Page 86 - The World About Us

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1.1.1
1.1.1
2.3.4
2.2.3

How does atmospheric circulation create climate zones?

As we saw on page 6, the global atmosphere Fig.225 Atmospheric circula on.
tends to circulate in six large cells: three are
north and three are south of the Equator. This
circula on in the Hadley, Ferrel and Polar cells
is shown in fig.225 as a simplified geographical
model.
Near the Equator intense hea ng of the
ground heats the air above it. This air expands
as it is warmed and begins to rise. The air rises
un l it reaches the tropopause, 18 km above
the Earth's surface. Here it begins to spread
north and south.
As the air reaches around 30 north and
south it begins to sink, joining air moving
towards the Equator from the Ferrel Cell. This
descending air creates a global zone of high
pressure called the Sub-Tropical High Pressure
(STHP).
Once at ground level much of this air returns
towards the Equator, as the Trade Winds. Some
of the air from the STHP moves polewards as
the South Westerlies in the Northern
Hemisphere or the North Westerlies in the
South. The Tri-Cellular Model. The ver cal scale of the circulatory
The Trade Winds blow steadily towards the Geographical models are simplified cells is also very exaggerated. The
Equator, where they meet the winds from the versions of real life processes and tropopause marks the top of the lowest
opposite hemisphere to create the region of low features. The model shown in Fig.225 is part of the atmosphere, called the
pressure called the Inter-Tropical Convergence very simplified. It does not show how troposphere. The tropopause is 18km
Zone (ITCZ). the cells move north and south or so above the Equator and around
throughout the year with the changing 12km above the Poles. If the circulatory
At around 60 , the air from the Ferrel Cells seasons, nor does it indicate the impact cells within the troposphere were
meets cold air moving outwards from the Arc c of the uneven distribu on of land, sea drawn to actual scale they would be
and Antarc ca. The warmer, less-dense tropical and mountain ranges in the Northern impossible to see when compared to
air rises above the colder polar air to create the
and Southern Hemispheres. the 12,742 km diameter of the Earth.
Polar Fronts.
What are the features of high and low atmospheric pressure?

Atmospheric pressure is measured in millibars (mb), with What happens in zones of low pressure?
an average of 1013mb across the surface of the planet. Normal
Low pressure is caused by rising air. At the Inter-Tropical
atmospheric pressure exists in a range between 975mb and
Convergence Zone (ITCZ) rising air is caused by the mee ng of
1050mbs, although highs of 1083mb and lows of 870mb have
the North-Easterly and South-Westerly Trade Winds, as well as
been recorded in extreme condi ons.
by convec on currents off the heated land and sea surface. At
What happens in areas of high pressure? the Polar Front, between the Ferrel and the Hadley Cells, the
low pressure is caused by warmer, tropical air rising over the
High pressure occurs when air in the upper
troposphere begins to descend towards the ground. This colder, denser polar or Arc c air.
air will already be rela vely dry, as it will have rained In both situa ons the rising air cools as it expands.
out most of its moisture at the ITCZ or Polar Front. As Condensa on of water vapour occurs to form cloud
the air descends, it warms as it becomes denser, droplets. This leads to rain or snowfall. The unse led weather
compressed by the Earth's gravity. This adiaba c warming of the UK is typical of mari me areas under the Polar Front.
results in the evapora on of any remaining moisture. There The UK is affected by around 100 mid-la tude depressions
are few clouds and li le rainfall. Falling air at around 30 each year. In each depression there is a warm front, created by
north and south of the Equator is associated with the mee ng warm air rising over cold air. This produces periods of persistent
of high level winds in the Ferrel and Hadley cells. Most of the rain. This is soon followed by a cold front. The cold front occurs
world's hot deserts are found under this band of descending where cold and dense Arc c air undercuts the warmer tropical air
air, known as the Sub-Tropical High Pressure (STHP). forcing it to rise and cool rapidly producing more intense rainfall.

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The global circulation of the atmosphere controls weather and climate.