英语听力文摘 English Digest 276、飞机的凝迹

（单词翻译）

　　Airplane Contrails
Have you ever seen a perfectly¹ cloudless sky?  Cloudless–that is–except for two or three long white lines smeared² across the perfect blue sky by airplanes like smudges on a pane³ of glass.  Why do airplanes sometimes leave contrails behind? Actually, contrails, short for condensation⁴ trails, are essentially⁵ clouds.  They are no different from the cirrus clouds（卷云） that naturally form in that region of the atmosphere.
High in the atmosphere, water can exist in one of two basic forms.  It can either be invisible water vapor⁶–a gas–or it can condense around microscopic⁷ dust particles into tiny droplets⁸ or ice crystals（冰晶）.  Every cloud you see is made of tiny droplets or crystals like this, and so is a contrail.

　　To make an airplane contrail, you need water and some kind of microscopic particle for the water to condense around.  When jet fuel（喷气燃料） burns, it releases both water vapor and microscopic particles（微观粒子） 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 relatively⁹ 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 (1)saturated 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!
Notes:
(1)saturate¹⁰ with v.使饱和
If a medium other than water is to be used, saturate it with oxygen in a similar manner.
若使用的介质不是水也用类似方法以氧饱和之。