Dryland Climates
Climatically, this is not the end of the story. Typically, where precipitation is low, its interannual
variability is high. This is well-illustrated by the chart below for North Africa south of
the Mediterranean coast (adapted from Nicholson et al., 1988). Long-term mean
annual rainfall, as shown by the colored bands, declines zonally northward toward the Sahara, and as it does so, rainfall interannual
coefficients of variation increase. This chart also shows rainfall regions, which are data-derived differences in how
and when the rain falls. These regions indicate that both
meridional and
zonal variability in rainfall occur.
Other climatic parameters of the Earth's drylands are distinctly
different from the remainder of the planet. You'll find information on
potential evaporation elsewhere, and on relative
humidity below. Relative humidity depends on the temperature of the air; as the chart below shows, warm air is able to contain very much more water vapor than can
cold air.
As air temperatures rise, then, the air can potentially hold more water vapor. Relative humidity, the measure of how much water vapor is held in the air compared to how much could be held (at a given temperature), is shown in the chart below. Note how the air over the deserts can clearly be seen to be relatively dry.
Definitions
This site last updated April 8, 1998