Contrail

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For other meanings of Vapor Trail please see Vapor Trail (disambiguation).

A C-141 Starlifter leaves vapour trails over Antarctica

Boeing 747 in cruise at roughly 35000 feet

Multiple contrails in an area with high airline traffic.

A satellite photo of Nova Scotia showing numerous contrails of jets travelling between the Eastern Seaboard and Europe.

These Contrails (Aviation Smog) over The Netherlands will soon look like normal, natural Cirrus cloud.

Contrails, becoming Cirrus. This 'ice curtain' clearly filters the Sun's radiation. The condensation particles in the airplane's exhaust contrails can foster cloud formation several times, at different altitudes.

Contrails are condensation trails (sometimes vapour trails): 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. Contrary to appearances, they are not air pollution as such, though might be considered visual pollution.

[edit] Condensation from engine exhaust

An aircraft engine's exhaust increases the amount of moisture in the air, which can push the water content of the air past saturation point. This causes condensation to occur, and the contrail to form.

Aviation fuel such as petrol/gasoline (piston engines) or paraffin/kerosene (jet engines) consists primarily of hydrocarbons. When the fuel is burned, the carbon combines with oxygen to form carbon dioxide; the hydrogen also combines with oxygen to form water, which emerges as steam in the exhaust. For every gallon of fuel burned, approximately one gallon of water is produced, in addition to the water already present as humidity in the air used to burn the fuel. At high altitudes this water vapour emerges into a cold environment, (as altitude increases, the atmospheric temperature drops) and the local increase in water vapour density condenses into tiny water droplets and/or desublimates into ice. These millions of tiny water droplets and/or ice crystals form the contrails. The energy drop (and therefore, time and distance) the vapour 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. At high altitudes, supercooled water vapour requires a trigger to encourage desublimation. The exhaust particles in the aircraft's exhaust act as this trigger, causing the trapped vapor to rapidly turn to ice crystals. Contrails will only occur when the outside air temperature around the aircraft is at or below -57 degrees Celsius.

[edit] Condensation from wing-tip pressure

Main article: wingtip vortices

The wings of an airplane cause a drop in air pressure in the vicinity of the wing (this is partly what enables a plane to fly). This drop in air pressure brings with it a drop in temperature, which can cause water to condense out of the air and form a contrail but only at higher altitudes. At lower altitudes, this phenomenon is also known as "ectoplasm." Ectoplasm is more commonly seen during high energy manouvers like those of a fighter jet, or on jet liners during takeoff and landing, at areas of very low pressure, including over the wings, and often around turbo-fan intakes on takeoff.

[edit] Contrails and climate

Contrails, by affecting cloud formation, can act as a radiative forcing. Various studies have found that 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 effect of contrails is a warming.^[1] However, the effect varies daily and annually, and overall the size 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.^[2]

[edit] September 11, 2001 climate impact study

It had been hypothesized that in regions such as the United States with heavy air traffic, contrails affected the weather, reducing solar heating during the day and radiation of heat during the night by increasing the albedo. The suspension of air travel for three days in the United States after September 11, 2001 provided an opportunity to test this hypothesis. Measurements did show that without contrails the local diurnal temperature range (difference of day and night temperatures) was about 1 degree Celsius higher than immediately before;^[3] however, it has also been suggested that this was due to unusually clear weather during the period.^[4]

[edit] See also

• Aviation and climate change
• Cirrus cloud
• Global dimming
• Ship tracks
• List of environment topics
• Chemtrail theory (A conspiracy theory that some contrail-like clouds are the result of high-altitude chemical spraying.)

[edit] References

1. ^ Ponater et al., GRL, 32 (10): L10706 2005
2. ^ 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:10.1038/nature04877.
3. ^ Travis et al., J. Climate, 17, 1123-1134, 2004
4. ^ Kalkstein and Balling Jr., Climate Research, 26, 1-4, 2004

[edit] External links

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Contrail

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• Contrails.nl: Pictures of Contrails and Aviation Cirrus (-Smog), from 1995 until now.
• Abstract of article in Nature announcing research results of contrail temperature change study
• Clouds Caused By Aircraft Exhaust May Warm The U.S. Climate
• Picture: Plane producing contrails
• Fascinating picture of contrails over the United States
• 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