-wave radiation, heating the atmosphere from the Earth outwards.
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Differences in Heating the Earth’s Surface (insolation) with Latitude
Tropical regions (low
latitudes are warmer
than Polar Regions
(high latitudes). This is
because tropical
regions receive and
absorb more solar
radiation, resulting in a
net gain in low
latitudes (Tropics),
whereas in Polar
Regions, which
receive and absorb
less solar radiation,
there is a net loss in
solar energy.
Tropical regions – low
latitudes = net gain in
solar radiation.
Polar regions – high latitudes = net loss in solar radiation.
Reasons why it is hot at the Tropics and cold at the Poles
1. In Polar Regions, due to the curvature of the Earth’s surface, the radiation has to pass
through a greater depth of atmosphere, which will absorb more heat by dust and cloud at
the Tropics.
2. The sun’s rays (solar radiation) are most direct
(they strike vertically) at the Tropics and so are
more concentrated, with less energy lost by
reflection as they pass through less
atmosphere.
3. In Polar regions, due the curvature of the earth,
the radiation is spread over a larger area than
at the Tropics.
4. The movement of the earth around the sun and the tilt axis mean that different parts of the
Earth receives more solar insolation than others at different times of the year. For
example: for 6 months of the year, the poles are in darkness and receive little or no
incoming solar radiation and consequently have very low temperatures.
5. Different albedo’s between the Tropics and the poles – darker tropical rainforests
absorbs radiation whereas lighter ice-caps at the Poles reflect much of the solar
radiation.
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Redistribution of Energy – An Introduction
If the only transfers were between the Earth’s surface and the atmosphere, the low latitudes at
the Tropics would become hotter and hotter, and the high Polar latitudes colder and colder, 38o
latitude (North and South of the equator is the dividing line:
Poleward of 380 N or S:
Less solar energy is received
and absorbed than terrestrial
(radiated / reflected from the
Earth surface) energy emitted
(sent back into the
atmospheres).
There is therefore an energy
deficit or loss.
Between 380 N and S:
More solar energy is received
and absorbed than energy
emitted.
There is therefore an energy
surplus or gain.
Why, then, do the polar areas N and S of 38o not get colder and the area between 38o N and S
hotter and hotter?
Because there are also energy transfers at a horizontal level, between the Tropics and the Polar
areas, redistributing (or transferring) energy from low latitudes where there is a surplus, to high
latitudes where there is a deficit.
Energy is redistributed by:
A series of cells (circulations of air or wind) in the atmosphere transfer warm tropical air to
the cold poles and return colder air (80%)
Ocean currents (20%)
The energy imbalance is what drives winds and ocean currents
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Atmospheric Circulation
Part A
The Earth’s Wind and Pressure Systems
Differences in air pressure cause winds to
blow from high to low pressure.
Due to the rotation of the Earth, these winds
are deflected to the right in the Northern
Hemisphere and to the left in the southern
hemisphere. This is called the Coriolis Force
or Effect.
An Explanation of How Atmospheric Cells and Related Surface Winds Assist in the
Redistribution of Energy over the Earth.
Over the major parts of the
Earth's surface there are
large-scale wind
circulations present. The
global circulation can be
described as the world-
wide system of winds by
which the necessary
transport of heat from
tropical to polar latitudes
is accomplished.
In each hemisphere there
are three cells (Hadley cell,
Ferrel cell and Polar cell) in
which air circulates through
the entire depth of the
troposphere. The
troposphere is the name
given to the vertical extent
of the atmosphere from the
surface, right up to between 10 and 15 km high. It is the part of the atmosphere where most of
the weather takes place.
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Three Cell Model - Thermal Circulation
Hadley cell
The largest cells extend from the
equator to between 30 and 40 degrees
north and south, and are named
Hadley cells, after English
meteorologist George Hadley.
Within the Hadley cells, the trade winds
blow towards the equator, then ascend
near the equator (creating low
pressure) as a broken line of
thunderstorms, which forms the Inter-
Tropical-Convergence Zone (ITCZ).
From the tops of these storms, the air
flows towards higher latitudes, where it
sinks to produce high-pressure regions
over the subtropical oceans and the
world's hot deserts, such as the Sahara
dessert in North Africa.
Ferrel cell
In the middle cells, which are known as the Ferrel cells, air
converges at low altitudes to ascend along the boundaries
between cool polar air and the warm subtropical air that generally
occurs between 60 and 70 degrees north and south. This often
occurs around the latitude of the UK, which gives us our unsettled
weather. The circulation within the Ferrel cell is complicated by a
return flow of air at high altitudes towards the tropics, where it joins
sinking air from the Hadley cell. These temperate cells do most of
the mixing of cold polar air and hot tropical air, especially at the
tops of the cells where high altitude winds, called jet streams,
blow.
The Ferrel cell moves in the opposite direction to the two other cells (Hadley cell and Polar cell)
and acts rather like a gear. In this cell the surface wind would flow from a southerly direction in
the northern hemisphere. However, the spin of the Earth induces an apparent motion to the right
in the northern hemisphere and left in the southern hemisphere. This deflection is caused by the
Coriolis effect and leads to the prevailing westerly and south-westerly winds often experienced
over the UK.
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Polar cell
The smallest and weakest cells are the Polar cells, which extend from between 60 and 70
degrees north and south, to the poles. Air in these cells sinks creating high pressure, over the
highest latitudes and cold air flows out towards the lower latitudes at the surface, where it is
slightly warmed and rises to return at altitude to the poles.
Three Cell Model – The Complete Picture
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Climate Maps
A global climate map gives a broad comparison of temperature /air pressure / prevailing wind / direction /
precipitation of the actual surface of the Earth.
Global Wind Belts
The 3-cell model also gives wind belts in each
hemisphere.
The north-east trade winds
The north-east trade winds blow from the sub-
tropical high pressure to the equatorial low
pressure areas, but are deflected to the right by
the Coriolis force.
The mid-latitude westerlies
The mid-latitude westerlies (or south-
westerlies) blow from the sub-tropical high
pressure belts to the mid-latitude low areas,
but again, are deflected to the right by the
Coriolis force.
The polar easterlies (or north-easterlies)
The polar easterlies (or north-easterlies) blow
from the polar high pressure belts to the mid-
latitude low areas, but again, are deflected to
the right by the Coriolis force.
Complications
The cell, pressure belts and
wind belts all move north
and south during the year in
line with the sun overhead.
They do not stretch all
round the world, but are
broken up by the
distribution of oceans and
land.
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Atmospheric Circulation
Part B
Ocean Currents
The oceans, like the atmosphere, play a vital role in transferring energy from the Tropics to higher
latitudes. Due to the coriolis force, ocean currents circulate in a clockwise direction in the
northern hemisphere and an anticlockwise direction in the southern hemisphere. Eventually the
ocean currents complete a circuit called a gyre.
This movement creates a global distribution of heat energy. Warm equator water moves north
and south toward the poles. Colder Arctic and Antarctic waters then move toward the equator and
are heated to repeat the cycle.
The oceans are so good at transferring heat energy because:
1. 70% of the earths surface is covered in water.
2. Can be heated to a great depth
3. Oceans retain heat
4. Water expands and rises when heated and sinks when cooled: a natural circulation from
the equator to the poles.
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Ocean Currents: Case Study of the Atlantic Ocean
In The North Atlantic:
Warm water in the Gulf of
Mexico move pole wards, as
the Gulf Stream.
As it moves north, it is
deflected by the Coriolis
Force to move towards the
northeast.
On reaching Europe, some
of the current deflects
southwards to become the
cold Canary Current.
Some of the current is
deflected as the North
Atlantic Drift.
The Canary Current is
dragged by north-easterly
trade winds to the equator
and moves west along the
equator as the North
Equatorial Current until it
rejoins the Gulf Stream to
complete the gyre.
The Labrador Current
brings cold water
southwards from the Polar
Regions.
In The South Atlantic:
Water moves the opposite way in the South Atlantic.
Water moves southwards from the Tropics as the Brazilian Current, but it is deflected by
the African coast northwards as the Bengueian Current and completes the anticlockwise
gyre as the South Equatorial Current, dragged by south-easterly trade winds.
Effects of the Atlantic Ocean Currents
Warm Currents:
Move pole wards
Raise coastal temperatures
Increase coastal rainfall because of greater evaporation from warm water.
Cool Currents
Move towards the equator
Reduce coastal temperatures
Reduce rainfall because of lower evaporation from the cool sea.
produces fogs as warm air, blowing over a cool current, is cooled from underneath
producing clouds at sea level.
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Reasons for and Explanation of Pattern of Ocean Currents
Ocean currents are mainly produced by:
energy transferred from winds to
water
temperature gradients in the sea
More Specifically:
relationship with pattern of
prevailing winds - energy, resulting
in movement, is transferred by
friction to the surface currents i.e.
winds drag surface water.
the rotation of the Earth - coriolis force deflects ocean currents to the right in the
Northern Hemisphere, and to the left in the Southern Hemisphere.
Large land masses (continents) break up oceanic circulation, and so there are only 2
complete double loops, or gyres, where there is sufficient room - in the Atlantic and the
Pacific. These are controlled by the powerful sub-tropical high pressure cells.
Denser, chilled water sinks (just as cold air sinks) to the ocean floor and moves towards
the less dense water at the `tropics (e.g. Labrador Current).
Differences in salinity (salt content)
o equatorial low pressure area: rainfall (salt free, of course!) reduces the salinity of
the sea
o subtropical high pressure area: drier climate, evaporation is greater than rainfall -
salinity will increase
In this way ocean cells transfer warm tropical equatorial water north and south, returning
cooler water at depth.
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Air Masses and Air Streams – An Introduction
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 the horizontal
gradients or changes in temperature and
humidity are relatively slight. That is to say that
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
transition that may be more sharply defined. This
transition zone or boundary is called a front. An
air mass may cover several millions of square
kilometres and extend vertically throughout the
troposphere.
When the air mass moves, it is know as an air stream. Why should an air mass move? Because
of differences in air pressure, called pressure gradients. The movement is always from high
pressure to low pressure.
Although the earth has many air masses, there are only 2 air masses to be studied:
Tropical Continental Air Mass (cT) Tropical Maritime Air Mass (mT)
Description Winter characteristics: very warm, dry weather Summer characteristics: extremely hot, dry weather Humidity is low (10% - 17%)
Description Hot / very hot weather. Very humid (65% - 82% relative humidity)
Explanation: Origin over the Sahara (i.e. large landmass in tropical latitudes) warm, dry stable air associated with sinking air which is warmed up, therefore no condensation.
Explanation: Origin over the Atlantic Ocean in tropical latitudes (warm, moist area). Unstable if it passes over land - heated at bottom, warm moist air rises and condenses - convectional rainfall.
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Air Streams
Air streams are air masses on the move! These are important because:
rainfall is determined by wind direction
in West Africa there are two main wind directions: SW and NE winds
South-Westerly Winds North-Easterly Winds
brought by Tropical Maritime air
streams
are trade winds
are on-shore winds
blow from the Atlantic Ocean]bring
very rainy, thundery weather
cloudy conditions
brought by Tropical Continental air
streams
are trade winds
are off-shore winds
blow from the Sahara Desert
bring very dry, dusty weather
calm sunny conditions
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The Inter-tropical Convergence Zone (ITCZ)
The equator, from about 5° north and 5° south, the
northeast trade winds and southeast trade winds
converge in a low pressure zone known as the Inter-
tropical Convergence Zone or ITCZ. Solar heating in the
region forces air to rise through convection, which
results in a plethora of precipitation. The ITCZ is a key
component of the global circulation system.
The location of the ITCZ varies throughout the year and
while it remains near the equator, the ITCZ over land
ventures farther north or south than the ITCZ over the
oceans due to the variation in land temperatures. The
location of the ITCZ can vary as much as 40° to 45° of
latitude north or south of the equator based on the
pattern of land and ocean.
In Africa, the ITCZ is located just south of the Sahel at about 10°, dumping rain on the region to
the south of the desert
There's a diurnal cycle to the precipitation in the ITCZ. Clouds form in the late morning and early
afternoon hours and then by 3 to 4 p.m., the hottest time of the day, convectional thunderstorms
form and precipitation begins. These storms are generally short in duration.If the ITCZ does not
migrate northwards sufficiently in July, the rains may not fall in the Sahel region -South Sahara -
causing drought in West Africa. The more frequent droughts are seen by some as part of the
process of desertification - turning huge areas of the Sahel into desert.
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Seasonal Migration of the ITCZ in Africa
The migration of the inter-tropical convergence zone (ITCZ) in Africa affects seasonal
precipitation patterns across that continent.
The ITCZ and it’s associated Trade wind belts swing north and south, following the
zone of maximum solar radiation i.e. the overhead sun.
This changing position of the ITCZ controls the times of year when rainfall occurs in
West Africa with maximum amounts falling 1 -2 months after the passage of the ITCZ.
In January the ITCZ has moved south, it’s associated Trade Winds (cT) brings warm
dry air, leading to a very dry season (drought). Only the coastal regions to the south
receive any rain.
In July, the ITCZ has moved north drawing mT air in over West Africa. Convection cells
in this moist air causes huge cumulo-nimbus clouds to develop. The heaviest rainfall is
associated with these.
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Distribution of Rainfall
Description of rainfall pattern from the coast of West Africa northwards
the coast of West Africa (close to the equator) receives the most rain, and it falls
throughout the year
the amount of rainfall decreases as you move northwards e.g. Minna 1328mm, Sokoto
691 mm Kidal 150mm
the rainfall pattern becomes more seasonal and unreliable as you move northwards.
Places near the coast (and Equator) have between 9 - 12 rainy months a year, but the
number of wet months decreases as you move northwards away from the equator, e.g.
Sokoto 4-6 months, Menaka 1-3 months
Explanation of rainfall pattern from the coast of West Africa northwards
in the late spring and summer, the ITCZ, a low pressure belt, moves northwards, following
the overhead sun, to the Tropic of Cancer
only areas south of the ITCZ, experiencing the warm moist Tropical Maritime (mT)
winds blowing from the Atlantic Ocean will receive rainfall
the rainfall total decrease northwards because:
- the winds have further to travel from the ocean and have already
dropped much of their moisture
- the wet season is shorter
coastal areas are influenced by the mT for most of the year and so have much higher
rainfall, falling throughout the year. They may experience two peaks as the ITCZ migrates
northwards in spring, and southwards in autumn.
areas north of the ITCZ experience the hot, dry, dust laden north east tropical
continental (cT) wind blowing from the Sahara Desert, known as the Harmattan. No rain
falls during this dry season, which lasts longer as you travel northwards.
the Sahara Desert does not have a wet season at all because it is always north of thr
ITCZ and so only experiences dry Tropical Continental winds.
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An Example of the seasonal migration of the ITCZ: Kano Nigeria