When a plane flies very high, hot wet air from its engines meets the very cold sky. The water turns into tiny bits of ice, and you see a long white line behind the plane. That line is a contrail. Though actually it is not steam, and it is not smoke from a broken engine — a healthy plane can leave one. The line may fade in a few minutes, or it may spread and look like a thin cloud. You will hear the word in weather talk and in news about planes and the sky. Some people mix it up with a made-up story called “chemtrails”; a contrail is just the normal ice line, nothing secret. British speakers also say vapour trail, which is the same thing in plainer pieces.

飛機雲（科學上稱凝結尾跡）