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Why do airplanes leave trails?

I drove home today during daylight hours so instead of seeing shooting stars, I saw jet trails. Of course, my mind being on meteors, I thought the airplane looked like a daytime shooting star, just kind of frozen in time. So what exactly is that white trail the plane leaves behind?

Jets leave contrails (condensation trails) in their wakes for the same reason you can sometimes see your breath. The hot, humid exhaust from jet engines mixes with the atmosphere, which at high altitude is of much lower vapor pressure and temperature than the exhaust gas. The water vapor contained in the exhaust condenses, and this mixing process forms a cloud similar to the one your hot breath makes on a cold day. The water vapor from the hot plane engine turns to ice mid-air which we see as white lines in the sky called contrails.

Jet engine exhaust contains carbon dioxide, oxides of sulfur and nitrogen, unburned fuel, soot and metal particles, as well as water vapor. The soot provides condensation sites for the water vapor. Any particles present in the air provide additional sites, too.

Depending on a plane's altitude, and the temperature and humidity of the atmosphere, contrails may vary in their thickness, extent and duration. The nature and persistence of jet contrails can actually be used to predict the weather. A thin, short-lived contrail indicates low-humidity air at high altitude, a sign of fair weather, whereas a thick, long-lasting contrail reflects humid air at high altitudes and can be an early indicator of a storm.

Either that or the chem-trail conspiracy theory... you can pick. :)

P.S. Sorry about the blurry video, apparently I bumped the focus ring when a GAINT spider bolted out from under the table I was setting up at. I was too busy keeping an eye on it to check the camera I guess, so enjoy the clear view of the brush in my hand... or again, maybe it's those chemtrails, you pick.

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