page_title: Chemtrail conspiracy theory
section_title: Contrails
hierarchical_section_title: Chemtrail conspiracy theory / Contrails
caption_attribution_description: English: Condensation Trails ("contrails") from Aircraft Engine Exhaust
context_page_description: The chemtrail conspiracy theory posits the erroneous belief that long-lasting condensation trails are "chemtrails" consisting of chemical or biological agents left in the sky by high-flying aircraft, sprayed for nefarious purposes undisclosed to the general public. Believers in this conspiracy theory say that while normal contrails dissipate relatively quickly, contrails that linger must contain additional substances. Those who subscribe to the theory speculate that the purpose of the chemical release may be solar radiation management, weather modification, psychological manipulation, human population control, biological or chemical warfare, or testing of biological or chemical agents on a population, and that the trails are causing respiratory illnesses and other health problems.
  The claim has been dismissed by the scientific community. There is no evidence that purported chemtrails differ from normal water-based contrails routinely left by high-flying aircraft under certain atmospheric conditions. Although proponents have tried to prove that chemical spraying occurs, their analyses have been flawed or based on misconceptions.
context_section_description: Contrails, or condensation trails, are "streaks of condensed water vapor created in the air by an airplane or rocket at high altitudes." Fossil fuel combustion (as in piston and jet engines) produces carbon dioxide and water vapor. At high altitudes the air is very cold. Hot humid air from the engine exhaust mixes with the colder surrounding air, causing the water vapor to condense into droplets or ice crystals that form visible clouds. The rate at which contrails dissipate is entirely dependent on weather conditions. If the atmosphere is near saturation, the contrail may exist for some time. Conversely, if the atmosphere is dry, the contrail will dissipate quickly.
  It is well established by atmospheric scientists that contrails can persist for hours, and that it is normal for them to spread out into cirrus sheets. The different-sized ice crystals in contrails descend at different rates, which spreads the contrail vertically. Then the differential in wind speeds between altitudes (wind shear) results in horizontal spreading of the contrail. This mechanism is similar to the formation of cirrus uncinus clouds. Contrails between 25,000 and 40,000 feet (7,600 and 12,200 m) can often merge into an "almost solid" interlaced sheet. Contrails can have a lateral spread of several kilometers, and given sufficient air traffic, it is possible for contrails to create an entirely overcast sky that increases the ice budget of individual contrails and persists for hours.
  Experts on atmospheric phenomena say that the characteristics attributed to chemtrails are simply features of contrails responding to diverse conditions in terms of sunlight, temperature, horizontal and vertical wind shear, and humidity levels present at the aircraft's altitude. In the US, the gridlike nature of the National Airspace System's flight lanes tends to cause crosshatched contrails, and in general it is hard to discern from the ground whether overlapping contrails are at similar altitudes or not. The jointly published fact sheet produced by NASA, the EPA, the FAA, and NOAA in 2000 in response to alarms over chemtrails details the science of contrail formation, and outlines both the known and potential impacts contrails have on temperature and climate. The USAF produced a fact sheet that described these contrail phenomena as observed and analyzed since at least 1953. It also rebutted chemtrail theories more directly by identifying the theories as a hoax and disproving the existence of chemtrails.
  Patrick Minnis, an atmospheric scientist with NASA's Langley Research Center in Hampton, Virginia, has said that logic does not dissuade most chemtrail proponents: "If you try to pin these people down and refute things, it's, 'Well, you're just part of the conspiracy'," he said.
  Analysis of the use of commercial aircraft tracks for climate engineering has shown them to be generally unsuitable.
  Astronomer Bob Berman has characterized the chemtrail conspiracy theory as a classic example of failure to apply Occam's razor, writing in 2009 that instead of adopting the long-established "simple solution" that the trails consist of frozen water vapour, "the conspiracy web sites think the phenomenon started only a decade ago and involves an evil scheme in which 40,000 commercial pilots and air traffic controllers are in on the plot to poison their own children."
  A 2016 survey of 77 atmospheric scientists concluded that "76 out of 77 (98.7%) of scientists that took part in this study said there was no evidence of a [secret large-scale atmospheric program (SLAP)], and that the data cited as evidence could be explained through other factors, such as typical contrail formation and poor data sampling instructions presented on SLAP websites."
caption: Contrails from propeller-driven aircraft engine exhaust, early 1940s
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