Anonymous asked: So I've always wondered this when I watch an airplane pass in the day: why do they leave a trail of water vapour in their wake? Does it have to be very humid air? Or cold air?
Those trails are actually called contrails, which is short for “condensation trails.” They’re formed when the hot humid air thrust out of jet exhaust collides with the wet, cold air of the upper atmosphere, condensing into little water droplets that quickly freeze into ice crystals.
Essentially, contrails are a type of cirrus cloud!
The air definitely has to be damp and cold—that’s why sometimes planes don’t leave contrails, or leave contrails that break off and restart again, because in dry air they don’t form, and the sky is a layered mix of air of different moisture levels.