The chemtrail conspiracy theory presupposes the erroneous belief that long-lasting contrails left by aircraft are, in fact, “chemtrails” intentionally sprayed for nefarious purposes.
The so-called “chemtrails” are just normal condensation trails produced by aircraft engine exhaust. Contrails are composed of water in the form of ice crystals. Depending on the temperature and humidity, contrails may be visible for a few seconds, or they may persist for hours, resembling natural clouds.
Chemtrail is just a conspiracy theory, like flat Earth. The proponents believe the conspiracy theory because of the desire to channel their prejudices, not because of a scientific basis. Just like with other conspiracy theories, a person who believes the chemtrail conspiracy theory also tends to become a victim of other conspiracy theories, including flat Earth, 5G, antivax, or those about COVID-19. And they will usually “connect the dots” between these conspiracy theories to support each other.