Desert Temperature Extremes
Why is it so hot in the daytime in deserts and so cold at night?
For the same reasons.
First, deserts are very dry and clear skies are normal.
The desert soil absorbs much of the Sun's energy because
there is so little water to evaporate (energy is needed
to evaporate water). That energy is then radiated to the
air (as long wavelengths, infrared) from the hot desert
soil; the air absorbs the energy and becomes hot.
At night, the dry air and clear sky allow energy to radiate
out towards space. There is little water vapor in the air
(water vapor is a good absorber of infrared energy) and so
the air loses energy and cools. The types of soils typically
found in deserts (sand, with little organic material) loses
energy easily and therefore the desert can cool off very
quickly at night.
David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory
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Update: June 2012