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.

Scientists say chemtrails don't exist

In a new study published in the journal Environmental Research Letters, researchers surveyed leading atmospheric scientists around the world about the existence of a secret spraying program.

The results showed that 76 of the 77 scientists said they had not encountered evidence of any such program. In addition, they agreed that the alleged evidence that people offer who believe the atmospheric spraying is happening, could be explained by other factors, such as normal airplane contrail formation. The only scientist who answered yes reported "high levels of atm[ospheric] barium in a remote area with standard 'low' soil barium."

The researchers were also shown four images that are often identified as chemtrails. All of them said they were just ordinary contrails, and they gave peer-reviewed citations to support their claims.

“We wanted to establish a scientific record on the topic of secret atmospheric spraying programs for the benefit of those in the public who haven't made up their minds," said study co-author Steven Davis of University of California Irvine, in a statement. "The experts we surveyed resoundingly rejected contrail photographs and test results as evidence of a large-scale atmospheric conspiracy."

The researchers say the goal of their study isn't to try to change the minds of those convinced chemtrails exist "these individuals usually only reject counter-evidence as further proof of their theories." Instead, they say they just wanted to offer objective science for a basis of discussion.

“Despite the persistence of erroneous theories about atmospheric chemical spraying programs, until now there were no peer-reviewed academic studies showing that what some people think are ‘chemtrails’ are just ordinary contrails, which are becoming more abundant as air travel expands. Also, it is possible that climate change is causing contrails to persist for longer periods than they used to,” said study co-author UC Irvine's Ken Caldeira. “I felt it was important to definitively show what real experts in contrails and aerosols think. We might not convince die-hard believers that their beloved secret spraying program is just a paranoid fantasy, but hopefully their friends will accept the facts.”

This story was originally published in 2013 and has been updated with new information.