A pejorative name for contrails - the lines left in the sky by aeroplanes.

Contrail is a combination of the words 'condensation' and 'trail', and is so named because of the way that they are formed. One of the byproducts of a plane's exhaust is water vapor, which rapidly cools to water and then to ice at high altitudes. These ice crystals then stay suspended in the air leaving the trails.

These trails contain other chemicals than just water, which has led to a conspiracy theorist backlash. Some consider them to be responsible for a widespread drop in public health due to air pollution, while others contend that governments specifically put certain chemicals in the trails to disseminate them over a wide area safely. Such actions would be ineffectual however, as strong winds at those altitudes means that most of the chemicals in a trail will never reach the people directly below them.