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，在寒冷的日子裡當你在戶外時會發生什麼事。當你吸氣時，空氣在你體內變得溫熱，當你吐氣時，你會看到一小朵雲形成。這是凝結尾（亦稱飛機雲）背後的關鍵原理。凝結尾是飛機在空中飛過時的遺留之物。但是，你可能很好奇為什麼在某些日子裡看得到凝結尾，在其他日子裡卻看不到。正如同你在冷天中的呼吸一樣，只有當飛機行經又冷又潮溼的空氣時才會出現凝結尾。
生活必備字詞：
principle n. 原理
leave behind... / leave... behind 留下……
humid a. 潮溼的
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本文節錄自《常春藤生活英語》雜誌，完整內容請見四月號。
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