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.

      干旱

      对于我们这些足够幸运能够有充足的食物和充沛的水资源的人来说，干旱是无法想象的。引起干旱的原因有许多，通常情况下是由降雨量少和高温引起的。干旱会使土壤含水量严重减少。那接下来我们就来谈谈为什么会发生这些气候事件，它们又是如何发生的。

      厚厚的大气层将地球包围着，并在地球的不同地区上下沉浮着。当空气上升时，水蒸气会被冷却，空气中的水分会越来越少。当空气下沉时，气温会逐渐升高，同时，还会吸收土壤中的水分，这样一来空气的湿度会随之增大，而土地却会变得干燥。下沉的空气对沙漠的形成可谓是“功不可没”，举世闻名的非洲撒哈拉大沙漠便是如此形成的。

      下沉的湿热气流会阻碍云的形成，如此一来，该地区会少雨且空气湿度低。当这些现象发生在雨水充足到足以浇灌植物且能保证有充足水源的地方的话，干旱可能会接二连三的降临。早年波及美国草原地区的大沙尘暴便是一次“名震全世界”的干旱。1988年夏天在美国又发生了一次可与之匹敌的大旱。美国中西部，北部平原及落基山脉的降水量比正常降雨量少了50%至85%。

      有时，过度开垦和水土流失也会导致干旱，但是也有这样的情况，即使是土壤被小心地保护起来了，这样的自然灾害同样会发生。另外，如果发生了干旱，人类几乎可以说是束手无策，没有办法人为地将其结束。保持水土不流失会有一定的帮助，但是，在再次降雨之前我们的科学也没什么办法。