Have you heard about chemtrails? It's a term some people use to describe the cloud-like trails that jets flying at high altitudes leave behind them. Those trails sometimes hold their shape for several hours. Because this seems counterintuitive and unnatural, some people have hypothesized that the jets are actually spraying chemicals at high altitudes on purpose.
What those chemicals are, why jets are spraying them and who is in charge varies depending on the person telling the story. Sometimes it's a government agency trying to keep the general population docile by dumping drugs from jets. Other times it's a corporation spraying down a region in an effort to create demand for its products.
But a recent paper published in Environmental Research Letters refutes these claims. The authors of the paper consulted with 77 scientists. Some were atmospheric chemists and others were geochemists. The two groups were told to examine data that conspiracy theorists frequently cite as evidence supporting the chemtrail hypothesis.
Seventy-six of those 77 experts concluded that the data wasn't suitable evidence. The atmospheric chemists said the trails behind jets form through simple condensation — they are contrails, not chemtrails. It's not that different from what happens when you exhale on a cold day. The warm, damp air in your lungs makes contact with the cold air and condenses, forming a cloud.
But those breath clouds only last for a short while before evaporating. Why do contrails sometimes hold their shape for hours? The primary reason is humidity. If the atmosphere around the jet is relatively humid, the contrails will hold together longer. Drier air will dissipate contrails faster. Very dry air will prevent contrails from forming at all. That's why you sometimes see contrails that appear to stop and start. It's not evidence of a spraying program; it's just that the jet is passing through a dry area in the atmosphere.
Meanwhile, the geochemists reviewed data about chemical levels in areas supposedly affected by chemtrail spraying and found no links there. They said that there were no unusual signs of chemical deposition and that the lab results reflect what you'd expect to find due to natural processes. There's no need for a secret spray campaign to explain the results.
Will this put to rest the chemtrail conspiracy theory? Probably not. The authors even acknowledge this in their paper. But they said it's important to provide a scientifically sound, peer-reviewed examination of the chemtrail hypothesis to educate people who have yet to form an opinion about the subject. For the dedicated chemtrail conspiracy theorist though, the paper will likely be seen as evidence of a cover-up.
And, in case you haven't checked them out, the guys from Stuff They Don't Want You to Know have covered this topic a bunch, including in the video below.