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Contrails are condensation trails (sometimes vapour trails): artificial clouds made by the exhaust of jet aircraft or wingtip vortices which precipitate a stream of tiny ice crystals in moist, frigid upper air. They are the cloud-like trails of water vapour that can be seen in the wake of airplanes, either from their exhaust or sometimes from their wing tips.
Contrail creation
Contrails are created in one of two ways:
1. First, the airplane's exhaust increases the amount of moisture in the air, which can push the water content of the air past saturation point (see saturation or dew point). This causes condensation to occur, and the contrail to form.
Airplane 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 superheated steam in the exhaust. For every gallon of fuel burnt, 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 steam emerges into a freezing environment, (as altitude increases, the atmospheric temperature drops) which lowers the temperature of the steam until each individual droplet freezes into tiny ice crystal. These millions of tiny ice crystals form the contrails. The time the steam/water droplets take to freeze accounts for the contrail forming some way behind the aircraft's engines.
2. The wings of an airplane cause a drop in air pressure in the vicinity of the wing (this is partly what allows 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.
Exhaust contrails tend to be more stable and long-lasting than wing-tip contrails, which are often disrupted by the aircraft's wake.
NASA experiment
In 1998, NASA scientists found that by circling a jet off the Pacific coast of the United States they were able to create contrails that eventually coalesced into a cirrus cloud covering 1,400 square miles; (3,600 km²). Satellite photographs have confirmed that on one occasion jet contrails produced by commercial aircraft over New England formed a cloud covering 13,000 square miles; (34,000 km²).
Contrails and climate
Contrails, by affecting cloud formation, can act as a radiative forcing. Perhaps surprisingly, various studies find that the overall effect is a warming (Ponater et al., GRL, 32 (10): L10706 2005). However, 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.
Sept. 11th, 2001 Climate Impact "Experiment"
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 in fact show that without contrails the local diurnal temperature range (difference of day and night temperatures) was about 1 degree Celsius higher than immediately before (Travis et al., J. Climate, 17, 1123-1134, 2004); however, it has also been suggested that this was due to unusually clear weather during the period (Kalkstein and Balling Jr., Climate Research, 26, 1-4, 2004).