The arid regions

Abstract

The arid regions of the world occupy 26--35{\%} of the Earth's land surface. This wide range reflects various definitions based on climatic conditions, vegetation types, or the potential for food production. Much of this land lies between 15{\textdegree} and 30{\textdegree} latitude (Figure 4.1). Here the air which is carried aloft along the intertropical convergence zone subsides to form the semipermanent high pressure cells that dominate the climate of the tropical deserts. Adiabatic warming of the air as it descends, coupled with intense radiation under cloudless skies, results in oppressive heat during the high sun season. The tropical deserts are bordered at lower latitudes by semi-arid grasslands which eventually merge with the tropical savannas as summer precipitation increases. Conversely, winter precipitation in the deserts increases towards higher latitudes, and in coastal regions the evergreen trees and shrubs of the Mediterranean climatic regions become more conspicuous. In Asia the tropical deserts form the southern border of the extensive temperate deserts of the continental interior.