Contrail appearance changes with weather, which leads some people to mistakenly believe they contain chemicals.

The way contrails look depends on humidity and temperature. When the air is very humid, contrails last longer and spread into thin cloud layers. This can make people think planes are spraying something secret. But scientists say this is just how water vapor behaves in the atmosphere, nothing more. These natural effects explain all so-called 'chemtrail' sightings.