【英语时差8,16】关于飞机凝结尾流（上）（在线收听）

 Have you ever seen a perfectly cloudless sky? Cloudless–that is–except for two or three long white lines smeared across the perfect blue sky by airplanes like smudges on a pane of glass. Why do airplanes sometimes leave contrails behind? Actually, contrails, short for condensation trails, are essentially clouds. They are no different from the cirrus clouds 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 vapor–a gas–or it can condense around microscopic dust particles into tiny droplets or ice crystals. Every cloud you see is made of tiny droplets or crystals like this, and so is a contrail.