From AMS Glossary
Aridity is a climatological condition in which the amount of precipitation received (supply) is exceeded, on average, by potential evapotranspiration (demand).
A number of physically based indices have been proposed to describe this deficiency that relate precipitation to temperature and humidity. For practical purposes, in the temperate and tropical zones, semiarid climates generally receive between 200 and 500 mm of precipitation per year on average, and arid climates receive less than 200 mm. However, definitions based on mean precipitation are not always satisfactory, because they do not express variability or the likelihood of drought. Rainfall in arid climates is extremely variable, with coefficients of variation for arid climates exceeding 50%, and coefficients for semiarid climates ranging between 30% and 50%. In broad economic terms, variability restricts the potential use of these lands. Arid climates are unsuitable for growing crops using rainfall alone, because crops may fail three or more years out of ten. Semiarid climates often support grasses that are suitable for grazing animals. Thus, livestock raising is often more appropriate, less risky, and more common than rainfed agriculture in semiarid areas.