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 (he 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.
Be gripped by：be heavily affected by sth.
Be in flux：be in a situation in which things are changing a lot and you cannot be sure what will happen
Take its toll (of sth.): cause loss, damage, etc crop failures:庄稼歉收
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