Precipitation and aridity

The key elements necessary for the formation of rainfall are moist, unstable air and ascent, which is often produced by a convergent pattern of airflow. Such convergence is associated with surface low-pressure systems. The factors that are classically considered to promote aridity are the converse: a lack of atmospheric moisture, stable air, subsidence, and divergent airflow. A fifth factor that must be considered is an absence of rain-bearing disturbances.

The first factor, a lack of sufficient moisture, is generally not the overriding influence in most arid climates. The case of the Sahara Desert clearly demonstrates this. Maps of precipitable water show that in summer the atmosphere above the Sahara contains about as much moisture as that over the wet southeastern USA. Likewise, the Namib Desert is a humid environment, although it is one of the “driest” deserts on earth in terms of rainfall. With the exception of the polar deserts, where a lack of available moisture plays a major role in the origin of the dry climate, an absence of moist air masses is a major factor in aridity only in the deep continental interior of Asia. Even there, however, the lee rain shadow effects of high terrain also play a major role.