Page 6 - The World About Us

Amazonia
1.1.1
2.1.1

How does atmospheric circulation affect biomes?

fig.6 Global patterns of atmospheric circulation. Air in the atmosphere does not stay in the same place.
It moves, or circulates, as wind. Moving air brings with it
the condi ons of the area of land or sea over which it has
Polar Cell Northern Polar Front
passed. Air passing over hot dry land will be warm and
90 N
o
lack moisture. Air moving over warm seas will quickly
become humid picking up moisture evaporated from the
o
60 N
surface of the water.
Sub-Tropical On a global scale, the atmosphere circulates in dis nct
Ferrel Cell
South High Pressure cells. There are three cells in the Northern Hemisphere and
Westerlies (STHP)
three in the Southern Hemisphere. The movement of air in
o
30 N STHP
these cells, the Hadley Cell, Ferrel Cell and Polar Cell, is
North driven by the differing amounts of energy the surface of the
Easterly Hadley Cell Earth receives at different la tudes.
Trade
Winds Near the Equator intense hea ng of the ground heats
Inter the air above it. This air expands as it is warmed and begins
Tropical to rise. The air rises un l it reaches a zone in the
0 o The Doldrums (ITCZ) Convergence
Zone atmosphere called the tropopause (the boundary between
South (ITCZ) the troposphere and stratosphere), about 9-18 km above
Easterly the Earth's surface. Here it begins to spread north and
Trade Hadley Cell south. As the air reaches around 30 north and south of the
Winds Equator it begins to sink, joining air moving towards the
o
30 S STHP Equator from the Ferrel Cell. This descending air creates a
global zone of high pressure called the Sub-Tropical High
Prevailing surface Pressure (STHP).
North
Ferrel Cell Air movement and the Equator, as the north-easterly and south-easterly Trade
Westerlies winds Once at ground level much of this air returns towards
o
60 S temperature change Winds. Some of the air from the STHP moves polewards as
High South Westerlies (Northern Hemisphere) or the North
o
90 S pressure
Polar Cell Southern Westerlies (Southern Hemisphere).
Polar Front Low
pressure The Trade Winds blow steadily towards the Equator,
where they meets the Trade Winds from the opposite
The diagram above is known as the Tri-Cellular Model of hemisphere. As these winds converge the air is pushed
Atmospheric Circula on. It is a simplified version of actual upwards, developing further the zone of low pressure called
circulatory pa erns but is very useful in recognising the the Inter-Tropical Convergence Zone (ITCZ).
most important winds and pressure zones, and essen al in
Towards the poles, at around 60 , the air from the Ferrel
understanding the distribu on of the world’s major biomes. Cells meets cold air moving outwards from the Arc c and
We will visit atmospheric circula on again when we begin to
Antarc ca. The warmer, less-dense tropical air rises above
inves gate extreme weather from page 72 onwards. the colder polar air to create the Polar Fronts.

fig.7 Hot deserts under descending air. fig.8 Woods and forest under rising air. fig.9 Grassland/scrub in transition zones.
Where air descends from the upper Where air rises it expands, becoming Where areas of the Earth lt beneath
atmosphere it is compressed by the ‘thinner’ and begins to cool. The areas of both rising and falling air
gravity of Earth and warms up. This adiaba c cooling of rising air results in throughout the year, those areas
adiaba c warming results in the the condensa on of water vapour into experience both heavy rain (under rising
evapora on of any moisture leaving cloud droplets. The result is heavy air) and dry spells (under falling air).
clear, blue skies and li le rainfall. downpours of rain. Plants here must adapt to drought.

6

Ecosystems consist of interdependent components.