Drought is characterized by various combinations of unusually low precipitation, low humidity, high temperatures, and high wind velocities. Extended droughts can reduce water supplies to the point where they are inadequate to support the demands of plants, animals, and humans.

Drought is a temporary condition that occurs in moist climates. This is in contrast to the conditions of normally arid regions, such as deserts, that usually experience low average rainfall or available water. Under both drought and arid conditions, individual plants and animals may die but the populations to which they belong survive. Both drought and aridity differ from desiccation, which is a prolonged period of intensifying drought in which entire populations become extinct. In Africa and Australia, periods of desiccation have lasted two to three decades. The loss of crops and cattle in these areas caused widespread suffering. Extensive desiccation may lead to desertification in which most plant and animal life in a region is lost permanently, and an arid desert is created.

Unlike a storm or a flood, there is no specific time or event that marks the beginning or end of a drought. Droughts are slow moving and have negative impacts on plants and animals. Droughts can threaten endangered species and lead to withered crops and wildfires. Hydrologists evaluate the frequency and severity of droughts based on measurements of river basins and other water bodies. Climatologists and meteorologists follow the effects of ocean winds and volcanoes on weather patterns that can cause droughts. Agriculturalists measure a drought's effects on plant growth. They may notice the onset of a drought long before hydrologists are able to record drops in underground water table levels. By observing weather cycles, meteorologists may be able to predict the occurrence of future droughts.

The severity of a drought is measured by both its duration and the ability of the living things in the affected vicinity to tolerate the dry conditions. In 2017, extreme drought affected 3 percent of the world's total land area, a figure that was only recorded 3 times before, in 1985, 1986, and in 2016. In 2019, Oxfam International reported that over 52 million people in 18 African nations were experiencing a hunger crisis due to extreme drought combined with conflict and poverty. Although a drought may end abruptly with the return of adequate rainfall, the effects of a drought on the landscape and its inhabitants may last for years.

Many factors affect the severity of a drought. Plants and animals are vulnerable to drought when stored water cannot replace the amount of moisture lost to evapotranspiration. Some plants and animals have mechanisms that enable them to tolerate drought conditions. Many desert annuals escape drought simply by having a short life span. The rest of the time, they survive as desiccation-resistant seeds. Some plants, such as cacti, evade drought by storing water in their tissues, while others, like Page 1418  |  Top of Articlemesquite trees, become dormant. Still others, such as the creosote bush, have evolved adaptations, such as reduced leaf size and a waxy coating over the leaves that protects against water loss. Many animals that live in areas prone to drought have developed drought survival techniques. Snakes and lizards forage and hunt at night, avoiding the drying effects of the Sun's rays. Other animals have adaptations that allow them to survive without drinking, obtaining all of the water they need from their food sources.

Studies of tree rings in the United States have documented droughts occurring as early as 1220. The thickness of annual growth rings of some tree species, such as red cedar and yellow pine, indicates the wetness of each season. The longest drought identified by this method began in 1276 and lasted 38 years. The tree ring method identified 21 droughts lasting five or more years during the period from 1210 to 1958.

The best-known American drought was the Dust Bowl on the Great Plains from 1931 to 1936, with 1934 and 1936 being the two driest years in the recorded history of the United States. The Dust Bowl encompassed an area approximately 399 mi (644 km) long and 298 mi (483 km) wide in parts of Colorado, New Mexico, Kansas, Texas, and Oklahoma. The United States experienced severe to extreme drought in over half of the country during 1987 to 1989. This drought was the subject of national headlines when it created conditions conducive to extensive fires in Yellowstone National Park during 1988. In 2011, parts of Texas and the American Southwest experienced their most intense droughts since the Dust Bowl and the 1950s extreme droughts. Australia experienced a period of crippling drought in many regions from 1995 to 2009. The event became known as the Millennium Drought because it spanned the turn of the twenty-first century.

Droughts have also had impacts in other regions of the world. A drought in northern China in 1876 dried up crops in an extensive region. Millions of people died of starvation. Russia experienced severe droughts in 1890 and 1921. The 1921 drought in the Volga River basin caused the deaths of up to five million people—more than had died during World War I, which had just ended. India normally receives most of its rain during the monsoon season, which lasts from June to September. Winds blowing in from the Indian Ocean bring most of the country's rainfall during this season. The monsoon winds did not come during two droughts in 1769 and 1865. An estimated 10 million people died in each of those droughts, many from diseases like smallpox, which was extremely contagious and deadly because people were already weakened from the lack of food.

Almost the entire continent of Africa suffered from droughts in the last quarter of the twentieth century. Ethiopia, usually considered the breadbasket of eastern Africa, experienced drought in the early 1980s. A dry year in 1981 resulted in low crop yields. Three years later, another dry year led to the deaths of nearly a million people. Drought conditions again threatened eastern Africa in 2002. An estimated 15 million people in Ethiopia, 3 million in Kenya, and 1.5 million in Eritrea faced famine as a result of the prolonged drought. According to the World Health Organization, drought is the cause of about half of all deaths from natural disasters. In February 2011, United Nations (UN) officials launched a global appeal to raise money to fund emergency food security programs in Ethiopia. Hard hit by prolonged drought, officials said the funds were needed to avoid famine for almost 3 million Ethiopians.

In July 2011, the UN Disasters Emergency Committee (DEC) declared famine in two southern areas of Somalia suffering under the most severe and prolonged drought in a half century. In 2019, 10 out of 33 areas in Somalia received less than average rainfall, and in these regions more than 15 percent of people experienced general acute malnutrition. According to a 2019 United Nations Children's Fund (UNICEF) report, up to 178,000 children were expected to suffer from severe acute malnutrition from July 2019–June 2020.

Between 1968 and 1973, the Sahel region in eastern Burkina Faso suffered a great drought. An estimated 50,000–200,000 people died as a consequence. Although the causes of these great African droughts are unknown, research is beginning to indicate that droughts could result from a combination of global and local climate patterns. Satellite imagery links El Niño conditions and vertical ocean mixing patterns to dry weather in Sahel. In addition, desertification may be a positive feedback mechanism driving the climate toward drought conditions. In regions where there are few surface water reservoirs, such as the Sahel, the major source of precipitation is transpiration from plants. As plants become sparse in drought conditions, this source of water for precipitation is diminished. The diminished precipitation further decreases the growth of vegetation.

Since 1994, drought and famine in North Korea have been worsened by the political situation. The government of North Korea has allowed its people to starve rather than negotiate with South Korea and famine relief organizations. Over 50 percent of the children in North Korea are suffering from malnutrition and lack of water, and innumerable children have perished. The drought may Page 1419  |  Top of Articleaccelerate political problems in the region if starving refugees flee to other countries.

Drought conditions in areas of Djibouti, Ethiopia, Kenya, Somalia, and Uganda, in many areas the worst in more than a half-century, are blamed for soaring starvation and malnutrition rates. By September 2011, areas of Kenya and Somalia were near famine conditions, according to UN experts, with more than 10 million people at risk across Africa before conditions improved.

Drought is a natural phenomenon and cannot be eradicated. Consequently, methods to mitigate its devastating effects are crucial. Crop and soil management practices can increase the amount of water stored within a plant's root zone. For example, contour plowing and terracing reduce the amount and velocity of water runoff after rainstorms. Vegetation protects the soil from the impact of raindrops, which causes both erosion and soil crusting (hardening of the soil surface that prevents rain from percolating into the soil where it is stored). Both living plants and crop residues left by minimum tillage reduce soil crusting, so the soil remains permeable and can absorb rainfall.

Other farming practices that lessen the impact of drought on crop production include strip cropping, windbreaks, and irrigation. Windbreaks or shelterbelts are strips of land planted with shrubs and trees perpendicular to the prevailing winds. Windbreaks prevent soil, with its moisture-retaining properties, from being blown away by wind. Plants can also be specifically bred to adapt to the effects of weather extremes. For example, shorter plants encounter less wind and better withstand turbulent weather. Plants with crinkled leaves create small pockets of still air that slow evaporation.

From a social standpoint, drought severity is influenced by the vulnerability of an area or population to its effects. Vulnerability is a product of the demand for water, the age and health of the population affected by the drought, and the efficiency of water supply and energy supply systems. Drought's effects are more pronounced in areas that have lost wetlands that recharge aquifers, are dependent on agriculture, have low existing food stocks, or whose governments have not developed drought-response mechanisms.