Definition of Drought
However, even this simple definition is complicated when attempts are made to compare droughts in different regions. For example, a drought in New Jersey would make for wet conditions in the deserts of Arizona. Therefore, drought should be considered relative to some long-term average condition of balance between precipitation and evapotranspiration (i.e., evaporation + transpiration) in a particular area—a condition often perceived as "normal."
Drought is also affected by the timing (i.e., principal season of occurrence, delays in the start of the rainy season, occurrence of rains in relation to principal crop growth stages) and the effectiveness of the rains (i.e., rainfall intensity, number of rainfall events). Likewise, other factors, such as temperature, wind, and relative humidity, can significantly aggravate or mitigate its severity. It is important to note, however, that weather isn't necessarily the sole cause of drought.
Drought can result from human activities that increase demand for water. Furthermore, a single rain event does not necessarily end a drought. It can take months of below-normal precipitation to create a drought, and it often takes one or more substantial rainfalls to make up the water deficit.
Improved Drought Forecasting
Numerous tools are used to forecast drought trends. For the shorter term, meteorologists at NOAA's Climate Prediction Center use (among other tools) predictions from the NWS numerical forecast models to forecast soil moisture two weeks into the future. For the longer term, they examine both statistical and dynamical models. For example, a statistical technique called the "constructed analogue," which combines bits and pieces of historical soil moisture information to create a synthetic analogue to current conditions, is used to forecast conditions several seasons into the future.
Another statistical tool is used which calculates the probabilities of the Palmer Drought Index being within various drought intensity categories for up to four months in advance based on historical weather data. Drought forecasters also look at soil moisture conditions during past El Niño and La Niña events to help forecast soil moisture conditions if these phenomena are active or are predicted to form when the drought forecast is being prepared. Research is also being conducted to relate drought to sea surface temperatures and atmospheric circulation features over various key areas in the Pacific and Atlantic oceans.
Drought forecasters work with the creators of CPC's long-lead seasonal temperature and precipitation outlooks to maintain consistency with those products, while also considering the impact of shorter-term (e.g., next several weeks) precipitation regimes on drought conditions. The CPC issues the U.S. Seasonal Drought Outlook each month (on the second or third Thursday of the month at 3:00 p.m. EST, on the day it releases the long-lead temperature and precipitation outlooks). These outlooks depict general large-scale expectations of average conditions that are not specific to relatively small areas. Short-term events (such as individual storms) cannot be forecast more than a few days in advance, so users are cautioned when using the Drought Outlook for applications, such as agriculture, that can be affected by such events. Initial drought areas—shown schematically—are approximated from the U.S. Drought Monitor.
For weekly updates on drought, users should see the latest Drought Monitor map and text. Users can find additional drought information on CPC's home page and NOAA's Drought page. Users are also encouraged to visit National Weather Service Weather Forecast Office sites, where a key word search for "Drought Statement" will lead you to abundant state and local drought information.
Relevant Web Sites
NOAA's Climate Prediction Center
NOAA's monthly U.S. Drought Outlook
Media Contact:
April 12, 2002 — Drought is a normal, recurrent feature of climate, although many erroneously consider it a rare and random event. It occurs in virtually all climatic zones and no region in North America is immune to periodic droughts—although its characteristics vary significantly from one region to another. However, it can be difficult to recognize the beginning or end of a drought because the definition of drought varies among agricultural, meteorological and hydrological concerns. Despite these differences, NOAA's monthly U.S. Drought Outlook provides valuable information about anticipated drought conditions across the United States. As a natural extension of the weekly U.S. Drought Monitor produced by NOAA, USDA, and the National Drought Mitigation Center, NOAA's Drought Outlook uses historical climate data and state-of-the-art computer model forecasts to help produce the nation's forecast for drought conditions.
Drought can be defined based on rainfall amount over some period of time, vegetation conditions, agricultural productivity, soil moisture, levels in reservoirs and stream flow, or economic impacts. In basic terms, a drought is a significant deficit in moisture availability due to lower than normal rainfall. This deficiency results in a water shortage for some activity, group or environmental sector.
Careful monitoring and forecasting of drought can ease its impacts by allowing people to take appropriate actions prior to onset. Improved remote sensing from satellites and radar—as well as the use of thousands of daily precipitation measurements—have improved the ability to monitor drought. But the most exciting developments in mitigating drought impacts may be advances made in forecasting the conditions that result in drought.
NOAA's Drought page
Carmeyia Gillis, NOAA's Climate Prediction Center, (301) 763-8000 ext. 7163