contrail

atmospheric science

Also known as: condensation trail, vapour trail

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History

contrail, streamer of cloud sometimes observed behind an airplane flying in clear, cold, humid air. A contrail forms when water vapor produced by the combustion of fuel in airplane engines condenses upon soot particles or sulfur aerosols in the plane’s exhaust. When the ambient relative humidity is high, the resulting ice-crystal plume may last several hours. The trail may be distorted by the winds, and sometimes it spreads outward to form a layer of cirrus cloud. On rare occasions, when the air is nearly saturated with water vapor, air circulation at the wing tips of an airplane may cause sufficient pressure and temperature reductions to cause cloud streamers to form.

Conspiracy and “chemtrails”

In the 1990s a popular conspiracy theory arose claiming that long-lasting contrails contained chemicals (“chemtrails”) that were sprayed by governments for purposes such as controlling the weather or dispersing drugs to influence the general population. Other conspirators have claimed that the trails are used for testing military weapons, or even to psychologically manipulate the general population. However, atmospheric scientists have explained that some contrails last longer than others because of factors such as the humidity of the air (which can vary quite sharply over short distances) and the temperature of the airplane’s exhaust.

Conspiratorial rhetoric involving chemtrails has also been used opportunistically by political officials. On one occasion, in the aftermath of Texas flooding in 2025, some politicians claimed that chemtrails were part of an irresponsible cloud seeding effort to induce flooding in the area. In reality, it is impossible for cloud seeding to cause such widespread extreme rainfall events, since current technology only operates at the scale of individual clouds (see also geoengineering).