国家地理-2008-09-11 旱灾的成因与影响（在线收听）

Its signs are subtle and slow: the earth dries; water levels fall; the rains do not come; and the land is gripped by drought. At its most basic, a drought occurs when more water is used than is replenished. It is a balance between supply and demand, with both natural and human factors in play.

The weather is constantly in flux. A low pressure system allows moist air to rise, cool and form rain clouds. A high pressure system traps the air beneath it, and banishes the clouds. Droughts form when changing wind patterns cause high pressure systems to last for months, or even years.

Aggravating the problem is society's demand for water. Farms are heavily dependent on water to irrigate crops and provide pasture for livestock. Urban areas also place huge demands on available water supplies. If the demand can't be reduced, then the drought begins to take its toll. Crops eventually wither and die. Soil erodes away into clouds of dust. Forest fires spread rapidly.

The damage to the environment has large-scale consequences for us human population. Short-term droughts cause stress on both the environment and people. Long-term droughts can lead to war, famine, disease or mass migrations.

In the 1930s, a severe drought in the Great Plains caused massive crop failures. In some places, the drought lasted 8 years. So much soil blew away  became known as the Dust Bowl. Over 50 million acres of land were affected, forcing many farmers to abandon their own property.

But by historical standards, the Dust Bowl was mild and short-lived. Some droughts have lasted for decades. The famines they create have killed over 40 million people in the 20th century alone.

Like other forms of weather, droughts are one of the earth's natural processes. There is very little we can do to stop them. The best we can do is prepare for when droughts do come before everything blows away.