Look! Up in the sky!

It’s a bird!

It’s a plane!

It’s a chemtrail!

The persistent high-altitude contrails are more than stripes in the stratosphere. They are the stuff of conspiracy theories on par with Area 51, the Roswell UFO crash and who shot JFK.

The chemtrail conspiracy theory claims that some contrails are chemical, biological or otherwise toxic elements sprayed at high altitudes by government agencies — of some sort — for the purpose of — well, something not good. It’s high-altitude crop dusting for nefarious purposes, the tin-foil hat crowd claims.

Normal jet airliner exhaust contrails quickly dissipate, the conspiracy theory holds. Chemtrails — which are loaded with toxic heavy metals and heaven knows what else — linger in the sky for hours.

The theory has no scientific basis, writes Grant Petty, a professor of atmospheric science and the University of Wisconsin-Madison.

“The lifetime and behavior of contrails depend on the environmental conditions in which they form,” Petty writes. “Growing contrails result when the humidity at high altitudes is at or above the saturation point with respect to ice. So when condensation is injected in the atmosphere at those altitudes, the microscopic particles of ice don’t simply evaporate, they persist and even grow, and they are spread by the winds into broader patches of cirrus cloud.”

If persistent contrails are more common, Petty suggests, it is because there is more high-altitude airliner traffic and the stratospheric humidity is higher because of the moisture left by the contrails of all that traffic.

Besides, Petty writes, such high-altitude spraying would be wildly inefficient.

“Stuff put into the stratosphere tends to stay there for many months to a year or longer unless the particles are large enough to fall with a significant speed,” he says.

And it would be basically impossible to target any specific spot on the surface.

The contrails left by air traffic do have an impact, however. The condensation trails from jet aircraft lower temperatures, according to one study. Researchers looked at temperature records at 4,000 weather stations during the period of Sept. 11-14, 2001 when commercial air traffic was halted following the terrorist attacks. The study found that the diurnal temperature range averaged across the United States increased.

There is also some concern that increasing airliner traffic may contribute to air pollution that contributes to global warming. By 2050, emissions from planes are expected to become one of the largest contributors to global warming, according to a 2006 report from the Royal Commission on Environmental Pollution, an independent group of scientists that advises the British government.

So there you have it.

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