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Airplane contrails and their effect on temperatures

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(Read caption) Airplane contrails above a lighthouse. Scientists have been studying whether contrails warm or cool temperatures.

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Maybe you did some airplane travel over the holidays, and maybe, once your plane reached cruising altitude, you noticed the vapor trails from other jetliners crisscrossing your path. Or possibly one day recently, you simply looked up and noticed many thin, white clouds crisscrossing the sky.

These are contrails, perhaps one of the most directly observable ways human activity can change the weather. They form when, as exhaust spews from jet engines, moisture condenses on particles of soot in the subfreezing air.

They usually appear above 26,000 feet where the air is less than -40 C (also -40 F.) But factors besides altitude also play a role in their formation. Depending on how much moisture is in air, for example, contrails last shorter or longer. Moisture availability also dictates whether they form at all, and how much they grow after formation.

As it turns out, they also impact temperatures at Earth's surface, although by how much and in which direction — up or down — is still being worked out.

Initially, scientists thought that contrails, like the naturally occurring cirrus clouds they resemble and sometimes seed, had an overall warming effect. Although contrails clearly reflected incoming sunlight, they also trapped heat from below that would otherwise escape into space. Scientists therefore thought contrails had a net warming effect.

Then Sept. 11, 2001 presented a unique opportunity to study what the sky looked like without airplanes and contrails. In the wake of the 9-11 terrorist attacks, the FAA prohibited commercial aviation over the United States for three days. That's when David Travis, an atmospheric scientist at the University of Wisconsin, Whitewater, thought to look at how temperatures might differ at temperature stations around the country.

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