【英语时差8,16】飞机凝结尾流（下）

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To make an airplane contrail, you need water and some kind of microscopic¹ particle for the water to condense around. When jet fuel burns, it releases both water vapor² and microscopic particles of exhaust. A contrail occurs when the jet’s water vapor condenses around its own exhaust particles. If you see a plane that’s leaving no contrail–or a very short one–this means the upper atmosphere is relatively³ dry. Dry air reabsorbs the water as invisible vapor almost as soon as it condenses. On the other hand, if you see a contrail that lasts a long time, this means the upper atmosphere is already saturated⁴ with as much water vapor as it can hold. Because of this, you sometimes can use contrails to predict the weather. Quickly fading ones suggest a dry upper atmosphere and fair weather. If you see a lot of long contrails in the sunset though, you might carry an umbrella the next day!

 

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