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Vapor Trail |
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Contrails or vapour trails are visible trails of condensed water vapour made by the exhaust of aircraft engines. As the hot exhaust gases cool in the surrounding air they may precipitate a cloud of microscopic water droplets. If the air is cold enough, this trail will comprise tiny ice crystals.
The wingtip vortices which trail from the wingtips and wing flaps of aircraft are sometimes partly visible due to condensation in the cores of the vortices. Each vortex is a mass of spinning air and the air pressure at the centre of the vortex is very low. These wingtip vortices are unrelated to the exhaust from the engines. They are sometimes known as vapour trails.
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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.citation needed 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 (-40°F).1
The wings of an aircraft cause a fall in air pressure in the vicinity of the wing. This brings with it a fall in temperature. The combined effect of the fall in pressure and temperature can cause water to condense out of the air and form trailing vortices. This effect is more common on humid days. Trailing vortices can sometimes be seen behind the wing flaps of airliners during takeoff and landing, during landing of the Space shuttle, and on the upper wing surface of military jet aircraft performing high-energy maneuvers. Additionally, the area around the intake of a turbo-fan engine will be at a lower pressure than the surrounding air, and may result in a condensation fog forming in the intake during high-thrust 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 engine. 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 trail behind the wingtips and wing flaps rather than behind the jet engines.
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.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 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.3
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
Condensation trails have been suspect of causing “regional-scale surface temperature” changes for some time6. Researcher David J. Travis, an atmospheric scientist at the University of Wisconsin-Whitewater, has published and spoke on the measurable impacts of contrails on climate change in the science journal Nature and at the American Meteorological Society 10th Annual conference in Portland, Oregon. The effect of the change in aircraft contrail formation on the 3 days after the 11th was observed in surface temperature change, measured across over 4,000 reporting stations in the continental U.S.6. Travis’ research documented an “anomalous increase in the average diurnal temperature change.6 The diurnal temperature change (DTR) is the difference in the days highs and lows at any weather reporting station 7. Travis observed a 1.8 degree Celsius departure from the two adjacent three-day periods to the 11th-14th.6. This increase was the largest recorded in 30 years, more than “2 standard deviations away from the mean DTR”6.
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.8
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