American Atacama is taken to be the driest land region on the planet containing
locationswhere it is manyyearsbetween rainfall events.
Three types of deserts are generallydelineated according to their causes.
Subtropical deserts inhabit the latitudes between 20°and 30°north and south of
the equator. They are caused by the year-round presence of subtropical highs in
the Hadley Circulation. The strong descent of air in the subtropical highs precludes
significant cloudiness and these are latitudes in which the solar angles are relatively
high year round. Thus, the subtropical deserts are the hottest places onEarth. The
Sahara is this type of desert.
A second type of desert is the middle latitude deserts. They are caused by the
summertime influence of the subtropical highs and also lack precipitation because
of topographic blockage and/or great distance inland from an oceanic source of
precipitable water. The middle latitude locations mean that summertemperatures
are not as high so that the imbalance between precipitation and evapotranspiration
is not nearly as great and the aridity not as great as in the subtropical deserts.
A noteworthy aspect of such regions is that winter brings cold temperatures and
some snow with the passage ofmiddle latitude cyclones. The Gobi is an out-
standing example of a middle latitude desert.
The third type of desert is coastal desert. Whereas one might suppose the pres-
ence of a coast would make for a water source and a wetter environment, there are
certain coasts that are impressively dry. Coastal deserts are found near the Tropics
of Cancer and Capricorn and, ironically, the aridity is amplified by the nearby
oceans. This desert type is found along coasts with cold currents bolstered by
upwelling. Air passing over these near-shore waters is cooled and stabilized, sap-
ping any chance to rise to initiate the precipitation processes. Accordingly, these
coastal deserts are quite a bit cooler than their inland counterparts. The Atacama
and eastern African Namib deserts are examples of this extraordinary dryness.
The first of several common misconceptions about the desert environment is that
they are covered by sand. Dry places have much slower rates of weathering of rocks
into the tiny pieces of clay so common in moist areas, so deserts have greater amounts
of sand at their surfaces. Yet, sand dunes are not the “average” desert surface. It is
believed that sand covers only about 15 percent of desert surfaces. More common
are rocky surfaces with high percentages of sand in the soil material. Of course, sand
dunes are the iconic symbols of deserts. Several types of sand dunes are classified
based on the local amount of sand supply and the seasonal nature of direction and
strength of wind. The world’s tallest dunes approach 500 m (Algeria). The largest
stretch of sand surface is the Rub’ al Kali (Empty Quarter), covering about 650,000
sq km of southern Saudi Arabia, Yemen, Oman, and the United Arab Emirates.
The fact that wind-crafted sand dunes are associated with deserts does not mean
that they are windier places than their moister counterparts. Wind is simply more96 Desert