Contrail
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Contrails or vapour trails are condensation trails and artificial cirrus clouds made by the exhaust of aircraft engines or wingtip vortices which precipitate a stream of tiny ice crystals in moist, frigid upper air. Being composed of water, the visible white streams are not air pollution. However, vapour trails or contrails generated by engine exhaust are inevitably linked with typical fuel combustion pollutants.
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[edit] Condensation from engine exhaust
The main products of hydrocarbon fuel combustion are carbon dioxide and water vapor. At high altitudes this water vapor emerges into a cold environment, and the local increase in water vapor can push the water content of the air past saturation point. The vapor then condenses into tiny water droplets and/or deposits into ice. These millions of tiny water droplets and/or ice crystals form the vapour trail or contrails. The energy drop (and therefore, time and distance) the vapor needs to condense accounts for the contrail forming some way behind the aircraft's engines. The majority of the cloud content comes from water trapped in the surrounding air.[citation needed] At high altitudes, supercooled water vapor requires a trigger to encourage deposition or condensation. The exhaust particles in the aircraft's exhaust act as this trigger, causing the trapped vapor to rapidly turn to ice crystals. Exhaust vapour trails or contrails usually occur at above 8000 metres (26,000 feet). where the temperature is below -40°C (-40°F).[1]
[edit] Condensation from decreases in pressure
The wings of an aircraft cause a drop in air pressure in the vicinity of the wing. This brings with it a drop in temperature, which can cause water to condense out of the air and form a vapour trail or contrail. This effect (the Prandtl-Glauert singularity), is more common on humid days, and can be seen on fighter jets performing high energy maneuvers, during shuttle launches, on the expanding surface "bubble" of nuclear explosions, or on airliners during takeoff and landing. Additionally, the area around a turbo-fan intake will be at a lower pressure than the surrounding air, and may result in a condensation fog forming there during high power settings.
These effects are compounded with the other explanation of contrails, which is the water vapor produced by the combustion of jet fuel. High altitude contrails are seen directly behind the one or two jets, and with aircraft with four jets, such as the Boeing 747, there are four contrails. The vapor trails caused by the first mentioned effect are usually seen at low altitude where the ambient humidity is higher, and they follow the wings rather that the jet engines.
[edit] Vapor trails or contrails and climate
Vapor trails or contrails, by affecting the Earth's radiation balance, act as a radiative forcing. Studies have found that vapour trails or contrails trap outgoing longwave radiation emitted by the Earth and atmosphere (positive radiative forcing) at a greater rate than they reflect incoming solar radiation (negative radiative forcing). Therefore, the overall net effect of contrails is positive, i.e. a warming.[2] However, the effect varies daily and annually, and overall the magnitude of the forcing is not well known: globally (for 1992 air traffic conditions), values range from 3.5 mW/m² to 17 mW/m². Other studies have determined that night flights are most responsible for the warming effect: while accounting for only 25% of daily air traffic, they contribute 60 to 80% of contrail radiative forcing. Similarly, winter flights account for only 22% of annual air traffic, but contribute half of the annual mean radiative forcing.[3]
[edit] September 11, 2001 climate impact study
The grounding of planes for three days in the United States after September 11, 2001 provided a rare opportunity for scientists to study the effects of contrails on climate forcing. Measurements showed that without contrails, the local diurnal temperature range (difference of day and night temperatures) was about 1 degree Celsius higher than immediately before;[4] however, it has also been suggested that this was due to unusually clear weather during the period.[5]
[edit] Distrails
Where an aircraft passes through a cloud, it can clear a path through it; this is known as a distrail. Because the plane's contrail is not yet visible, (because of its height, contrails usually form at 22,000-28,000 feet, depending on the temperature and other factors) the cloud looks like a tunnel seen horizontally and vertically, assuming the cloud is very thin, looks like it has been divided.[6]
[edit] See also
- Aviation and climate change
- Cirrus cloud
- Global dimming
- Ship tracks
- List of environment topics
- Chemtrail conspiracy theory
[edit] Related matters
- Aurora aircraft are hypothesized high-technology Black project aircraft which leave "donuts-on-a-rope" contrails.
[edit] References
- ^ NASA, Contrail Education FAQ
- ^ Ponater et al., GRL, 32 (10): L10706 2005
- ^ Stuber, Nicola; Piers Forster, Gaby Rädel, Keith Shine (June 15, 2006). "The importance of the diurnal and annual cycle of air traffic for contrail radiative forcing". Nature 441: 864-867. doi:.
- ^ Travis et al., J. Climate, 17, 1123-1134, 2004
- ^ Kalkstein and Balling Jr., Climate Research, 26, 1-4, 2004
- ^ Distrail on Earth Science Picture of the Day
[edit] External links
- Contrails.nl: Pictures of Contrails and Aviation Cirrus (-Smog), from 1995 on.
- Abstract of article in Nature announcing research results of contrail temperature change study
- Clouds Caused By Aircraft Exhaust May Warm The U.S. Climate
- Contrails over the USA
- Effects of contrails on ground astronomy
- Contrail simulator (Java applet) — interactively shows how temperature and humidity of the surrounding air affect contrail formation and characteristics
- Contrails: What's Left Behind Is Bad News, article by Nick Onkow from March 4, 2006
- Night flights give bigger boost to global warming
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