Under normal conditions, the temperature of the
atmosphere decreases with increasing height. Occasionally,
conditions are set up such that the temperature actually
increases with height, a condition known as a temperature
inversion, or simply an inversion. A radiation temperature
inversion may occur on clear, cold, generally windless nights.
On these nights the ground may undergo strong radiation
cooling, cooling the dense surface layer of air. At the same
time the slightly higher layers of air that are not in contact
with the ground do not cool as much, forming a radiation
temperature or a nocturnal inversion.
Atmospheric inversions often cause increased air pollution.
The temperature profile of the atmosphere helps determine
the atmospheric stability—when the temperature
increases with height, the air is very stable. Any air parcel
that tries to rise up into the inversion layer will at some point
be cooler and heavier than the air around it in the inversion
and will not be able to rise any further. This, and warm air
from smokestacks, cars, and other pollutants that normally
rises and is removed from the local area and dispersed, gets
trapped below the inversion and stays close to the source.
This can form particularly acute conditions in places where
cities are located in valleys and the air has no way to escape,
increasing levels of toxic chemicals.
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