Dryland Climates

It should come as little surprise that precipitation in the Earth's drylands is, by definition, low. The chart below shows global annual average daily precipitation; both cold and hot deserts show up very clearly, with precipitation averaging less than 1 mm per day.

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.

References
Nicholson, S.E., J. Kim, and J. Hoopingarner, 1988. Atlas of African Rainfall and Its Interannual Variability. Department of Meteorology, Florida State University, Tallahassee, FL.

Definitions
A period of record's standard deviation ¸ by its annual rainfall mean: coefficient of variation
Along lines of longitude: meridional
Along lines of latitude: zonal
[actual vapor pressure ¸ saturation vapor pressure]´100%: relative humidity


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This site last updated April 8, 1998