Environmental Engineering Reference
In-Depth Information
The troposphere, the stratosphere and the polar
vortex:
There are only two layers of the atmosphere we need to think about in this discussion: the
troposphere and the stratosphere. The troposphere extends upwards from Earth's surface to
an altitude of about 18 kilometres over equatorial regions and to about 8 kilometres over
the High Arctic and Antarctic. The upper boundary is known as the
tropopause
and is the
altitude at which air ceases to cool with height. Above the tropopause lies the stratosphere,
where temperature increases with altitude. This is caused by the presence of the ozone max-
imum layer at an altitude of between 15 and 35 kilometres. Stratospheric ozone is formed
naturally at these altitudes, where solar ultraviolet radiation breaks oxygen (O
2
) molecules
into free oxygen atoms that are then able to combine with intact oxygen molecules to pro-
duce ozone (O
3
). Ozone in the stratosphere absorbs solar UV radiation. Therefore, the stra-
tospheric temperature gradient runs in the opposite direction to that seen in the troposphere,
which is warmest close to Earth. This temperature trend is part of the definition of the tropo-
sphere (cooling with height) and the stratosphere (warming with height). The upper bound-
ary of the stratosphere (the stratopause) lies at an altitude of about 50 kilometres. Above this
level, temperature decreases with altitude out into space.
The stratospheric polar vortex is a large-scale region of low pressure air that is con-
strained by a strong west-to-east jet stream that circles the polar region. It usually has two
centres: one over Baffin Island and the other over north-east Siberia. The polar vortex ex-
tends from the upper troposphere through the stratosphere. Low values of ozone and cold
temperatures are associated with the air inside the vortex, which acts as a barrier to prevent
the movement of air from the South.
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