Contrail

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Qantas Boeing 747-400 VH-OJU over Starbeyevo Kustov

Propeller tip vortices being generated by P-40N1 Warhawk VH-ZOC at Temora

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Bomber stream

Contrails, short for "condensation trails," are line-shaped clouds produced by aircraft engine exhaust or changes in air pressure, typically at aircraft cruise altitudes several miles above the Earth's surface. Contrails are composed primarily of water, in the form of ice crystals, that condenses from the aircraft's exhaust and the surrounding atmosphere.

Formation

Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. The mixing is a result of the turbulence generated by the engine exhaust. For a contrail to form, the air must be at a high relative humidity; the exhaust temperature must be below a critical value that depends on the ambient pressure and temperature; and the ambient air temperature must be below about −40°C. There are two types of contrails: short-lived (or non-persistent) contrails and persistent contrails, which can last for hours and spread across the sky, forming cirrus clouds that can affect weather patterns.

Types

Short-lived Contrails

Short-lived contrails evaporate quickly because they form in air that is not saturated with moisture. These contrails are often seen as short lines that follow the aircraft and disappear within minutes.

Persistent Contrails

Persistent contrails form in air that is supersaturated with moisture. These can last for hours and spread out to form thin cirrus clouds. The persistence and spread of contrails depend on the temperature, humidity at altitude, and the amount of particulates present in the engine exhaust.

Environmental Impact

Contrails and the cirrus clouds they evolve into can have a significant impact on the Earth's environment. They trap outgoing infrared radiation, contributing to the greenhouse effect and global warming. The extent of their impact is still under study, but they are considered to contribute to climate change. Efforts to mitigate the environmental impact of contrails include modifying flight routes, changing the altitude of flights, and developing engines and fuels that produce fewer particulates.

Research and Mitigation

Research into contrails and their effects is ongoing. Scientists are studying the properties of contrails and their impact on the atmosphere and climate. Mitigation strategies being explored include altering flight paths to avoid areas where contrails are likely to form or persist, optimizing flight altitudes based on atmospheric conditions, and developing more efficient engines that produce fewer emissions.

See Also

• Cloud formation
• Aviation and the environment
• Global warming
• Cirrus clouds