Deserts occur in specific latitudes ( 5-35° north
and south of the equator) because of the general
thermodynamics of our planet. Solar radiation
hits the earth with highest intensity near the
equator. Because the earth's axis is tilted 3.5°
with respect to the plane of its orbit, during part of
the year the zone of maximum solar interception
shifts northwards, towards the Tropic of Cancer,
and during part of the year it moves southwards,
towards the Tropic of Capricorn. Thus, the warm
tropics form a belt around the equator from
latitude 3° north to latitude 3° south, where the
tropical heat generates rising, unstable air.
climbs, the air condenses the moisture evaporated
from the warm tropical seas and forests, and
produces the heavy downpours that characterize
the wet tropics. As it moves away from the
equator at high altitudes, the air cools again and
eventually starts descending towards the midlatitudes,
some 3000 km away from the equator
both north and south. The air masses heat in their
descent and, having lost their moisture during
their tropical ascent, they become extremely dry.
Thus, by contrast with the equatorial forests, the
mid-latitude arid fringes that run alongside the
tropical belt have a more stable atmosphere.
These are the "horse" latitudes, where calm, dry
air often dominates.
Additionally, because of the
stable atmosphere, not only are winds slack, but
rainstorms seldom develop. For this reason most
of the world's large deserts occur along the belt
that separates the tropics from the temperate
regions (Goudie and Wilkinson 1977).