They’re etched across the sky at high altitude, lines of white, ethereal substance gradually dispersing into wisps and eventually disappearing.

Aviation officials call them “contrails” – condensation trails of frozen water vapor that’s part of the exhaust from aircraft engines, harmless byproducts of flight. Others insist that they’re “chemtrails” – chemical or biological agents sprayed from above for secret or even sinister purposes, such as changing the weather or controlling populations.

Now, the US Environmental Protection Agency has weighed in on the side of what critics of chemtrail conspiracies call reason and science.

On its web page about greenhouse gas emissions from aircraft, the EPA points readers to an “Aircraft Contrails Factsheet,” which states plainly:

“Contrails are line-shaped clouds or ‘condensation trails,’ composed of ice particles, that are visible behind jet aircraft engines, typically at cruise altitudes in the upper atmosphere. Contrails have been a normal effect of jet aviation since its earliest days. Depending on the temperature and the amount of moisture in the air at the aircraft altitude, contrails evaporate quickly (if the humidity is low) or persist and grow (if the humidity is high). Jet engine exhaust provides only a small portion of the water that forms ice in persistent contrails. Persistent contrails are mainly composed of water naturally present along the aircraft flight path.”

“EPA is not aware of any deliberate actions to release chemical or biological agents into the atmosphere,” the agency states, noting that the ice particles in contrails melt and evaporate as they fall to earth, posing no threat to human health.

The EPA does note one potentially harmful effect of contrails: “Contrail cloudiness might contribute to human-induced climate change. Climate change may have important impacts on public health and environmental protection.” That will depend on flight traffic and routes in coming decades as well as other weather changes tied to global warming.

Such assertions do not satisfy the proponents of theories about chemtrails, some of whom hold other beliefs about shadowy attempts by big government to control peoples’ lives.

Mark Pitcavage, director of investigative research at the Anti-Defamation League, who has studied antigovernment radical groups for many years, points to the case of Jerad and Amanda Miller – the couple who shot and killed two police officers at lunch and a third person in a Las Vegas Wal-Mart before being killed themselves.

“If you examine Jerad Miller’s Facebook page, you will see his references to the New World Order, as well as all the subsidiary conspiracy theories: that the government will declare martial law, that it will or already has set up concentration camps for Americans, that there will be gun confiscation, that the government is poisoning the American people using chemtrails, etc.,” Mr. Pitcavage told the Monitor last summer.

Of course, most believers in chemtrails are not violent criminals nor do they necessarily hold to other government conspiracies. But they do insist that what appears to be simple water vapor from aircraft exhaust, crystallized at high altitude, is something else.

Under the headline “Chemtrails Exposed: A History of the New Manhattan Project,” Peter Kirby of the Centre for Research on Globalization in Quebec recently writes: “Evidence suggests that today’s chemtrail spraying operations consist of airplanes saturating our atmosphere with nano-sized particles influenced by electromagnetic energy for the purpose of weather modification.”

“Motives are plenty. Most notably, significant direct benefits can be gained by playing financial markets which rise and fall with the weather such as the weather derivatives and catastrophe reinsurance markets; not to mention agricultural and energy commodities,” Mr. Kirby writes. “Weather routinely changes the course of human history. It determines what we do every day. It determines the outcomes of wars and influences elections. Control of the weather is God-like power. Money and power junkies want it.”

The web site abovetopsecret.com includes a lively discussion of chemtrails.

Still, most scientists side with the EPA.

“Chemtrails are said to last much longer than normal contrails from before 1995, but proponents are curiously oblivious of photographs of long-lasting contrails from as far back as World War II,” writes Dave Thomas, research scientist at the New Mexico Institute of Mining and Technology, in the journal Skeptical Inquirer. “The supposedly ominous ‘grid patterns’ of contrails are easily explained as the expected effect of wind movement across frequently used east/west and north/south aircraft travel lanes. And one of the defining characteristics of ‘chemtrails’ – gaps in the trails, supposedly caused by turning the ‘sprayers’ on and off – is quite simply explained as normal humidity variations in the atmosphere.”

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EPA scientists may reject the notion of chemical or biological agents released into the atmosphere. But if you have any doubts, they say, “Please email us at otaq@epa.gov or call the contrail information line at 734-214-4432.”

Full disclosure: Monitor staff writer Brad Knickerbocker is a former military pilot who flew jets in the US Navy.