Present-day desert environments - Climatic Change in Deserts - page 14

2.2 Causes of present-day aridity
There are four main reasons why deserts are lacking in rain (Mabbutt, 1977 ; Cooke
et al., 1993 ; Thomas, 1997 ; Laity, 2008 ; Parsons and Abrahams, 2009 ; Nicholson,
2011 ). The two most important factors are location in a latitude dominated by dry
subsiding air and location inland far from sources of moist maritime air. The remaining
two factors are location in the rain shadow of high mountain ranges and location on
a coast flanked by cold ocean currents or cold upwelling ocean water.
The first factor is a direct product of the global atmospheric circulation system,
which is determined by solar radiation modulated by latitude, the distribution of land
and sea, and the topography of the land. Solar radiation is greatest at the equator
because the sun is most directly overhead there for much of the year. Away from the
equator, progressively more incoming radiation is reflected or absorbed by the earth's
atmosphere, because the sun's rays travel ever more obliquely through the atmosphere
as a result of the curvature of the earth. Because of the tilt of the earth's axis, the sun
is directly over each of the tropics once a year - at the summer solstice. If the axial tilt
were greater, the sun would appear to travel further from the equator during summer,
and the converse would apply if the tilt were less.
The tropical anticyclonic deserts, such as the Sahara, are a direct result of the
atmospheric circulation cells (often termed Hadley cells ) located between the equator
and the tropics ( Figure 2.1 ), so their location is determined by latitude rather than by
the regional distribution of land and sea. For a full discussion, seeWebster ( 2004 ). The
two polar deserts also come under the influence of semi-permanent anticyclones and
of cold, dry subsiding air. Because the distribution of high pressure cells (anticyclones)
is closely related to latitude, the oceans in both polar and strictly tropical latitudes
receive very little precipitation and are the arid marine counterparts of the continental
deserts.
In equatorial latitudes, incoming short wave solar radiation heats both land and
sea throughout the year. The air above both land and sea is constantly warmed by
convection and outgoing long-wave terrestrial radiation. As the air becomes warmer,
it moves upwards by convection, expands and eventually cools adiabatically ,thatis,
without heat entering or leaving the air mass. The moist adiabatic lapse rate is about
5 C cooler after an ascent of 2 km.

Given that warm air can store more water vapour than an equivalent volume of cold air,
the rising air soon reaches dew point, that is, it becomes saturated with respect to water
vapour and excess water vapour condenses to form clouds. Cumulonimbus clouds can
attain a thickness of 3-5 km, which means that during the rainy season equatorial
regions are constantly cloudy, in contrast to the deserts, where there is little or no
respite from the sun. Convectional uplift induces further cooling, leading to additional
condensation of water droplets that eventually coalesce into larger drops and fall as

° C/km (3 ° C/1,000 feet), so the air mass will be 10 °