Plane engines get hot. Much hotter than the atmosphere outside the plane, in fact. So when the exhaust leaves the plane, the water vapor freezes mid-air, causing a trail of ice behind the plane known as "contrails".

Or, if you believe in the "chemtrail" conspiracy theory, they are chemicals deliberately sprayed into the atmosphere by a government agency of your choosing, for unknown purposes. Proponents, claim they are part of military tests, dispense chemicals that make you sick in order to bump the profits for drug companies, or are used for mind control purposes. Some even go as far as to suggest chemtrails could wipe out humanity by causing "biblical flooding". Advocates include Alex Jones and Chuck Norris, which should be all the warning you need.

Unfortunately, this doesn't seem to be the case, as a new study has found that 10 percent of Americans believe the chemtrail conspiracy is "completely true" and 20-30 percent believing it's "somewhat true". Social media appears to be the cause of the rise.

The study, published in Nature, found that social media is helping the spread of conspiracy theories. The authors of the study analyzed mentions in public social media posts across Facebook, Twitter, Google Plus (which apparently still exists), Tumblr and other online platforms from 2008 to 2017.

Specifically, they looked for terms relating to geoengineering, including "climate engineering" and "solar geoengineering". They found that when it comes to online discussions around geoengineering, conspiracy theories dominated at the expense of science.

"Conspiratorial views have accounted for around 60% of geoengineering discourse on social media over the past decade," the authors wrote in their study.

"Of that, Twitter has accounted for over 90%, compared to around 75% of total geoengineering mentions."

The study found that a surprisingly low number of posts relating to geoengineering were neutral, at just 6 percent, which is having a damaging effect on rational discussion.

"Anonymity of social media appears to help it spread, so does the general ease of spreading unverified or outright false information. Online behavior has important real-world reverberations, with implications for climate science communication and policy." 

The study also looked at a national poll of 36,000 respondents in the US, which found that 10 percent of the population believe the chemtrail conspiracy is "completely true".

The poll, which also found that 20 to 30 percent believe it is "somewhat" true, found that the conspiracy was not related to any particular political affiliations. The authors say that belief in this conspiracy is damaging to discussions around climate change.

"[This] renders rational conversations around solar geoengineering and its potential role in climate policy even more difficult than it would be absent the chemtrails conspiracy," the authors concluded. "It also shows some of the broader implications of this online community of conspiracy with implications well beyond climate policy."