U.S. Drought Monitor Indicators
Using Precipitation Indices to Determine Severity, Long-term Impact
Although drought is difficult to define or forecast, climatologists can determine the impact and intensity of droughts using drought indicators and precipitation indices.
Droughts As Natural Hazards
Droughts, although less apparent than other natural hazards, result in billions of dollars of damage and destruction in the United States. Although the immediate affects may not be as drastic or spectacular as other weather-related disasters such as tornadoes or hurricanes, droughts affect more people altogether than any other natural hazard, according to the National Climate Data Center (NCDC). No region in North America is unsusceptible to droughts. El Nino, specifically La Nina has been known to cause extensive drought conditions.
Drought By Definition
The definition of drought is not a clear and simple one. Drought is not determined by the number of dry days but by the amount of moisture before and after a dry period. It is dependent on specific conditions and the region. A drought in the desert is not the same as a drought in a rain forest. The National Climate Data Center defines drought as "a significant deficit in moisture availability due to lower than normal rainfall". A drought is defined by rainfall, crop productivity, soil moisture, reservoir levels and other factors. However, since rainfall records have only been kept in the last century, it is difficult to determine the severity or to compare droughts over long periods of time. Paleoclimatology is used to study the past climate, including analyzing tree rings, sediment, archaeology and historical documents in order to determine drought severity and prediction.
Drought Indicators
Drought are monitored by the National Climate Data Center, which contains the largest active archive of weather data in America. The U.S. Drought Monitor provides updates and predictions of drought impact on agricultural and hydrological areas. Drought are monitored by
- Dryness Categories
- Drought Intensity
- Drought or Dryness Types
Climatologists use two main indices to determine drought severity and impact.
- Palmer Drought Severity Index (PDSI) - measures drought intensity and duration by measuring precipitation, air temperature and local soil moisture. The index uses a scale to determine severity, ranging from - 4.0 for extreme drought to +4.0 meaning extremely moist conditions.
- Standard Precipitation Index (SPI) - This index relies on precipitation amounts alone. It can determine short-term and long-term applications. The index displays data over 1, 3, 6, 9, and 12 month durations.
1930s Dust Bowl Drought
The Dust Bowl of the 1930s is considered to be the major drought of the 20th century. This devastating drought lasted seven years, happening in three waves: 1934, 1936 and 1939-40. The Dust Bowl drought was so severe and widespread, covering over half of the United States, that it is considered a natural disaster. The sustained drought conditions left the soil depleted of moisture. The soil lifted into the air by the wind, formed huge dust clouds, called "black blizzards". These dust clouds killed agriculture, resulting in a great migration of the region's population from the Great Plains to the West Coast.
