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A jet forming contrails in a blue sky
GenusCirrus (curl of hair), cirrocumulus, or cirrostratus
Altitude7,500 to 12,000 m
(25,000 to 40,000 ft)
ClassificationFamily A (High-level)
AppearanceLong bands

Contrails (/ˈkɒntrlz/; short for "condensation trails") or vapor trails are line-shaped clouds produced by aircraft engine exhaust or changes in air pressure, typically at aircraft cruising altitudes several miles above the Earth's surface. Contrails are composed primarily of water, in the form of ice crystals. The combination of water vapor in aircraft engine exhaust and the low ambient temperatures that exist at high altitudes allows the formation of the trails. Impurities in the engine exhaust from the fuel, including sulfur compounds (0.05% by weight in jet fuel) provide some of the particles that can serve as nucleation sites for water droplet growth in the exhaust. If water droplets form, they might freeze to form ice particles that compose a contrail.[1] Their formation can also be triggered by changes in air pressure in wingtip vortices or in the air over the entire wing surface.[2] Contrails, and other clouds directly resulting from human activity, are collectively named homogenitus.[3]

Depending on the temperature and humidity at the altitude the contrails form, they may be visible for only a few seconds or minutes, or may persist for hours and spread to be several miles wide, eventually resembling natural cirrus or altocumulus clouds.[1] Persistent contrails are of particular interest to scientists because they increase the cloudiness of the atmosphere.[1] The resulting cloud forms are formally described as homomutatus,[3] and may resemble cirrus, cirrocumulus, or cirrostratus, and are sometimes called cirrus aviaticus.[4] Some persistent spreading contrails contribute to climate change.[5]

Condensation trails as a result of engine exhaust[edit]

Contrails of a Boeing 747-400 from Qantas at 11,000 m (36,000 ft)

Engine exhaust is predominantly made up of water and carbon dioxide, the combustion products of hydrocarbon fuels. Many other chemical byproducts of incomplete hydrocarbon fuel combustion, including volatile organic compounds, inorganic gases, polycyclic aromatic hydrocarbons, oxygenated organics, alcohols, ozone and particles of soot have been observed at lower concentrations. The exact quality is a function of engine type and basic combustion engine function, with up to 30% of aircraft exhaust being unburned fuel.[6] (Micron-sized metallic particles resulting from engine wear have also been detected.) At high altitudes as this water vapor emerges into a cold environment, the localized 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 distance behind the aircraft. 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 condense rapidly. Exhaust contrails usually form at high altitudes; usually above 8,000 m (26,000 ft), where the air temperature is below −36.5 °C (−34 °F). They can also form closer to the ground when the air is cold and moist.[7]

A 2013–2014 study jointly supported by NASA, the German aerospace center DLR, and Canada's National Research Council NRC, determined that biofuels could reduce contrail generation. This reduction was explained by demonstrating that biofuels produce fewer soot particles, which are the nuclei around which the ice crystals form. The tests were performed by flying a DC-8 at cruising altitude with a sample-gathering aircraft flying in trail. In these samples, the contrail-producing soot particle count was reduced by 50 to 70 percent, using a 50% blend of conventional Jet A1 fuel and HEFA (hydroprocessed esters and fatty acids) biofuel produced from camelina.[8][9][10]

Condensation from decreases in pressure[edit]

A vintage P-40 Warhawk with propeller tip vortex condensation

As a wing generates lift, it causes a vortex to form at the wingtip, and at the tip of the flap when deployed (wingtips and flap-boundaries are discontinuities in airflow.) 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.

At 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.

The tips of rotating surfaces (such as propellers and rotors) sometimes produce visible contrails.[11]

In firearms, a vapor trail is sometimes observed when firing under rare conditions due to changes in air pressure around the bullet.[12][13] A vapor trail from a bullet is observable from any direction.[12] Vapor trail should not be confused with bullet trace, which is a much more common phenomenon (and is usually only observable directly from behind the shooter).[12][14]

Radiative forcing[edit]

MODIS tracking of contrails generated by air traffic over the southeastern United States

Contrails, by affecting the Earth's radiation balance, act as a radiative forcing: they trap outgoing longwave radiation emitted by the Earth and atmosphere more than they reflect incoming solar radiation. In 1992, the warming effect was estimated between 3.5 mW/m2 and 17 mW/m2.[15] Global radiative forcing has been calculated from the reanalysis data, climate models, and radiative transfer codes; estimated at 12 mW/m2 for 2005, with an uncertainty range of 5 to 26 mW/m2, and with a low level of scientific understanding.[16]

The effect varies daily and annually: night flights contribute 60 to 80% of contrail radiative forcing while accounting for 25% of daily air traffic, while winter flights contribute half of the annual mean radiative forcing while accounting for 22% of annual air traffic.[17] Contrail cirrus may be air traffic's largest radiative forcing component, larger than all CO2 accumulated from aviation, and could triple from a 2006 baseline to 160-180 mW/m2 by 2050 Without interventions.[18][19]

Condensation trails may cause regional-scale surface temperature changes for some time.[20][verification needed] NASA researched atmospheric and climatological effects of contrails, including effects on ozone, ice crystal formation, and particle composition, during the Atmospheric Effects of Aviation Project (AEAP).[21]

Bomber contrails affected climate during World War II.[22][23] A 0.8 °C (1.4 °F) hotter temperature was recorded near airbases.[24]

Diurnal temperature variation[edit]

The sky above Würzburg without contrails after air travel disruption in 2010 (left) and with regular air traffic and the right conditions (right)

The diurnal temperature variation is the difference in the day's highs and lows at a fixed station.[25] Contrails decrease the daytime temperature and increase the nighttime temperature, reducing their difference.[26]

When no commercial aircraft flew across the USA following the September 11 attacks, the diurnal temperature variation was widened by 1.1 °C (2.0 °F).[20] Measured across 4,000 weather stations in the continental United States, this increase was the largest recorded in 30 years.[20] Without contrails, the local diurnal temperature range was 1 °C (1.8 °F) higher than immediately before.[27] This was maybe due to unusually clear weather during the period.[28] In the southern US, the difference was diminished by about 3.3 °C (6 °F), and by 2.8 °C (5 °F) in the US midwest.[29]

Head-on contrails[edit]

A contrail from an airplane flying towards the observer can appear to be generated by an object moving vertically.[30][31] On 8 November 2010 in the US state of California, a contrail of this type gained media attention as a "mystery missile" that could not be explained by U.S. military and aviation authorities,[32] and its explanation as a contrail[30][31][33][34] took more than 24 hours to become accepted by U.S. media and military institutions.[35]


A distrail is the opposite of a contrail

Where an aircraft passes through a cloud, it can disperse the cloud in its path. This is known as a distrail (short for "dissipation trail"). The plane's warm engine exhaust and enhanced vertical mixing in the aircraft's wake can cause existing cloud droplets to evaporate. If the cloud is sufficiently thin, such processes can yield a cloud-free corridor in an otherwise solid cloud layer.[36] An early satellite observation of distrails that most likely were elongated, aircraft-induced fallstreak holes appeared in Corfidi and Brandli (1986).[37]

Clouds form when invisible water vapor (H2O in gas phase) condenses into microscopic water droplets (H2O in liquid phase) or into microscopic ice crystals (H2O in solid phase). This may happen when air with a high proportion of gaseous water cools. A distrail forms when the heat of engine exhaust evaporates the liquid water droplets in a cloud, turning them back into invisible, gaseous water vapor. Distrails also may arise as a result of enhanced mixing (entrainment of) drier air immediately above or below a thin cloud layer following passage of an aircraft through the cloud, as shown in the second image below:


See also[edit]


  1. ^ a b c "Aircraft Contrails Factsheet" (PDF). FAA.Gov. Archived (PDF) from the original on 28 September 2006. Retrieved 13 October 2015.
  2. ^ "vapour trail". Encyclopædia Britannica. Encyclopædia Britannica Inc. Retrieved 17 April 2012.
  3. ^ a b Sutherland, Scott (23 March 2017). "Cloud Atlas leaps into 21st century with 12 new cloud types". The Weather Network. Pelmorex Media. Retrieved 24 March 2017.
  4. ^ "Cirrus Aviaticus – Cirrus – Names of Clouds". Retrieved 13 October 2021.
  5. ^ Timperley, Jocelyn. "The fastest ways aviation could cut emissions". Retrieved 11 June 2021.
  6. ^ Ritchie, Glenn; Still, Kenneth; Rossi Iii, John; Bekkedal, Marni; Bobb, Andrew; Arfsten, Darryl (2003). "Biological and health effects of exposure to kerosene-based jet fuels and performance additives". Journal of Toxicology and Environmental Health, Part B. 6 (4): 357–451. doi:10.1080/10937400306473. PMID 12775519. S2CID 30595016.
  7. ^ "Contrail Education – FAQ". Archived from the original on 8 April 2016.
  8. ^ "The Week in Technology". Aviation Week & Space Technology. 20–24 March 2017. Paper published in Nature, Rich Moore & Hans Schlager, authors.
  9. ^ Sean Broderick (24 December 2017). "Biofuels Could Reduce Contrail Formation, Research Finds". Retrieved 13 October 2021.
  10. ^ Richard H. Moore; et al. (15 March 2017). "Biofuel blending reduces particle emissions from aircraft engines at cruise conditions" (PDF). Nature. 543 (7645): 411–415. Bibcode:2017Natur.543..411M. doi:10.1038/nature21420. PMC 8025803. PMID 28300096. S2CID 4447403. Archived (PDF) from the original on 27 April 2019.
  11. ^ "Photos from the field". Vertical Magazine, April/May 2014, p. 39. Accessed: 8 July 2014.
  12. ^ a b c Vapor trail and Bullet trace Sniper Country accessed 13 October 2021
  13. ^ Vapor Trail vs Bullet Trace YouTube accessed 13 October 2021
  14. ^ Language Lessons: TRACE Breach Bang Clear accessed 13 October 2021
  15. ^ Ponater, M.; et al. (2005). "On contrail climate sensitivity". Geophysical Research Letters. 32 (10): L10706. Bibcode:2005GeoRL..3210706P. doi:10.1029/2005GL022580.
  16. ^ Lee, D.S.; et al. (2009). "Aviation and global climate change in the 21st century" (PDF). Atmos. Environ. 43 (22): 3520–3537. Bibcode:2009AtmEn..43.3520L. doi:10.1016/j.atmosenv.2009.04.024. PMC 7185790. PMID 32362760. Archived (PDF) from the original on 16 July 2016.
  17. ^ Stuber, Nicola; et al. (15 June 2006). "The importance of the diurnal and annual cycle of air traffic for contrail radiative forcing". Nature. 441 (7095): 864–7. Bibcode:2006Natur.441..864S. doi:10.1038/nature04877. PMID 16778887. S2CID 4348401.
  18. ^ Michael Le Page (27 June 2019). "It turns out planes are even worse for the climate than we thought". New Scientist. Retrieved 13 October 2021.
  19. ^ Bock, Lisa; Burkhardt, Ulrike (2019). "Contrail cirrus radiative forcing for future air traffic". Atmospheric Chemistry and Physics. 19 (12): 8163. Bibcode:2019ACP....19.8163B. doi:10.5194/acp-19-8163-2019.
  20. ^ a b c Travis, D.J.; A. Carleton; R.G. Lauritsen (August 2002). "Contrails reduce daily temperature range". Nature. 418 (6898): 601. doi:10.1038/418601a. PMID 12167846. S2CID 4425866.
  21. ^ "The Atmospheric Effects of Aviation Project (AEAP)". Archived from the original on 20 May 2000. Retrieved 2 February 2019.
  22. ^ Irfan, Umair (7 July 2011). "World War II Bomber Contrails Show How Aviation Affects Climate". (ClimateWire). Retrieved 13 October 2021.
  23. ^ Parry, Wynne (7 July 2011). "WWII Bombing Raids Altered English Weather". Retrieved 13 October 2021.
  24. ^ Ryan, A. C.; et al. (2012). "World War II contrails: A case study of aviation-induced cloudiness". International Journal of Climatology. 32 (11): 1745–1753. Bibcode:2012IJCli..32.1745R. doi:10.1002/joc.2392. S2CID 129296874.
  25. ^ Perkins, Sid. (11 May 2002), "September's Science: Shutdown of airlines aided contrail studies", Science News, Science News Online, retrieved 13 October 2021
  26. ^ Bernhardt, J.; Carleton, A.M. (14 March 2015), "The impacts of long-lived jet contrail 'outbreaks' on surface station diurnal temperature range", Journal of International Climatology, 35 (15): 4529–4538, Bibcode:2015IJCli..35.4529B, doi:10.1002/joc.4303, S2CID 128789946
  27. ^ Travis, D.J.; A.M. Carleton; R.G. Lauritsen (March 2004). "Regional Variations in U.S. Diurnal Temperature Range for the 11–14 September 2001 Aircraft Groundings: Evidence of Jet Contrail Influence on Climate". J. Clim. 17 (5): 1123. Bibcode:2004JCli...17.1123T. doi:10.1175/1520-0442(2004)017<1123:RVIUDT>2.0.CO;2.
  28. ^ Kalkstein; Balling Jr. (2004). "Impact of unusually clear weather on United States daily temperature range following 9/11/2001". Climate Research. 26: 1. Bibcode:2004ClRes..26....1K. doi:10.3354/cr026001.
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  30. ^ a b McKee, Maggie (9 November 2010). "Mystery 'missile' likely a jet contrail, says expert". New Scientist. Archived from the original on 10 November 2010. Retrieved 10 November 2010.
  31. ^ a b West, Mick (10 November 2010). "A Problem of Perspective – New Year's Eve Contrail". Archived from the original on 12 November 2010. Retrieved 10 November 2010.
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  33. ^ Pike, John E. (November 2010). "Mystery Missile Madness". Retrieved 11 November 2010.
  34. ^ Bahneman, Liem (9 November 2010). "It was US Airways flight 808". Archived from the original on 13 November 2010. Retrieved 10 November 2010.
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  37. ^ Corfidi, Stephen; Brandli, Hank (May 1986). "GOES views aircraft distrails" (PDF). National Weather Digest. 11: 37–39. Archived (PDF) from the original on 21 April 2017. Retrieved 13 October 2021.

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