What Causes Drought?

Brian Barth
Drought conditions

Drought is increasingly on the minds of Americans and concerned citizens everywhere. As of summer 2015, the western United States continues to suffer through the worst drought in recorded history, and climate change is producing more extreme weather variations in many other regions. Understanding what actually causes a lack of precipitation helps shape public policies and consumer choices that have the potential to exacerbate or reduce the effects of drought.

Physical Causes of Drought

The NASA Earth Observatory lists three factors that cause drought. These are interrelated, naturallyoccurring phenomena that determine the location, severity and frequency of droughts.

Temperature of Oceans and Land

Cumulus cloud formation
Temperature affects cloud formation.

Precipitation is the result of the natural process in which:

  • Water evaporates from the earth's surface whether from bodies of water or from land.
  • The moisture then condenses in the atmosphere.
  • Finally, the moisture becomes concentrated and falls to the earth once more.

The process is driven by the heat of the sun; the hotter it is, the greater the rate of evaporation. Thus, if the temperature of the ocean or the surface of the land is relatively cool in a certain area, drought may occur in regions that rely on those sources of moisture. For example, cold temperatures in the Pacific Ocean near the equator are usually correlated with low rainfall in the western and central US.

Air Circulation Patterns in the Atmosphere

Weather pattern
Drought results from changing weather patterns.

Large scale weather patterns, including the distribution of rainfall, are largely driven by the patterns of air circulation in the atmosphere. As hot air rises and expands, it creates a contrasting flow of air from cooler areas where air condenses and sinks. This gives rise to air currents that move moisture around the atmosphere and result in different patterns of rainfall in different regions.

When there is an anomaly in surface temperatures, typical patterns of air circulation change, which means precipitation patterns also change. This leads to higher-than-average rainfall in some areas and drought in others. El Nino is a prime example of a major fluctuation in air currents, which is often associated with drought in locations such as Australia, India, Brazil and Hawaii.

Quantity of Moisture in the Soil

Parched earth
Hot, dry soil leads to drought.

Soil moisture influences cloud formation, or the lack of it, at a more local level. When the soil is moist, surface air temperatures stay cooler because more of the sun's energy is absorbed in the process of evaporation. If the ground is dry, there is no local source for the moisture that causes clouds to form. This leads to hotter surface temperatures which make the soil even drier. The cycle builds on itself and results in long term droughts.

The Human Connection

In addition to the meteorological factors that cause drought, human activity can also be a cause. How much water humans consume, and the timing of that consumption, factors into how much water is available at a later date for people, plants and animals. Thus, drought can largely be viewed as an imbalance between supply and demand.

Hydrological Drought

Large dam
Low water levels in an Arizona reservoir.

As opposed to meteorological drought, hydrological drought is caused by a greater demand for water in a given region than is available. Sources for water may include natural lakes and rivers, manmade reservoirs and groundwater. Demand for water comes from municipalities that supply tap water, water for agricultural and water for industrial use such as hydroelectric plants.

Wildlife and aquatic organisms also depend on certain water levels in lakes and rivers to survive, and vegetation depends on certain levels in the water table. A drought occurs when all of these combined 'demands' become greater than the available water supply for an extended period.

Timing

Melting snow
A lack of snow can lead to drought.

The timing of precipitation and water demand has a lot to do with when a drought occurs. Even if the overall water supply is low, drought is usually less of a concern in the winter months because demand is far less than in the summer.

Agricultural droughts tend to occur when there's not enough water in springtime to help establish seedlings and ensure the success of the crops. When the distribution of precipitation occurs more in the summer than in the winter, a lot of the water is quickly lost to evaporation and runoff, rather than being stored as snowpack. This causes drought conditions later on when people or natural systems are accustomed to having water available from snowmelt.

Climate Change and Drought

Climate change demonstration
People can protest climate change.

As human-caused climate change is increasingly accepted as a scientific fact, its effects on drought are being studied in detail. As the meteorological mechanisms that cause drought make clear:

  • Abnormal fluctuations in temperature correspond to abnormal variations in precipitation.
  • Increases in temperature have the potential to cause more frequent and severe droughts, which puts climate change in the crosshairs as a major cause of drought in the present day.

According to the World Resources Institute, there is conclusive scientific evidence that global warming makes the planet hotter on average, which makes heat waves more intense and droughts more severe. In 2012, about two-thirds of the continental United States suffered from drought, and the frequency and severity of droughts is expected to increase across the country in during the next few decades.

Guarding Against Dry Times

The causes of drought are complex, interrelated and, increasingly, manmade. However, there are many ways to conserve water, which is becoming more and more of a priority in regions that are afflicted by drought. In fact, people everywhere should consider practicing water conservation as a preventative measure against future droughts.

What Causes Drought?