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HIGHER GEOGRAPHY

PHYSICAL CORE

ATMOSPHEREATMOSPHERE

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By the end of this topic you should be able to:

explain with the aid of an annotated diagram, why Tropical latitudes receive more of thesun’s energy than Polar regions

explain why there is a net gain of solar region in the Tropical latitudes and a net loss towards the poles

describe the role of atmospheric circulation in the redistribution of energy over the globe

describe and explain the earth’s energy exchanges shown on a diagram

describe the factors which affect the amount of sunlight reflected from the earth’s surface

describe and account for the generalised pattern of atmospheric circulation and globalwinds, or ocean currents shown on a world map

describe the variations in world temperature for the last 100 years (shown eg. on a graph)and suggest both physical and human reasons for these variations

describe and explain the origin, nature and weather characteristics of Tropical Maritime (mT)and Tropical Continental (cT) air masses which affect West Africa

with reference to the Inter-Tropical Convergence Zone and the movement of air masses,describe and account for the variations in West African rainfall.

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GMTs

describe and interpret climate maps, diagrams and graphs

construct and analyse climate graphs

describe and explain climate graphs

comment on the accuracy of statements which describe climate patternsshown on maps etc.

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Troposphere = main zone of weather and climate.

lapse rate= decrease in temperaturewith altitude= 6.4ºC for every 1000metres

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Mt Everest (8800metres)Calculate the difference in temperature between sea level and the summit of the mountain.

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Nitrogen - 78%Oxygen - 21%

Carbon dioxide - 0.036% …...and rising!! Water vapour - variable - up to 4% over tropical oceans.(as humidity increases the relative amounts of othergases decrease).

ATMOSPHERIC GASES

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Global extremes of Temperature

58ºCSan Luis

Potosi, MexicoAl Aziziyah,

Libya

-88ºCVostok

Antarctica In the absence of an atmosphere the Earth would average about 30ºC

less than it does at present. Life (as we now know it) could not exist.

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absorbed by clouds and dust, water vapour and other gases in the atmosphere

absorbed by surface

reflected by clouds and dust, water vapour and other gases

in the atmosphere

reflected by surface

100%

23%

25%

52%

6%

46%

SOLAR INSOLATION

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absorbed by surface

reflected by atmosphere

reflected by surface

100%

23%

25%

52%

6%

46%

absorbed by atmosphere

solar insolation

reaches surface

TOTAL ALBEDO = 25 + 6= 31%

TOTAL ABSORPTION = 23 + 46= 69%

SOLAR INSOLATION

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The Earth's atmosphere is put into motion because of the differential heating of the Earth’s

surface by solar insolation. The Poles receive less heat than the Tropics because:

1. Insolation has to pass through more of the Earth’s atmosphere

2. the angle of incidence of insolation

and 3. higher levels of surface albedo.

ENERGY SURPLUS and DEFICIT

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3

1

2

3

Insolation has to pass through more of the Earth’s atmosphere

The angle of incidence of insolation -energy is spread out over a larger area becausethe sun’s rays strike the surface at a lower angle.

Higher levels of surface albedo - the ice-cap reflects more solar insolation

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In theory an imbalance in energy receipt could resultin lower latitudes becoming warmer and higher

latitudes becoming even colder.

In reality energy is transferred from lower latitudes (areas of surplus)to higher latitudes (areas of deficit)

BY

1. ATMOSPHERIC CIRCULATIONand

2. OCEAN CURRENTS

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0º Equator

90º Pole

SURPLUS

DEFICIT

1. ATMOSPHERIC CIRCULATION

2. OCEAN CURRENTS

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0º Equator 90º Pole

surplus deficit

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0º Equator 90º Pole

TRANSFER of ENERGY by ATMOSPHERIC CIRCULATION

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TRANSFER of ENERGY by OCEAN CURRENTS

0º Equator

90º Pole

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0º Equator 90º PoleLP HP

At the Equator the atmosphere is heated Air becomes less dense and rises. Rising air creates low pressure at the equator. Air cools as it rises because of the lapse rate. Air spreads. As air mass cools it increases in density and descends. Descending air creates high pressure at the Poles. Surface winds blow from HP to LP.

SINGLE CELL MODEL

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warm air is less dense therefore lighter air rises in the Tropics this creates a zone of LOW PRESSURE air spreads N and S of the Equator air cools and sinks over the Poles this is a zone of HIGH PRESSURE air returns as surface WINDS to the Tropics

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This was later improved and a three cell modelwas developed.

The single cell model of atmospheric circulationwas developed to explain the transfer of energy

from the Tropics to the Poles.

SINGLE CELL MODEL

Today the three cell model is also considered to bean oversimplification of reality.

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ITCZ = Inter-tropical convergence Zone (Low Pressure)

STH = Sub-tropical High (High Pressure)

HADLEY CELL ITCZ

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0º Equator 90º Pole30º 60ºLP HP LP HP

Hadley Cell

Polar Cell

Ferrel Cell

THREE CELL MODEL

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Warm air rises at the Equator - Inter-Tropical Convergence Zone (ITCZ).

Equatorial air flows to ~30º N then sinks to the surface and returns as a surface flow to the tropics.

This is the Hadley cell. Cold air sinks at the North Pole. It flows S at the surface

and is warmed by contact with land/ocean, by ~60º N it rises into the atmosphere.

This the Polar cell.

Between 60º N and 30º N there is another circulation cell. This is the Ferrel cell.

The Hadley cell and the Polar cell are thermally direct cells.The Ferrel cell is a thermally indirect cell.

ENERGY TRANSFER

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ENERGY TRANSFER

Hadley Cell

Polar Cell

Ferrel Cell

Heat energy is transferred from the Hadley Cell to the Ferrel Cell and from the Ferrel Cell to the Polar Cell.

In this way heat is transferred from the Equator where there is an energy surplus

to the Poles where there is an energy deficit.

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0º Equator 90º Pole30º 60ºLP HP LP HP

conv

erge

nce

dive

rgen

ce

conv

erge

nce

dive

rgen

ce

winds blow from high pressure zones to low pressure zones

WINDS

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CONVERGENCE and …………DIVERGENCE

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Coriolis occurs because the Earth rotates.

Earth rotates about its axis every 24 hours. Distance around the equator is ~25,000 miles

the earth is travelling east at ~ 1,000 miles per hour. Distance around the Earth at 40ºN ~19,000 miles

the earth is travelling east at ~800mph.

The Coriolis effect results from this difference in velocity.

In the Northern hemisphere the Coriolis effect deflects movement to the right.

In the Southern hemisphere Coriolis effect deflects movement to the left.

The combination of atmospheric cells and Coriolis effect lead to the wind belts.

Wind belts drive surface ocean circulation

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WIND

Coriolis effect

High Pressure

Low Pressure

pressure gradient force

Winds are named by the direction they blow from.

PLANETARY WINDS

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Be very, very careful what you put that head, because you will never, ever get it out.

Thomas Cardinal Wolsey (1471-1530)

The water in a sink rotates one way as it drains in the northern hemisphere and the other way in the southern hemisphere. Called the Coriolis Effect, it is caused by the rotation of the Earth.

This is NOT true!

The Coriolis force is so small, that it plays no role in determining The Coriolis force is so small, that it plays no role in determining the direction of rotation of a draining sink anymore than it doesthe direction of rotation of a draining sink anymore than it doesthe direction of a spinning CD.the direction of a spinning CD.

CORIOLIS

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90ºS

90ºN

0º Equatorial Low - Doldrums LP

30ºNSub-tropical High - Horse Latitudes HP

30ºSSub-tropical High - Horse Latitudes HP

60ºSTemperate Low LP

60ºN Temperate Low LP

Polar easterlies

South westerlies

NE Trades

Polar easterlies

North westerlies

SE Trades

WIND BELTS

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90ºS

Polar easterlies

South westerlies

NE Trades

Polar easterlies

North westerlies

SE Trades

0º convergence Inter-tropical convergence zone LP

30ºSdivergence Sub-tropical High HP

30ºNdivergence Sub-tropical High HP

60ºN convergence LP

60ºSconvergence LP

WIND BELTS

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WIND BELTS

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Northern Hemisphere

Polar Easterlies Blowing from the Polar High Pressure zone to about 60ºN

Westerlies Blowing from Sub-Tropical High Pressure zone to about 60ºN

Northeast Trade Winds Blowing from Sub-Tropical High Pressure zone to Equatorial Low Pressure zone.

Southern Hemisphere

Southeast Trade Winds Blowing from Sub-Tropical High Pressure zone to Equatorial Low Pressure zone.

Westerlies Blowing from Sub-Tropical High Pressure zone to about 60ºS

Polar Easterlies Blowing from the Polar High Pressure zone to about 60ºS

WIND BELTS

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Series of High and Low pressure centres approx. every ? latitude ? pressure zones associated with descending air ( ? ) Low pressure zones associated with ? air (convergence)

? circulation cells in each hemisphere: ? ? ? Polar Cell

Wind is the horizontal movement of air arising from differences in ? . Very little wind at the Equator ( ? ) because air is being convected ? .Little wind at 30ºN and S (Horse Latitudes) because direction of air movement is down.

Winds always blow from an area of ? Pressure to ? Pressure.

Winds are affected by the ? Effect.Coriolis is a consequence of motion on a rotating sphere. Acts to the ? of direction of motion in Northern HemisphereActs to the ? of direction of motion in the Southern Hemisphere

Major wind belts of the Earth surface0 to 30ºN ? ?

? Southeast Trades 30 to 60ºN/S ?

60 to 90ºN/S Polar ?

SLIDE 37

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Series of High and Low pressure centres approx. every 30º latitudeHigh pressure zones associated with descending air (divergence) Low pressure zones associated with rising air (convergence)

Three circulation cells in each hemisphere: Hadley Cell thermally direct Ferrel Cell thermally indirect Polar Cell thermally direct

Wind is the horizontal movement of air arising from differences in pressure. Very little wind at the Equator (Doldrums) because air is being convected upward.Little wind at 30ºN and S (Horse Latitudes) because direction of air movement is down.

Winds always blow from an area of High Pressure to Low Pressure.

Winds are affected by the Coriolis Effect.Coriolis is a consequence of motion on a rotating sphere. Acts to the Right of direction of motion in Northern HemisphereActs to the Left of direction of motion in the Southern Hemisphere

Major wind belts of the Earth surface0 to 30ºN Northeast Trades

0 to 30ºS Southeast Trades 30 to 60ºN/S Westerlies 60 to 90ºN/S Polar easterlies

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23º

The most intense heating of the sun, occurring at the so-called thermal equator, annually moves

between the tropics.

On or around June 20th each year the sun is overhead at 23½ºN, the Tropic of Cancer.

On or around December 20th the the sun is at overhead at 23½ºS, the Tropic of Capricorn.

These two dates are the solstices.

Twice a year, at the equinoxes, on or around March 20th and September 20th the overhead

sun crosses the equator.

This annual north to south and back again "shift" of the thermal equator shifts the belts of

planetary winds and pressure systems to the north and to the south as the year turns.

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0ºEQUATOR

23½ºNTROPIC of CANCER

23½ºSTROPIC of CAPRICORN

December Winter Solstice

September Autumn Equinox

June Summer Solstice

March Spring Equinox

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ITCZ

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ITCZ JULYITCZ

JANUARY

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The location of the ITCZ varies throughout the year The ITCZ over land moves farther north or south than the ITCZ over the oceans

due to the variation in land temperatures.

ITCZ JULY ITCZ JANUARY

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http://www.cla.sc.edu/geog/faculty/carbone/modules/newmods/africa-itcz/

The blue shading on the map shows the areas of highest cloud reflectivity,

which correspond to the average monthly position of the ITCZ.

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The migration of the inter-tropical convergence zone (ITCZ) in Africa affects seasonal precipitation patterns across that continent.

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DESERT

SAVANNA

RAINFOREST

dry all year

ITCZ moves north in summer

dry ‘winter’wet ‘summer’

wet all year

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Tropical rainforest

savanna

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The further North of the Equator in tropical Africa:-

the lower the annual rainfall

the more the rainfall is concentrated in the summer months

the more variable the rainfall.

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0º 20ºN10ºN

RAINFORESTGUINEA

SAVANNASAHEL

SAVANNADESERT

rainfall decreasesseasonality increasesvariability increases

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LAGOS SOKOTO TIMBUKTUA B C A B C A B C

JAN 27 28 65 20 0 13 22 0 22FEB 29 41 69 22 0 13 24 0 19MAR 29 96 72 31 0 11 28 2 18APR 28 143 72 34 10 17 32 0 15MAY 28 274 76 33 51 31 35 5 18JUN 26 460 80 30 89 41 36 23 31JUL 25 282 80 28 147 55 32 79 45AUG 25 69 76 26 236 64 30 81 57SEP 25 140 77 27 145 59 32 38 45OCT 26 208 76 28 13 37 31 3 23NOV 27 69 72 27 0 18 28 0 17DEC 28 25 68 25 0 15 22 0 19annual 27 1835 74 28 691 31 29 231 27

A = average monthly temperature CB = mean monthly rainfall mmC = % relative humidity at midday

LAGOS

SOKOTO

TIMBUKTU

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savanna savanna climateclimatetropical summer rain

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savanna savanna vegetationvegetation

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savanna ‘parkland’

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savanna ‘parkland’

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savanna ‘parkland’

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savanna ‘parkland’

58baobab tree

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acacia tree

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acacia thorns

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desertification

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4 forces: solar heating surface winds Coriolis effect and surface windsresult in a clockwise circulation of water in the Northern hemisphere.

This circulation is known as a GYRE.

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OCEAN CURRENTS IN THE NORTH ATLANTIC

0º Equator

90º Pole

1

2

3

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6

1

2

3

4

5

6

NORTH EQUATORIAL CURRENT

GULF STREAM

NORTH ATLANTIC DRIFT

LABRADOR CURRENT

CANARIES CURRENT

NORTH ATLANTIC DRIFT

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The greenhouse effect is the name applied to the process which causes the surface of the Earth to be warmer than it would have been in the

absence of an atmosphere.

Global warming or the enhanced greenhouse effectenhanced greenhouse effect

is the name given to an expected increase in the magnitude of the greenhouse effect, whereby the surface of the Earth will amost inevitably become hotter than it is now.

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About 70% of the sun's energy is radiated back into space.

But some of the infrared radiation is trapped by greenhouse gases

and warms the atmosphere,

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Water vapour accounts for 98% of the natural Greenhouse effect.

Water vapour has lower ‘radiative forcing’ properties than some other atmospheric gases such as

carbon dioxide, methane and nitrous oxide which are naturally present in the atmosphere

in small quantities.

Since the Industrial Revolution the proportion of these gases has increased significantly.

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Enhanced Greenhouse Effect

164%

219%

311%

46%

CO2CFCs N2O

CH4

1 Carbon Dioxide > fossil fuels, vehicle emissions, forest clearance2 Methane > rice cultivation, biomass burning, digestive fermentation,

termites, sewage, landfill, natural gas production3 CFCs > aerosol propellants, refrigerants, foaming agents4 Nitrous oxide > nitrogen fertilisers, industrial pollution

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Carbon Dioxide: 280 ppm 360 ppm (+30%)

Methane: 0.70 ppm 1.80 ppm (+145%)

Methane c25 x effect of CO2

CFCS (chlorofluorocarbons) recent significant decrease due to concern about OZONE LAYER

CFCs c10,000 x effect of CO2

BUT

CONCENTRATION CHANGES SINCE 1750

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°C Temperature anomalies from the period 1961-1990