Dear Tom,
I have observed high-altitude jets dispensing heavy, non-dissipating trails behind them, unusual in that they expand across the entire sky, turning a once-blue sky into a complete cover of milky white. Tom, these are not condensation trails.
— Jeff Blondell
Dear Jeff,
Ah, but they are. Water vapor is one of the gaseous products of combustion and, when added to the air in the exhaust of jet engines, condenses into visible clouds of ice crystals — the wispy, cirruslike clouds that we see as condensation trails (contrails) behind high-flying jets.
Temperatures and humidity at flight altitude determine whether contrails form and how long they last. In dry air, contrails will not form, or they will be small and evaporate in a few seconds. In moist air, they persist for hours, often “seeding” their own further growth.
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