What Causes Drought?
QUICK ANSWER
Droughts are caused by extended periods of below-average precipitation, often lasting months or years. The underlying causes include shifts in atmospheric pressure patterns, changes in ocean temperatures (like La Niña), warming temperatures that increase evaporation, and natural variability in regional climate patterns. Multiple factors usually combine to cause major droughts.
Droughts are caused by persistent shortfalls in precipitation, but the underlying meteorological causes are complex and varied. Some droughts develop quickly during a single dry season; others develop slowly over years. Understanding what causes droughts helps explain why some regions face frequent dry conditions and how natural climate cycles produce drought patterns even without human influence.
What is a drought?
A drought is an extended period of below-average precipitation that results in water shortages affecting people, agriculture, ecosystems, or the economy. Droughts are defined relative to normal conditions in a region, so what counts as drought in a wet area might be normal in a desert. Severity is typically measured by duration, geographic extent, and impact. The Palmer Drought Severity Index and the US Drought Monitor classify droughts on multi-level scales from abnormally dry to exceptional drought. Long-term droughts can last years; flash droughts develop in weeks.
What atmospheric patterns cause droughts?
Several atmospheric patterns produce droughts. Persistent high-pressure systems blocking storm tracks divert precipitation around affected regions. Shifts in the polar jet stream can leave certain areas without their normal precipitation sources. Changes in atmospheric circulation alter where moisture flows from oceans. The position of subtropical high-pressure belts affects where precipitation reliably reaches. When these patterns persist for months or years, they produce drought. Some droughts occur during otherwise normal periods when one specific atmospheric feature shifts; others result from multiple aligned patterns.
How does climate variability contribute?
Natural climate variability significantly affects drought patterns. The El Niño-Southern Oscillation (ENSO) cycle is the most important: La Niña phases typically cause drought in the southwestern US and parts of South America, while bringing wet conditions to Australia and Indonesia. The Pacific Decadal Oscillation affects long-term western US precipitation. The Atlantic Multidecadal Oscillation influences hurricane activity and Sahel rainfall. These natural cycles have been operating for thousands of years and produce drought patterns even before considering human-caused climate change.
What types of droughts exist?
Droughts are classified into several types. Meteorological drought is defined by precipitation deficit alone. Agricultural drought involves soil moisture deficits affecting crops. Hydrological drought affects rivers, lakes, and groundwater supplies. Socioeconomic drought occurs when water supply systems can't meet demand. These overlap but develop at different rates: meteorological drought begins first when rain stops; agricultural drought follows within weeks; hydrological drought may take months to develop. Snow drought (low snowpack) is increasingly recognized as a distinct type affecting water supply later when expected meltwater doesn't materialize.
What is a drought?
A drought is an extended period of below-average precipitation that results in water shortages affecting people, agriculture, ecosystems, or the economy. Droughts are defined relative to normal conditions in a region, so what counts as drought in a wet area might be normal in a desert. Severity is typically measured by duration, geographic extent, and impact. The Palmer Drought Severity Index and the US Drought Monitor classify droughts on multi-level scales from abnormally dry to exceptional drought. Long-term droughts can last years; flash droughts develop in weeks.
What atmospheric patterns cause droughts?
Several atmospheric patterns produce droughts. Persistent high-pressure systems blocking storm tracks divert precipitation around affected regions. Shifts in the polar jet stream can leave certain areas without their normal precipitation sources. Changes in atmospheric circulation alter where moisture flows from oceans. The position of subtropical high-pressure belts affects where precipitation reliably reaches. When these patterns persist for months or years, they produce drought. Some droughts occur during otherwise normal periods when one specific atmospheric feature shifts; others result from multiple aligned patterns.
How does climate variability contribute?
Natural climate variability significantly affects drought patterns. The El Niño-Southern Oscillation (ENSO) cycle is the most important: La Niña phases typically cause drought in the southwestern US and parts of South America, while bringing wet conditions to Australia and Indonesia. The Pacific Decadal Oscillation affects long-term western US precipitation. The Atlantic Multidecadal Oscillation influences hurricane activity and Sahel rainfall. These natural cycles have been operating for thousands of years and produce drought patterns even before considering human-caused climate change.
What types of droughts exist?
Droughts are classified into several types. Meteorological drought is defined by precipitation deficit alone. Agricultural drought involves soil moisture deficits affecting crops. Hydrological drought affects rivers, lakes, and groundwater supplies. Socioeconomic drought occurs when water supply systems can't meet demand. These overlap but develop at different rates: meteorological drought begins first when rain stops; agricultural drought follows within weeks; hydrological drought may take months to develop. Snow drought (low snowpack) is increasingly recognized as a distinct type affecting water supply later when expected meltwater doesn't materialize.
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