下面的新托福考试阅读练习资料中，主要为大家介绍一些关于干旱的知识。

托福考试阅读练习资料:Drought

　　For those of us lucky enough to always have enough food and water, it may be hard to imagine living through a drought. A drought occurs when for a variety of reasons, usually including a lack of rainfall and warm temperatures. Soil moisture severely dwindles. Why and how do these meteorological events occur?

　　As air swirls around the earth, it rises and sinks over different areas of the globe. When air rises it cools and loses moisture. When air sinks, it grows warmer and absorbs moisture, drying out the land. Sinking air helps to form deserts, such as the Sahara Desert in Africa.

　　Warm, sinking air can hamper cloud formation, which results in less rain and lower overall humidity. When these conditions occur in an area that normally receives enough rain to allow plants to grow and keep water sources replenished, a drought can ensue. The Dust Bowl that affected the prairie regions of the United States was one infamous drought. An equally severe drought occurred in the United States in the summer of 1988. Rainfall over the mid-west, Northern Plains, and the Rockies was fifty to eighty-five percent below normal.

　　Sometimes over-farming and soil erosion contribute to drought, but there are also natural disasters that can occur even when soil is carefully preserved. And when a drought hits, there’s little that can be done to end it. Water and soil conservation can help, but until the rains come again there’s not much science can do.