What is a contrail?
For grown-ups: the science
A contrail is a cloud that forms when water vapor from an airplane cools and turns into tiny ice crystals.
Contrails form when hot exhaust from jet engines, which carries a lot of water vapor, mixes with the extremely cold, low-pressure air high in the sky and cools very quickly. The water vapor uses tiny particles as a starting point and freezes into ice crystals, which is why we see a cloud. This is the same idea as seeing your breath turn white outside, where water vapor in the air cools and turns into tiny droplets. The difference is that breath is mostly made of tiny water droplets, while contrails are mostly made of ice crystals. Whether a contrail lingers or spreads out depends on things like the moisture, temperature, and wind high in the sky. Scientists are still studying how much contrails, especially the ones that spread and stick around, affect global warming, and the exact impact isn't settled yet.
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Transcript
There's a white line trailing behind that airplane way up in the sky—how does it get there?
Hot exhaust streams out of the airplane's engines. That exhaust is packed with water vapor.
Way up high, the air is super cold and thin. When the exhaust cools down, its water vapor turns into tiny ice crystals.
Your breath turns white in winter the same way, as vapor cools into droplets. A contrail looks white too, since it's made of icy crystals bunched together.
So a contrail is really a cloud made when water vapor turns into tiny ice crystals.