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Contrails are condensation trails: artificial clouds made by the exhaust of jet aeroplanes or wingtip vortices which precipitate a stream of tiny ice crystals in moist, frigid upper air.

In 1998, NASA scientists found that by circling a jet off the Pacific coast of the United States they were able to create contrails that eventually coalesced into a cirrus cloud covering 1,400 square miles. Satellite photographs have confirmed that on one occasion jet contrails produced by commercial aircraft over New Mexico formed a cloud covering 13,000 square miles.

It had been hypothesized that in regions such as the United States with heavy air traffic, contrails affected the weather, reducing solar heating during the day and radiation of heat during the night. The suspension of air travel for 3 days in the United States after September 11, 2001 provided an opportuntity to test this hypothesis. Measurements did in fact show that without contrails daily temperatures were about 1 degree Centigrade higher during the day and 1 degree lower at night.

A conspiracy theory postulates that "mysterious white tanker planes, a counterpart to the black helicopters of paranoid legend, crisscrossing the country spraying "chemtrails" that make people sick. Often the chemtrails form an X, which is "read by satellites to track dispersal patterns," we learn. In many cases the contrails are accompanied by a cobweblike cloud of "angel hair" filaments descending from the sky. Other times clear or brown Jell-O-like goop spatters the landscape. Some think the goop and the filaments result from improperly adjusted spray nozzles on the mysterious aircraft." 1 (http://www.straightdope.com/columns/000616)

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