<a href=http://en.wikipedia.org/wiki/Aridity_index>http://en.wikipedia.org/wiki/Aridity_index</a>;

An aridity index (AI) is a numerical indicator of the degree of dryness of the climate at a given location. A number of aridity indices have been proposed (see below); these indicators serve to identify, locate or delimit regions that suffer from a deficit of available water, a condition that can severely affect the effective use of the land for such activities as agriculture or stock-farming.