Those white streaks behind jets are contrails, short for condensation trails. They are thin ice clouds that form when hot, moist exhaust from engines mixes with very cold air at high altitude [1][2][3].
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As a kid I used to look up and wonder why some planes left bright white lines behind them while others did not. The basic idea is simple. Jet fuel produces water vapor when it burns. At cruising altitude the air is often far below freezing. When the warm, moist exhaust meets that cold air, water vapor quickly condenses and freezes around tiny particles from the exhaust. The result is a line of ice crystals, which we see as a contrail [1][3].
NASA and other agencies describe contrails as a type of cirrus cloud that humans create. They form only when the humidity and temperature are in the right range. If the air is dry, the trail will either not appear or will fade away very quickly. If the air is very moist, contrails can spread out and hang around for hours, blending into thin cloud sheets that are even visible from satellites [1][2][3].That also explains why some days the sky is full of lines and other days it is clear even though plenty of planes are flying. The number of flights matters, but the cold and moisture conditions at cruising altitude matter more. Lower altitude planes usually will not create contrails because the air is not cold enough [1][2].
In Short
- Next time you see a contrail, watch how long it lasts. A quick fading trail means the air is dry. A spreading one means there is more moisture high up [1][2].
- If you enjoy photography, contrails can be a fun way to show how busy the sky really is on clear days.
Bottom Line
Contrails are man made ice clouds, not mysterious chemicals. They form when hot engine exhaust mixes with very cold, moist air at high altitude, and they come and go based on the weather conditions in that part of the sky [1][2][3].
