Contrails

aircraft · clouds · water vapor · sky

Airplanes make clouds. Contrails are created when hot, wet air from jet engines meets the super cold air at cruising altitude, miles above Earth. The water vapor instantly freezes into ice crystals, forming the white trails you see streaking across the sky.

Made of ice crystals. Despite looking like smoke, contrails are actually tiny frozen water droplets. They form the same way regular clouds do, except they appear in the atmosphere where planes fly.

Cold air is key. Contrails only form when it is extremely cold at high altitude. If the air is too warm, the water vapor disappears before it can freeze into visible ice crystals.

Pressure changes matter too. Besides hot engine exhaust, contrails can also form from the pressure changes caused by airplane wings moving through the air. This can create clouds even when engines alone wouldn't.

Persistence varies. Some contrails disappear within seconds, while others linger for hours and spread across the sky. How long they last depends on how humid the air is at that altitude.