What are the white trails behind planes?
Planes fly so high that the air is colder than a freezer. Their hot, damp exhaust freezes into a line of tiny ice crystals, which looks like a white cloud trail.
Contrails are clouds made by aircraft. Hot, damp exhaust meets very cold air and freezes into tiny ice crystals.
What they are
Engine exhaust contains water vapour. At cruising height the air is far below freezing, often −40 °C or colder, so the vapour freezes straight away and makes a visible white line.
Why some vanish and some stay
If the surrounding air is dry, the ice evaporates within seconds and the trail is short. If the air is already damp, the ice stays and the trail can spread into a wide sheet of thin cloud that lasts for hours.
Reading the sky
Long-lasting trails show that there is a lot of moisture high up. Short ones show dry air.