ELI5: how are contrails formed

What are contrails?

Contrails are white lines that planes leave behind in the sky, kind of like the trail of steam you see when you boil water on the stove [2]. These lines are actually made of ice clouds [2].

Why do planes make them?

When a plane flies really high where it's super cold, the hot water vapor from its engines comes out into the freezing air [3]. The water vapor sticks to tiny bits of soot (like dust from the engine) and turns into ice crystals, just like how your breath becomes foggy on a cold day [2][3].

What makes them visible?

You can only see the contrails when the water vapor actually turns into ice crystals [4]. If the air is too dry up there, the water vapor might not stick around and the contrail won't form [4].

How long do they last?

Some contrails disappear quickly like breath on a window, while others can spread out and stick around for hours depending on how wet and cold the air is [1].