英語聽力文摘 干旱知識ABC

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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.

干旱

對于我們這些足夠幸運能夠有充足的食物和充沛的水資源的人來說，干旱是無法想象的。引起干旱的原因有許多，通常情況下是由降雨量少和高溫引起的。干旱會使土壤含水量嚴重減少。那接下來我們就來談談為什么會發(fā)生這些氣候事件，它們又是如何發(fā)生的。

厚厚的大氣層將地球包圍著，并在地球的不同地區(qū)上下沉浮著。當空氣上升時，水蒸氣會被冷卻，空氣中的水分會越來越少。當空氣下沉時，氣溫會逐漸升高，同時，還會吸收土壤中的水分，這樣一來空氣的濕度會隨之增大，而土地卻會變得干燥。下沉的空氣對沙漠的形成可謂是“功不可沒”，舉世聞名的非洲撒哈拉大沙漠便是如此形成的。

下沉的濕熱氣流會阻礙云的形成，如此一來，該地區(qū)會少雨且空氣濕度低。當這些現(xiàn)象發(fā)生在雨水充足到足以澆灌植物且能保證有充足水源的地方的話，干旱可能會接二連三的降臨。早年波及美國草原地區(qū)的大沙塵暴便是一次“名震全世界”的干旱。1988年夏天在美國又發(fā)生了一次可與之匹敵的大旱。美國中西部，北部平原及落基山脈的降水量比正常降雨量少了50%至85%。

有時，過度開墾和水土流失也會導致干旱，但是也有這樣的情況，即使是土壤被小心地保護起來了，這樣的自然災害同樣會發(fā)生。另外，如果發(fā)生了干旱，人類幾乎可以說是束手無策，沒有辦法人為地將其結束。保持水土不流失會有一定的幫助，但是，在再次降雨之前我們的科學也沒什么辦法。