The topic of contrails is a hazardous one in certain circles, depending on said circle's fondness for goofy conspiracy theories. Here's what we do know: Contrails are those streaks of thin white clouds cutting across the sky, and they're produced by aircraft. How do they form? What are they made of? Are the Illuminati involved?
Julian Huguet gets up in the air for today's DNews dispatch.
The boring truth is that contrails are clouds, basically -- or more specifically, they're ice. Water vapor freezes into ice crystals when the temperature is low enough, and when the crystals have something to latch onto and form around.
That's where aircraft exhaust comes in. Contrails are made up mostly of water vapor that's already in the air as the aircraft passes by. Materials in the aircraft exhaust -- oxides, sulfates, soot and even little metal flakes -- provide the "seeds" that the water vapor needs to start crystallizing into ice.
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The exhaust also adds in some additional water vapor of its own. Jet fuel is made up primarily of hydrocarbons. When the hydrogen in the fuel mixes with oxygen during combustion, the chemical process kicks out new molecules of carbon dioxide and H20, known to its friends as water. It's the same phenomenon that produces the water dripping out of your car's tailpipe.
Contrails form naturally so long as the ambient air is cold and moist enough when an aircraft passes through. If it's too dry or warm, you won't see any contrails at all. Contrails are occasionally formed by an airplane's wing tips, too. That's because the physics of air movement around the wings, at certain angles, produces a drop in air pressure sufficient to lower the immediate temperature. Those lower temperatures turns water vapor into ice.
It's all relatively straightforward, but you can't tell some people that. Tenacious conspiracy theorists contend that contrails are actually chemtrails -- chemicals being dumped into the atmosphere by (A) the government, or (B) big business, or (C) aliens, for the purpose of (A) mind control, or (B) weather modification, or (C) something.
Check out Julian's video for a delightfully simple counterargument that can be used to fend off the tin foil hat crowd, citing evidence visible to the naked eye. Science marches on.
-- Glenn McDonald
NASA: Contrail Science
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