Contrail

Aviaticus cloud
Engine exhaust contrails
Genus	Cirrus (curl of hair), cirrocumulus, or cirrostratus
Altitude	Usually above 5,000 m (Usually above 16,500 ft)
Classification	Family A (High-level)
Appearance	long bands
Precipitation cloud?	No

Sinuous vapor trail over south-east Poland.

Contrails (/ˈkɒntreɪlz/; short for "condensation trails") or vapor trails are long thin artificial (man-made) clouds that sometimes form behind aircraft. Their formation is most often triggered by the water vapor in the exhaust of aircraft engines, but can also be triggered by the changes in air pressure in wingtip vortices or in the air over the entire wing surface.^[1] Like all clouds, contrails are made of water, in the form of a suspension of billions of liquid droplets or ice crystals.

Depending on the temperature and humidity at the altitude the contrail forms, they may be visible for only a few seconds or minutes, or may persist for hours and spread to be several miles wide. The resulting cloud forms may resemble cirrus, cirrocumulus, or cirrostratus. Persistent spreading contrails are thought to have a significant effect on global climate.^[2]

Condensation from engine exhaust

Contrails from a Qantas Boeing 747-400 as it passes over Moscow at 11,000 m (36,000 ft)

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 raise the relative humidity of the air past saturation point. The vapor then condenses into tiny water droplets which freeze if the temperature is low enough. These millions of tiny water droplets and/or ice crystals form the contrails. The time taken for the vapor to cool enough to condense accounts for the contrail forming some way behind the aircraft's engines. 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 condense. Exhaust contrails rarely occur below 8,000 m (26,000 ft), only if the temperature there is below −40 °C (−40 °F), and if the relative humidity is over 60%.^[3]

Condensation from decreases in pressure

As a wing generates lift, it causes a vortex to form at each wingtip, and sometimes also at the tip of each wing flap. These wingtip vortices persist in the atmosphere long after the aircraft has passed. The reduction in pressure and temperature across each vortex can cause water to condense and make the cores of the wingtip vortices visible. This effect is more common on humid days. Wingtip vortices can sometimes be seen behind the wing flaps of airliners during takeoff and landing, and during landing of the Space shuttle.

The visible cores of wingtip vortices contrast with the other major type of contrails which are caused by the combustion of fuel. Contrails produced from jet engine exhaust are seen at high altitude, directly behind each engine. By contrast, the visible cores of wingtip vortices are usually seen only at low altitude where the aircraft is travelling slowly after takeoff or before landing, and where the ambient humidity is higher. They trail behind the wingtips and wing flaps rather than behind the engines.

During high-thrust settings the fan blades at the intake of a turbofan engine reach transonic speeds, causing a sudden drop in air pressure. This creates the condensation fog (inside the intake) which is often observed by air travelers during takeoff. For more information see the Prandtl-Glauert singularity effect.

Anti-contrail measures

Military aircraft take precautions to avoid contrails which greatly enhance visual detection ranges, including choice of altitude.

Contrails and climate

Contrails, by affecting the Earth's radiation balance, act as a radiative forcing. 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). Global radiative forcing has been calculated from the reanalysis data, climatological models and radiative transfer codes. It is estimated to amount to 0.012 W/m² for 2005, with an uncertainty range of 0.005 to 0.0026 W/m², and with a low level of scientific understanding.^[4] Therefore, the overall net effect of contrails is positive, i.e. a warming effect.^[5] 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.^[6]

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 °C (1.8 °F) higher than immediately before;^[7] however, it has also been suggested that this was due to unusually clear weather during the period.^[8]

Condensation trails have been suspected of causing "regional-scale surface temperature" changes for some time.^[9][10] Researcher David J. Travis, an atmospheric scientist at the University of Wisconsin-Whitewater, has published and spoken on the measurable impacts of contrails on climate change in the science journal Nature and at the American Meteorological Society's 10th Annual conference in Portland, Oregon. The effect of the change in aircraft contrail formation on the three days after the 11th was observed in surface temperature change, measured across over 4,000 reporting stations in the continental United States.^[9] Travis' research documented an "anomalous increase in the average diurnal temperature change".^[9] The diurnal temperature range (DTR) is the difference in the day's highs and lows at any weather reporting station.^[11] Travis observed a 1.8 °C (3.24 °F) departure from the two adjacent three-day periods to the 11th–14th.^[9] This increase was the largest recorded in 30 years, more than "2 standard deviations away from the mean DTR".^[9]

The September 2001 air closures are deeply unusual in the modern world, but similar effects have provisionally been identified from Second World War records, when flying was more tightly controlled. A 2011 study of climate records in the vicinity of large groups of airbases found a case where contrails appeared to induce a statistically significant change in local climate, with a temperature variance around 0.8 °C, suggesting that examination of historic weather data could help study these effects.^[12]

Head-on contrails

A contrail from an airplane flying towards the observer can appear to be generated by an object moving vertically.^[13][14] On November 8, 2010 in California, U.S., a contrail of this type gained wide media attention as a "mystery missile" that could not be explained by U.S. military and aviation authorities,^[15] and its explanation as a contrail^{[13][14][16][17]} took more than 24 hours to become accepted by U.S. media and military institutions.^[18]

Distrails

A distrail is the opposite of a contrail. This photograph of a young distrail was taken on November 22, 2012 in Hong Kong.

Distrail is 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 a distrail looks like a tunnel through the cloud if the cloud is very thin.^[19]

Distrails are created by the elevated temperature of the exhaust gases absorbing the moisture from the cloud. Clouds exist where the relative humidity is 100% but by increasing the temperature the air can hold more moisture and the relative humidity drops below 100%, even for the same absolute moisture density, causing the visible water droplets in the cloud to be converted back into water vapor.

Gallery

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  B-17s over Europe, 1944.

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  Airliner contrails, some new, some old and much spread out by wind shear.

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  Contrail spread out by upper winds, Manchester, England, May 13, 2010.

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  The image on the left shows the sky above Würzburg, Germany without contrails after the grounding of air traffic in 2010 and the image on the right shows the sky with regular air traffic on a day when conditions were right for contrails to form.

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  Iridescent contrails from a Boeing 747. This effect happens when water molecules interact with the aircraft and the sun shines through them.

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  Contrails from an S7 Airlines Tupolev Tu-154M

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  MODIS tracking of contrails generated by air traffic over the southeastern United States on January 29, 2004.

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  Multiple contrails above Nova Scotia.

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  A Airbus A340 of Lufthansa leaves contrails behind

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  A contrail over southwest Virginia

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  A contrail over southwest Virginia

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  Two F-15 Eagle interceptors approaching Two Soviet MiG-29s

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  A Boeing 747 of Japan Airlines displays contrails.

See also

References

External links

Wikimedia Commons has media related to Contrails.

Look up contrail in Wiktionary, the free dictionary.

• 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
• Contrails over the USA
• Reference site for debunking weird stories about contrails
• 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
• NASA Contrail Education
• What is a contrail and how does it form?, National Weather Service
• Vapor Trails March 1943 article