Transcript atmosphere - WHOLE

HIGHER GEOGRAPHY
PHYSICAL ENVIRONMENTS
ATMOSPHERE
1
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 the
sun’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 global
winds, 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.
2
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 patterns
shown on maps etc.
3
Troposphere = main zone
of weather and climate.
lapse rate
= decrease in temperature
with altitude
= 6.4ºC for every
1000metres
4
Mt Everest (8800metres)
Calculate the difference in temperature between sea level
and the summit of the mountain.
5
ATMOSPHERIC GASES
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 other
gases decrease).
6
Global extremes
of Temperature
58ºC
San Luis
Potosi, Mexico
Al Aziziyah,
Libya
-88ºC
Vostok
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.
7
SOLAR INSOLATION
reflected by clouds
and dust, water vapour and other gases
in the atmosphere
100%
26%
18%
absorbed by clouds
and dust, water vapour and other gases in the atmosphere
56%
reflected by surface
6%
absorbed by surface
50%
8
SOLAR INSOLATION
100% solar insolation
26% reflected by atmosphere
18% absorbed by atmosphere
TOTAL ALBEDO = 26 + 6
= 32%
56% reaches surface
TOTAL ABSORPTION = 18 + 50
= 68%
6%
reflected by surface
50% absorbed by surface
9
ENERGY SURPLUS and DEFICIT
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.
10
3
2
1
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 because
the sun’s rays strike the surface at a lower angle.
Higher levels of surface albedo - the ice-cap reflects
more solar insolation, and vegetation at the equator
11
reflects far less.
In theory an imbalance in energy receipt could result
in 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 CIRCULATION
and
2. OCEAN CURRENTS
12
90º Pole
DEFICIT
1. ATMOSPHERIC CIRCULATION
2. OCEAN CURRENTS
SURPLUS
0º Equator
13
SINGLE CELL MODEL
 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.
0º Equator
LP
90º Pole
HP
14
 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
15
surplus
0º Equator
deficit
90º Pole
16
TRANSFER of ENERGY by ATMOSPHERIC CIRCULATION
0º Equator
90º Pole
17
THREE CELL MODEL
Hadley
Cell
0º Equator
LP
Polar
Cell
Ferrel
Cell
30º
HP
60º
LP
90º Pole
HP
18
ENERGY TRANSFER
Hadley
Cell
Ferrel
Cell
Polar
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.
19
ENERGY TRANSFER
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.
20
TRANSFER of ENERGY by OCEAN CURRENTS
90º Pole
0º Equator
21
SINGLE CELL MODEL
The single cell model of atmospheric circulation
was developed to explain the transfer of energy
from the Tropics to the Poles.
This was later improved and a three cell model
was developed.
Today the three cell model is also considered to be
an oversimplification of reality.
22
HADLEY CELL
ITCZ
ITCZ = Inter-tropical convergence Zone (Low Pressure)
STH = Sub-tropical High (High Pressure)
23
convergence
divergence
convergence
divergence
WINDS
0º Equator
LP
30º
HP
60º
LP
90º Pole
HP
winds blow from high pressure zones to low pressure zones
24
CONVERGENCE and …………DIVERGENCE
25
26
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
27
PLANETARY WINDS
High Pressure
Coriolis effect
WIND
pressure gradient force
Low Pressure
Winds are named by the direction from which they blow.
28
90ºN
WIND BELTS
Polar easterlies
Temperate Low
LP
60ºN
South westerlies
Sub-tropical High - Horse Latitudes
HP
30ºN
NE Trades
Equatorial Low - Doldrums
LP 0º
SE Trades
Sub-tropical High - Horse Latitudes
HP
30ºS
North westerlies
Temperate Low
LP
60ºS
Polar easterlies
29
90ºS
WIND BELTS
Polar easterlies
convergence
LP
60ºN
South westerlies
divergence
Sub-tropical High
HP
30ºN
NE Trades
convergence
Inter-tropical convergence zone
LP
0º
SE Trades
divergence
HP 30ºS
Sub-tropical High
North westerlies
convergence
LP
60ºS
Polar easterlies
30
90ºS
WIND BELTS
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
31
Series of High and Low pressure centres approx. every
? latitude
?
pressure zones associated with descending air (
?
)
Low pressure zones associated with
?
air (convergence)
SLIDE 37
?
circulation cells in each hemisphere:
?
?
direct
?
thermally indirect
Polar Cell
thermally direct
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 Hemisphere
Acts to the
?
of direction of motion in the Southern Hemisphere
Major wind belts of the Earth surface
0 to 30ºN
?
?
?
Southeast Trades
30 to 60ºN/S
?
60 to 90ºN/S Polar
?
32
Series of High and Low pressure centres approx. every 30º latitude
High 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 Hemisphere
Acts to the Left of direction of motion in the Southern Hemisphere
Major wind belts of the Earth surface
0 to 30ºN Northeast Trades
0 to 30ºS Southeast Trades
30 to 60ºN/S Westerlies
60 to 90ºN/S Polar easterlies
33
34
35
23º
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.
36
June Summer Solstice
23½ºN
TROPIC of CANCER
September Autumn Equinox
March Spring Equinox
0º
EQUATOR
December Winter Solstice
23½ºS
TROPIC of CAPRICORN
37
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
38
39
The migration of the inter-tropical convergence zone (ITCZ) in Africa affects
seasonal precipitation patterns across that continent.
40
THE ITCZ AND PRECIPITATION IN SUMMER
South
North
Stable, dry, cT air from the
Sahara moves southwards.
It causes the dry season Occasional
across this area
rainfall
Heaviest
rainfall
ITCZ
Moist unstable mT air now reaches far inland,
bringing rain to about 17ºN
20ºN
15ºN
10ºN
GULF OF
GUINEA
5ºN
Based on a diagram from “Core Themes in Geography”, by Broadley & Cunningham
41
THE ITCZ AND PRECIPITATION IN WINTER
North
South
Stable, dry, cT air from the Sahara moves southwards.
It causes the dry season across this area
mT air right at the ITCZ is
prevented from rising by
stable cT air
ITCZ
Warm land surface makes the mT
air unstable. It rises and rain fallsGULF
a OF
GUINEA
short way behind the ITCZ
20ºN
15ºN
10ºN
Based on a diagram from “Core Themes in Geography”, by Broadley & Cunningham
5ºN
42
DESERT
SAVANNA
dry all year
dry ‘winter’
wet ‘summer’
RAINFOREST
wet all year
ITCZ moves north in summer
43
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.
44
RAINFOREST
0º
GUINEA
SAVANNA
SAHEL
SAVANNA
10ºN
rainfall decreases
DESERT
20ºN
seasonality increases
variability increases
45
savanna ‘parkland’
46
47
3 forces:
 solar heating
 surface winds
 Coriolis effect
result in a clockwise circulation of
water in the Northern hemisphere
and an anticlockwise circulation in
the Southern hemisphere.
This circulation is known as a
GYRE.
48
OCEAN CURRENTS IN THE NORTH ATLANTIC
90º Pole
1
NORTH EQUATORIAL CURRENT
4
2
GULF STREAM
3
NORTH ATLANTIC DRIFT
4
NORTH ATLANTIC DRIFT
5
LABRADOR CURRENT
6
CANARIES CURRENT
5
3
6
2
1
0º Equator
49
50
51
52
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 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.
53
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,
54
55
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.
56
Enhanced Greenhouse Effect
3
11%
4
6%
CFCs N2O
CH4
2
19%
CO2
1
64%
1 Carbon Dioxide > fossil fuels, vehicle emissions, forest clearance
2 Methane > rice cultivation, biomass burning, digestive fermentation,
termites, sewage, landfill, natural gas production
3 CFCs > aerosol propellants, refrigerants, foaming agents
4 Nitrous oxide > nitrogen fertilisers, industrial pollution
57
CONCENTRATION CHANGES SINCE 1750
Carbon Dioxide: 280 ppm
Methane: 0.70 ppm
360 ppm (+30%)
1.80 ppm (+145%)
Methane c25 x effect of CO2
CFCS (chlorofluorocarbons)
recent significant decrease due to concern
about OZONE LAYER
BUT
CFCs c10,000 x effect of CO2
58
59
60
61
°C Temperature anomalies from the period 1961-1990
62