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.




principle n. 原理

leave behind... / leave... behind  留下……

humid a. 潮溼的




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壹蘋派生活美語 藍色慢活區教你長壽的祕訣(一)