Millions of people all around the world are affected by drought every year, often with devastating consequences.
Drought is the condition of abnormally dry weather in a region where rain is usually expected. This scarcity of rain causes a serious imbalance in the hydrological system which, for example, leads to water-supply reservoirs and wells drying up which in turn leads to crop damage. Drought severity is measured by its duration, the degree of moisture deficiency and the size of the affected area. Droughts can last from a few weeks (partial drought) to many years.
Some areas tend to be more severely affected by droughts than others. Areas bordering permanently arid regions of the world, at latitudes of about 15-20 degrees suffer catastrophic droughts. This happens because in permanently arid areas warm, tropical air masses become hotter and drier as they reach the earth. When the prevailing westerlies experience a poleward shift in direction, the high-pressure, anti-cyclonic conditions of the permanently arid regions affects areas that normally experience seasonally wet low-pressure weather, a drought follows. The most severe drought of the 20th century, in the African Sahel region which lasted for 12 years was caused by a southward shift in the westerlies. Such droughts can be aggravated by nonclimatic pressures like overcropping, overpopulation, the lack of timely relief measures, and poor internal and international relations.
Drought cannot be reliably predicted, however precautionary measures can be taken such as the building of dams and reservoirs, the studying of drought cycles and education to prevent the overgrazing, overcropping and overpopulating of drought-risk areas.