METEOROLOGIST JEFF HABY
Deserts are famous for having the hottest temperatures on Earth. But what makes it so hot in a desert? Deserts occur where there
is a lack of moisture and thus an abundance of sunlight. With the relative lack in moisture, there is less evaporation. There are
also less clouds to reflect the sunlight away. With no evaporative cooling and a relative lack of vegetation to use the sunlight,
most all of the sunlight goes into warming the ground surface. This causes very hot afternoon temperatures. The largest deserts
occur where global high pressure systems persist. Between the equator and the mid-latitudes is where these subtropical high
pressure systems persist. The lack of storm systems and clouds helps warm the ground.
Deserts also tend to occur in lower elevation regions surrounded by higher elevations. As air sinks over a mountain or higher terrain, it
warms adiabatically. This warming adds to the already warm and dry conditions found at a desert. The sinking air compresses
and warms. The subtropical high contributes to sinking air while local impacts such as air flow down from high to low elevations
compresses the air even more. Sinking air also dries the air (decreases relative humidity). This contributes to less clouds
and precipitation. The dry air reinforces the already sparse vegetation and dry soil.
A somewhat less known fact about deserts is that the temperature swings from the high temperature to the low temperature are often
extreme. The dry conditions that help contribute to high afternoon temperatures also contributes to cold overnight lows. Once
the sun goes down, there is a relative lack of moisture in the air to trap outgoing long wave radiation. Thus, the ground
cools rapidly. The clear skies, light wind and dry air helps the air temperature cool off significantly at night. Unbearable
heat during the day can turn into unbearable chill at night.
Another somewhat less known fact about deserts is that there are “cold deserts” also. A desert by definition receives less than 10
inches of rainfall per year. There are other conditions besides climatological high pressure with hot and dry air that can lead
to a desert. When the air is very cold it will not have much moisture. Thus, areas in the polar region that have very cold air
will not receive significant precipitation. These cold deserts also support less vegetation.