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Why are deserts so hot?
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| Deserts are hot primarily because of the lack of water. When the sun shines on the
ground, all of the absorbed sunlight goes into raising the ground's temperature. If there was moisture
in the soil, much of this heat would go into evaporation of some
of the water, pumping water vapor in the air, and keeping the soil cooler that it would otherwise be.
This cooling is from the "latent" heat of evaporation that is required to change liquid water into
water vapor. Associated with the lack of water is a lack of vegetation, which also evaporates water as part of
photosynthesis -- a process called evapotranspiration. The vegetation itself doesn't cool the desert -- it's the water
being processed by the vegetation. In fact, since vegetation is darker than most desert soils, if a desert
could have vegetation that didn't need water to live, the desert would actually be hotter since it would
absorb more sunlight! Similarly, if deserts were covered by dark soil rather than light soil or sand, they would also be hotter.
The common view that deserts are so hot because of the bright sand is, therefore, incorrect.
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| Interesting facts:
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DESERTS ARE COLD AT NIGHT:Because of the lack of water in the ground,
and little water vapor in the air, most deserts can get quite cool at night. This is because (1) dry ground
does not retain as much heat as moist ground, and (2) water vapor
is the most important greenhouse gas, so dry air allows the surface to cool rapidly at night
through loss of infrared radiation to outer space. In fact, it has been estimated that the Sahara Desert
actually loses more infrared radiation than it gains solar radiation from the sun.
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