Climate of extremes - Antarctica: Global Science from a Frozen Continent - page 109

cools, due to the decrease in pressure with increasing altitude and the resulting
expansion of the air, similar to air that is let out of a tyre. As the air cools clouds, and
eventually precipitation, can develop. In fact, most of the clouds and precipitation that
occur in the atmosphere are the result of rising motion. Knowing this, it is
not surprising that the region near the equator, associated with the rising branch of the
Hadley cell, is quite cloudy with frequent thunderstorms. Similarly, the rising branch
of the Ferrell and polar cells near 50 to 60º latitude are also characterised by
extensive cloudiness and precipitation. As air sinks in the atmosphere it warms and
this warming causes any clouds which may be present to dissipate, thus suppressing
the development of precipitation. Not surprisingly, the regions near 30º and the
poles, which are characterised by sinking motion between the Hadley and Ferrell cells
and in the polar cell, have generally cloud free skies and receive little precipitation.

Solar radiation: warming the atmosphere

When solar radiation is absorbed by the ground the ground warms, in the same
way that you will be warmed if you stand in bright sunlight. The majority of solar
radiation that is absorbed by the Earth (i.e. the atmosphere and the ground) is
absorbed by the ground. As a result the surface of the Earth is heated and in turn
warms the air in contact with the ground, similar to the way a pot of water placed
on a stove is warmed by contact with the hot burner. As one moves up through
the atmosphere, away from the ground, the warming in uence of the ground

decreases, and as a result the temperature of the atmosphere also decreases.
This decrease of temperature with increasing altitude in the atmosphere is
typical for the lowest layer of the atmosphere, known as the troposphere. This
tendency for temperature to decrease with increasing altitude eventually ceases,
usually at altitudes between 10 to 18 km above sea level. Above this height temperature
either remains constant with altitude or actually increases with altitude. This
situation, where temperature increases with altitude, is known as an inversion. The
layer of the atmosphere above the troposphere in which temperature is either constant
or increasing with altitude is known as the stratosphere. From the discussion above we
see why temperature decreases with altitude in the troposphere, but why does the
temperature increase with altitude in the stratosphere?
It turns out that a large fraction of the UV radiation that makes up solar radiation
is absorbed by the gas ozone. The maximum concentration of ozone is found in
the stratosphere. As a result, the stratosphere is heated by the absorption of UV
solar radiation by ozone. The region of maximum absorption is in the upper
stratosphere, and thus this is the region of the stratosphere with the warmest
temperatures. As sunlight passes through the stratosphere, more and more of the
UV solar radiation is absorbed, leaving less and less to continue down through the