Introduction

Meteorological phenomena that pose hazards in dryland regions include droughts, floods, snow, wildfires, and dust storms. Drought is a protracted phenomenon; abnormally dry conditions must generally persist for a month or more before a drought situation arises. It results from changes in large-scale circulation patterns, usually quasi-continental or greater in spatial extent. Floods and dust storms are usually coupled to individual weather events, although floods may also result from long periods of excessive rainfall. Flood events are generally linked to smaller, mesoscale meteorological patterns, although their occurrence may be made more likely by conducive, large-scale atmospheric conditions. Drought is often a prelude to major dust storms in semi-arid regions, since the aridity promotes the mobilization of dust. Wildfires are also associated with prolonged drought. Snow is a relatively rare phenomenon in the warm drylands and is associated with very unusual weather conditions. Because it occurs infrequently, the inhabitants of these regions are generally not equipped to cope with it.

All of these hazards have major physical and economic consequences. Some of the impacts are qualitatively the same for several of these hazards. Drought reduces the surface vegetation cover and leaves the soil prone to erosion by both wind and water; crops fail and herds die off. Floods, fires, and dust storms similarly destroy the vegetation and remove or destroy fertile soil; agricultural and economic consequences can be comparable to those due to drought. The impact of floods can be exacerbated by their frequent occurrence during what is normally the dry season, a time when vegetation cover is reduced and the ground is more prone to runoff and erosion.