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Here is a marvelous picture from the NASA MODIS satellite showing conditions this afternoon over the Northwest (you can see the Columbia River directly below). There are some high cirrus and cirrostratus, but what caught my eye was the extraordinary show of jet contrails (click on the image for a better view).

Contrails occur because composition in jet engines produces water vapor and small particles. The water vapor is injected into the very cold upper atmosphere, where the vapor rapidly condenses into small water droplets or ice crystals. Particles produced by combustion can help this condensation process.

Contrails are promoted by an atmosphere close to saturation--so that is why contrails are particularly evident on days with cirrus or cirrostratus around (indicating upward motion and air at or close to saturation). Below is this afternoon radiosonde sounding (balloon-launched weather instruments) at Salem, Oregon, temperature is red and dewpoint is dashed blue. Above 500 mb (roughly 18,000 ft), the temperature and dewpoint are relatively close, indicating an atmosphere close to saturation.

Several times of year I get emails from people claiming that contrails are really chemtrails, which have been injected into the atmosphere by some dark government conspiracy. Obviously complete nonsense, and my attempts to point out the weaknesses of chemtrail theories rarely convince them. It is similar to those folks who feel that Elvis is still alive--you can't convince them otherwise.
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