Rainfall in arid and semi-arid regions

Abstract

Arid and semi-arid regions are associated with dry climates. Dry climates imply little rainfall and strong evaporative conditions, frequently leading to serious water deficits and drought conditions. The boundaries of such regions have been identified through numerous climate classifications. Meigs (1953) prepared maps for UNESCO showing the distribution of arid and semi-arid homoclimates on the different continents. The basis for Meigs' division was the system developed by Thornthwaite (1948), who used monthly precipitation values and evapotranspiration estimates to calculate a moisture index, which provides a measure of the adequacy of precipitation in relation to the needs of plants. Meigs (1953) noted that the world's arid and semi-arid regions are seen to occur in five great provinces. Each was described as 'a core of desert, partly surrounded by semi-arid lands bordering the west coasts of the continents, chiefly from 15 to 35 latitude, and extending inland and poleward as far as 55 latitude' (Slatyer & Mabbutt 1964).