Drought is an extended period of time in which no precipitation falls. In sunny weather with wind and high temperatures, a lot of moisture can evaporate, which quickly increases the water shortage. The history is also important: if it was also dry earlier in the year, the shortage will increase. Agriculture supplements this in many places through artificial irrigation, which affects the availability of water. When the demand for water exceeds the natural supply, water scarcity occurs.
Prolonged drought can have major consequences and has left its mark on history. The decline of the Akkadian Empire is one of the earliest examples of a known civilization. Drought may have also played a role in the collapse of the Maya Empire, the decline of the Pueblo, Fremont, and Cahokia cultures during the megadroughts of western North America, and the decline of Angkor.
With a society growing in complexity, drought can also be disruptive in modern times. Agriculture, with its high water requirement, is an important sector that is severely damaged by drought, such as an annual average of between 6 and 8 billion dollars in the United States, but for example rising to 40 billion in direct and indirect costs in 1988. In Europe, it was estimated in 2007 that damage over the previous thirty years was about EUR 100 billion, involving an average of 11% of the population and 17% of the surface. An outlier was 2003, when the damage amounted to around 11 billion euros. In China, damage from the 2001 drought was estimated at $6.4 billion.
The great importance of water can also lead to water conflicts. A recent example is the conflict in Darfur, where drought contributed to the Baggara moving their cattle further south in search of water.
The World Meteorological Organization (WMO) distinguishes meteorological drought, hydrological drought and agricultural drought. Meteorological drought is a prolonged decrease in precipitation from normal. Hydrological drought is when it has an effect on watercourses such as rivers and streams. Agricultural drought occurs when agriculture is seriously disadvantaged by the lack of precipitation.
In addition to these deviations from average levels, an aridity index also distinguishes between areas with a dry climate. The desert climate and the steppe climate are examples of this, B climates in the Köppen climate classification. Other climates may have dry seasons.
Many areas with seasons have drier periods. Especially in the tropics and subtropics this is a pronounced phenomenon with a dry season in which there is hardly any precipitation. In the trade winds, this alternates with the monsoon.
High pressure areas are accompanied by drier weather. At higher latitudes, these can block low pressure areas with precipitation. These blockages are more common in the spring and on the eastern side of the oceans, such as Alaska and Scotland and Norway in the Northern Hemisphere.
Orography can promote drought, because air currents across a ridge on the windward side can cause orographic precipitation, while the leeward side is in the rain shadow of the mountains.
During El Niño, the west coast of South America experiences abundant precipitation. On the other side of the Pacific, El Niño leads to a period of exceptional drought in India, Indonesia and Australia. El Niño is a phase in the El Niño Southern Oscillation (ENSO). This is a phenomenon that affects the air currents over the Pacific Ocean. It is associated with drought because it causes the air to be pushed downwards, thus holding back precipitation.
Worst droughts in the world
2006-2010: drought in Syria, possibly also