Contrails artificial clouds that sometimes form behind aircraft. Their formation is most often triggered by the water vapor in the exhaust of aircraft engines but can also be triggered by the changes in the air pressure over the entire wing surface.

Some believe that the government is spraying something that creates unusual looking contrails, this is referred to as the chemtrail conspiracy theory. This is the 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,^[3] 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.

Chemtrail conspiracy theories began to circulate after the United States Air Force (USAF) published a 1996 report about weather modification.

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.

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