What is Drought?

What's a drought? Most of us think of it as “no rainfall", but it's not that simple. Drought is when you have less rainfall than you expected over an extended period of time, usually several months or longer. Drought is a normal part of climate, and it can occur almost anywhere on earth. For example, Arizona and Maine have very different climates (see figures), but drought occurs in both states. Drought's features and effects vary from place to place because of different geographical features and cultures, which affect how people use water.

A shortage of rainfall can result in major impacts on agriculture, city water supplies, tourism and recreation, energy (power) production, river navigation, and the environment. (See How Drought Affects Us for more information.)

If you are a farmer, drought means that you do not have enough water in the soil for crops to grow normally or for pastures to produce enough grass for livestock. For farmers who rely on irrigation to produce their crops, drought may be a shortage of water in reservoirs, streams, or groundwater, and irrigation may be restricted. If you live in a city, drought may result in a shortage of water for watering grass, trees, and other plants. Often during drought, people in cities are asked to conserve water used inside the home and outside. Has your town or city experienced drought recently and have residents been asked to conserve water?

In the United States, although drought can cause serious economic impacts, it usually does not result in food shortages. For example, a drought may affect the production of vegetables or fruits, which may result in an increase in prices for these foods. In some countries, however, drought may result in severe food shortages that lead to serious problems of malnutrition and famine. Providing food to these affected areas is a major activity of international relief organizations, like the Red Cross.

There are many definitions of drought because its characteristics and impacts differ from one location to another. A general definition that we use at the National Drought Mitigation Center is "a deficiency of rainfall over a period of time, resulting in a water shortage for some activity, group, or environmental sector." See the National Drought Mitigation Center's overview of drought and (for more in-depth information) definitions of drought.

Another way to think of drought is in terms of supply and demand. The water that we receive from rainfall is the supply that we have available. We often store water in reservoirs and lakes. We also sometimes have water available in underground aquifers. We use water for many activities. During drought, the amount of available water is reduced. So, we must reduce our use or demand for water so that there is enough available to meet our basic needs until rainfall amounts return to normal.

What Causes Drought?
Drought has many causes. It could be a lack of rainfall; it could be a lack of snowfall from mountains far away; or it could be caused when water supplies aren't sufficient to meet everybody's needs.

To understand why we don't always get the rainfall or snowfall we need or expect, we first need to understand weather and climate. Winds cause weather patterns to move around the globe, including the clouds that bring rain. Over the years, these patterns become routine, creating what we know as our climate. But sometimes these patterns change, and we get less (or more) rainfall than we are expecting. In temperate locations, like the United States, the main winds are called the jet stream, which moves around the atmosphere in a pattern of ridges and troughs. (For more information, see The Science of Jet Streams.) The jet stream's behavior changes with each of the four seasons, and in general you can count on a regular pattern in the jet stream, unique in the spring, summer, fall, and winter, that behaves in the same manner. But sometimes these jet stream patterns change and cause unusual weather, with some areas getting less (or more) precipitation than they are expecting. If an area has a jet stream pattern that has large ridges and/or troughs that remain in place for a long time, then that area will experience drought, because the ridges will block air masses and fronts that bring rain. On the other hand, if the jet stream has no ridges or troughs for a long period of time, then the area will have a wet spell. No one is sure why these patterns change, but it's thought that conditions such as snow and ice cover, vegetation (plants), soil moisture, and sea surface temperatures have something to do with it. Conditions like these can cause or force changes in local or regional wind flow patterns, and these changes in turn can affect wind patterns over other parts of the earth. This kind of long-distance relationship between winds is known as a teleconnection. El Niño and La Niña, two weather phenomena that involve changes in ocean currents, are examples of teleconnections. (For an explanation of the El Niño and La Niña phenomena, see What is an El Niño? and What is La Niña?)

In general, clouds form when the wind forces air to move upward; clouds disappear when the wind forces air to move downward. But sometimes a ridge of high atmospheric pressure will build over a region (like the southeastern United States during the summer). In these high pressure regions, air moves downward, preventing clouds from forming and producing rain. If these ridges stay in place for an abnormally long time, this can also result in a drought. For example, in 1993, a ridge of high pressure became established over the southeastern United States, causing severe drought that cost $1.3 billion in damage.

People can also play a big role in drought. All living things, including humans, require a certain amount of water to live. But humans also have developed lifestyles that require additional water. When drought occurs, this additional water can make the difference between having enough water and running out.

Can We Predict When Drought Will Occur?
Predicting drought depends on our ability to forecast precipitation and temperature. Scientists don't know how to predict drought a month or more in advance for most parts of the world.

Scientists are studying how interacting weather events, or teleconnections, can influence the formation of various regional and global weather patterns. Because these patterns tend to be repeated, studying their occurrence can help us improve our ability to predict changes in climate, particularly in the tropics. One teleconnection that leads to certain predictable weather patterns is the El Niño/Southern Oscillation (ENSO) and the associated El Niños and La Niñas.

We may soon be able to improve our ability to predict drought in some areas of the world. In tropical areas, for example, meteorologists have conducted research that has given them a better understanding of the climate system. For example, we now know that many of the long-lasting changes or variations in global weather patterns are associated with variations in tropical sea surface temperatures. The results of one study, the Tropical Ocean Global Atmosphere (TOGA) project, suggest that when an ENSO event occurs, we may now be able to predict certain climatic conditions more than a year in advance by studying the ENSO's characteristics and effects. Many tropical regions are greatly influenced by ENSO events, and for these areas, more reliable long-term forecasts would help people plan ahead to reduce risks. Farmers and governments, for example, could plan for the possibility of drought by making different planting decisions or making arrangements for additional water supplies.

In regions outside of the tropics, the relationship between precipitation and ENSO events is not well understood, and meteorologists do not believe that we can make reliable forecasts for these regions a season or more in advance.

Climatology
Weather is the condition of the earth's atmosphere over a brief period of time. For example, we speak of today's weather or the weather this week. Day-to-day weather over a longer period of time is climate. Climographs can help us understand the climate of various cities or regions. A climograph is a graph that shows monthly average temperature and precipitation for some location.

Climate influences our daily lives in many ways. For example:

• People in Minneapolis–St. Paul expect a white Christmas, and people in New Orleans expect very warm, humid summers. And a traveler going to Orlando, Florida, in March will not pack the same kind of clothing as a traveler going to Vail, Colorado, in March.
• Our houses are designed for the climate in which we live.
• Farmers make plans for planting crops based on the length of the growing season (from the last killing freeze in the spring to the first freeze in the fall).
• Utility companies need to understand the climate for their area so that they will have enough power for their customers' winter heating and summer cooling needs.

A climatologist studies and tries to explain the impacts of climate so that society will be able plan its activities, design its buildings and infrastructure, and prepare for bad weather conditions. Although climate is not weather, it is defined by the same terms, such as temperature, precipitation, wind, and solar radiation.

When you hear or read a weather forecast, you may notice the word normal, describing either precipitation or temperature. Climate is usually defined by the weather conditions that people expect or consider "normal". Climatologists calculate "normal" temperature and precipitation values by taking temperature and precipitation readings for a 30-year period and averaging them for a variety of time periods (for example, daily and monthly). But we need to remember that normals can be made up of a wide variety of temperature and rainfall values, so "normal" actually may not be all that common! For example, many people consider warm, sunny days normal in southern California. History (and climatology) tell us that this is not the full story. Although sunny weather is considered common in southern California, severe floods and droughts also occur there, so it would be more accurate to say that precipitation in southern California varies widely, and that rain is most likely to occur between October and April.