Contrails or vapour trails are condensation trails and artificial clouds made by the exhaust of aircraft engines or wingtip vortices which precipitate a stream of tiny ice crystals in moist, frigid upper air.
Condensation from engine exhaust
The main products of hydrocarbon fuel combustion
are carbon dioxide and water vapour. At high altitudes this water vapour emerges into a cold environment, and the local increase in water vapour can push the water content of the air past saturation point
. The vapour 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 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 deposition or condensation. The exhaust particles in the aircraft's exhaust act as this trigger, causing the trapped vapour to rapidly turn to ice crystals. Exhaust vapour trails or contrails usually occur above 8000 metres (26,000 feet). where the temperature is below -40°C
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 sorts of vapour trails contrast with the other major type of contrails, which are caused by the combustion of jet fuel. Contrails produced from jet engine exhaust are seen at high altitude, directly behind each jet. With aircraft with four jets, such as the Boeing 747, there are four contrails. In contrast, vapour trails caused by a drop in air pressure are usually seen at low altitude where the ambient humidity is higher, and they follow the wings rather than the jet engines.
Vapour trails or contrails and climate
Vapour 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
. 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 mostly 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.
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; however, it has also been suggested that this was due to unusually clear weather during the period.
Short for dissipation trail. 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.