Why do some jet contrails vanish in seconds while others linger for hours?
Show answer & explanation
Answer: The air they cross is humid enough to keep the ice crystals
Some planes leave smoke that lingers when their engines run dirty — Contrails aren't smoke; they're clouds of ice crystals made from the water in engine exhaust. Every jet leaves a similar amount of water, so the difference comes from the air, not the engine.
The air they cross is humid enough to keep the ice crystals ✓ — A contrail forms when hot, moist exhaust freezes into ice in the frigid air. In dry air the crystals quickly turn back into vapor, but in air already near saturation they survive, and can even spread into thin cirrus clouds for hours.
Dirty air high up holds the trail together — Pollution high up doesn't glue trails together. Whether the ice survives depends on how much water vapor the surrounding air already holds.
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