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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