There are many concepts of what a desert is. However, if we omit polar deserts, the major role is played by available moisture—that is, moisture that is potentially available for biological use. This is not determinable solely by the amount of precipitation. Evapotranspiration (evaporation of water into the atmosphere and transpiration, which is the loss of water through the leaves and other organs of plants) returns moisture to the air, rendering it unavailable to organisms. For this reason, definitions of deserts usually include (directly or indirectly) some measure of precipitation versus evaporation. One definition of a desert climate is where evaporation exceeds precipitation.
Under this definition, several desert characteristics emerge, often including a lack of permanent lakes and waterways that are ephemeral except close to their source, resulting in closed basins. Exceptions to these characters generally result from special conditions, such as through-flowing rivers that receive most of their water from highland areas or from outside the region. The Rio Grande is an example of such a river.
Other common characters are reductions in relative humidity and cloud cover, both of which tend to further increase evaporation and contribute to large temperature fluctuations.
The simplest measure of climatic dryness is merely a ratio of precipitation to evaporation. However, seasonality of precipitation also is of importance. The Chihuahuan Desert is relatively well watered (relatively mesic), but some 60 to 80% of the precipitation falls during the summer months when evaporative losses are greatest, with only about 3 to 20% falling as winter precipitation Morafka (1977a). Morafka also notes that Tamayo (1962) maps the limits of the Chihuahuan Desert as coinciding with the area of the Mexican Plateau having an average annual relative humidity of less than 50% and with cloudy days averaging less than 60 per year.
Why the aridity? Several factors contribute. For one, the Chihuahuan Desert lies at the latitudes where air converging from the south and north sinks. As the air sinks, it becomes compressed, which heats it and thus decreases its relative humidity.
A second factor is its position between the highlands of the Sierra Madre Occidental and the Sierra Madre Oriental. Moisture-bearing winds originating over oceanic waters are forced by these high mountain ranges to rise. As air rises, it expands and cools. The cooling forces the air to drop much of its load of moisture on the windward slopes of the highlands. An additional factor here is that condensation of moisture releases heat, with the result that as the air sinks and becomes compressed on the desert side of the mountains, it is not only drier, but also hotter than it started out.
A third factor lies in the distance of the desert from sources of moisture. All else being equal, areas deeper within a continent tend to receive less moisture merely because there is a greater chance that the air will have dropped much of its moisture closer to the coast.
Last Update: 20 May 2006.