Think of what happens when you are outside on a very cold day. As you breathe in, the air becomes warm inside your body, and when you breathe out, you see a small cloud form. This is the key principle behind condensation trails, or contrails. Contrails are left behind by airplanes passing in the sky. However, you may wonder why you see contrails on some days and not on others. As with your breath on a cold day, contrails only appear when planes are flying through cold, humid air.
回想一下，在寒冷的日子里当你在户外时会发生什么事。当你吸气时，空气在你体内变得温热，当你吐气时，你会看到一小朵云形成。这是凝结尾（亦称飞机云）背后的关键原理。凝结尾是飞机在空中飞过时的遗留之物。但是，你可能很好奇为什么在某些日子里看得到凝结尾，在其他日子里却看不到。正如同你在冷天中的呼吸一样，只有当飞机行经又冷又潮湿的空气时才会出现凝结尾。
生活必备字词：
principle n. 原理
leave behind... / leave... behind 留下……
humid a. 潮湿的
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本文节录自《常春藤生活英语》杂志，完整内容请见四月号。
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