龙卷风相信大家都知道，但是它是怎么形成的大家知道么？本期带来的托福听力背景材料就为大家解释厄尔尼诺和拉尼娜现象与龙卷风的关系。
It's been a light year for tornadoes in the U.S.—just a few dozen so far. By this time in 2011, a notoriously deadly tornado year, there had already been more than 120 twisters. Why the variability?
在美国那是至今几十年为止龙卷风的光年。2011年的这个时候，那是一个臭名昭著致命龙卷风的一年，已经超过有120个龙卷风。为什么会这么多呢？
For decades, scientists have hypothesized that tornado frequency may be related to the fluctuation of sea surface temperatures in the equatorial Pacific—known as El Niño and La Niña. But the link wasn't clear in the peak springtime tornado season. Because observations of tornadoes and hail tend to be most prevalent where people can see them—meaning the data isn't that comprehensive.
几十年来，科学家假设龙卷风跟在赤道附近海平面起伏的温度有关-被称为厄尔尼诺和拉尼娜现象。但是它们之间的关系在春天龙卷风的旺季并不明显。因为对龙卷风和冰雹的观察很是流行，对于那些可以看到他们的-意味着，这些数据是不全面的。
So researchers took a step back, looking instead at the basic ingredients of tornadoes and hail: factors like wind shear, humidity and temperature. During La Niña, they indeed saw a boost in the conditions that breed springtime twisters and hail in the southeastern U.S. I
所以研究者后退一步，不看龙卷风的基本要素：像乱流、潮湿和温度。在拉尼娜现象中，他们的确看到在额外的条件下，在春季美国东南部的产生了龙卷风和冰雹
Study author John Allen, a climatologist at the Earth Institute at Columbia University, says the data might be used to create yearly tornado forecasts, like this one: "So the forecast for this spring is a 60 percent chance of being around about normal, because we've got a relatively weak El Nino pattern, which tends to suppress a little bit. So we'd probably say that it's going to be at or below normal."
研究作者约翰·艾伦，哥伦比亚大学地球研究所的气候学家，说这些数据可以用来预测年度的龙卷风，像这一个，今年春季气候正常的几率为60%，因为我们处于一个相对较弱的厄尔尼诺现象的模式中，这样会抑制一点。所以我们可能说它将是或低于正常。
Insurance companies or governments could use that data to gauge risk, he says—but no forecast can guarantee safety. "One tornado will still wreck a house. It will still end somebody's life, or destroy their life. And this is the thing to remember. We're saying there’s a lower chance, but even in a quiet year we’re still seeing about 800 tornadoes in the United States. And you know, if the right conditions match up, a tornado can form anywhere on this planet."
保险公司和政府运用这些数据统计风险，他说-预报可以确保安全。一个龙卷风仍然可以摧毁一个房子。它将会结束人的生命，或结束他们的生活。一定要记住这件事。我们说有一个较低的机会，即使是平静的一年，我们仍然看到了800个龙卷风在美国。而且你知道，如果条件匹配，龙卷风可以形成在这个星球上的任何地方。”
以上就是小编整理的关于龙卷风的托福听力背景材料，希望能够帮助大家备考，前程百利祝大家都能取得理想的托福考试成绩。
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