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"Vapor trail" redirects here. For other uses, see Vapor Trail (disambiguation).
For other uses, see Contrail (disambiguation).
Aviaticus cloud
Engine exhaust contrails
Engine exhaust contrails
Genus Cirrus (curl of hair), cirrocumulus, or cirrostratus
Altitude 7,620 to 12,192 m
(25,000 to 40,000 ft)
Classification Family A (High-level)
Appearance long bands
Precipitation cloud? No

Contrails (/ˈkɒntrlz/; short for "condensation trails") or vapor trails are long, thin artificial 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 changes in air pressure in wingtip vortices or in the air over the entire wing surface.[1] 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 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. The resulting cloud forms may resemble cirrus, cirrocumulus, or cirrostratus, and are sometimes called cirrus aviaticus. Persistent spreading contrails are thought by some, without overwhelming scientific proof, to have a significant effect on global climate.[2] According to the FAA, in conjunction with scientific experts at the EPA, NASA and NOAA, "Contrail cloudiness might contribute to human-induced climate change. Climate change may have important impacts on public health and environmental protection .... Changes in cloudiness resulting from human activities are important because they might contribute to long-term changes in the Earth’s climate. Contrails’ possible climate effects are one component of aviation’s expected overall climate effect." [3]

Condensation from engine exhaust[edit]

Contrails from a Qantas Boeing 747-400 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 condense rapidly. Exhaust contrails usually form at very high altitudes; usually above 8,000 m (26,000 ft), where the air temperature is below −36.5 °C (−34 °F), and the relative humidity is over 60%. They can also form closer to the ground when the air is very cold and has enough moisture.[4]

Condensation from decreases in pressure[edit]

Main article: Wingtip vortices



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.

C-27J Spartan with propeller tip vortices condensation.

The tips of rotating surfaces (such as propellers and rotors) sometimes produce visible contrails.[5]

Contrails and climate[edit]

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). NASA conducted a great deal of detailed research on atmospheric and climatological effects of contrails, including effects on ozone, ice crystal formation, and particle composition, during the Atmospheric Effects of Aviation Project (AEAP).[6] 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 (watts per square meter) for 2005, with an uncertainty range of 0.005 to 0.026 W/m2, and with a low level of scientific understanding.[7] Therefore, the overall net effect of contrails is positive, i.e. a warming effect.[8] 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.[9]

September 11, 2001 climate impact study[edit]

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;[10] however, it has also been suggested that this was due to unusually clear weather during the period.[11]

Condensation trails have been suspected of causing "regional-scale surface temperature" changes for some time.[12][13] 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[clarify] was observed in surface temperature change, measured across over 4,000 reporting stations in the continental United States.[12] Travis' research documented an "anomalous increase in the average diurnal temperature change".[12] The diurnal temperature range (DTR) is the difference in the day's highs and lows at any weather reporting station.[14] Travis observed a 1.8 °C (3.24 °F) departure from the two adjacent three-day periods to the 11th–14th.[12] This increase was the largest recorded in 30 years, more than "2 standard deviations away from the mean DTR".[12]

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.[15]

Head-on contrails[edit]

A contrail from an airplane flying towards the observer can appear to be generated by an object moving vertically.[16][17] 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,[18] and its explanation as a contrail[16][17][19][20] took more than 24 hours to become accepted by U.S. media and military institutions.[21]


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

Where an aircraft passes through a cloud, it can clear a path through it; this is known as a distrail (short for "dissipation trail"). The plane's warm engine exhaust causes existing water droplets to evaporate, leaving a clear wake through an otherwise cloudy sky.[22]

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.


See also[edit]


  1. ^ "vapour trail". Encyclopædia Britannica. Encyclopædia Britannica Inc. 2012. Retrieved 17 April 2012. 
  3. ^
  4. ^ NASA, Contrail Education FAQ
  5. ^ "Photos from the field" page 39 Vertical Magazine, April/May 2014. Accessed: 8 July 2014.
  6. ^ |url=
  8. ^ Ponater, M.; S. Marquart, R. Sausen and U. Schumann (2005). "On contrail climate sensitivity". Geophysical Research Letters 32 (10): L10706. Bibcode:2005GeoRL..3210706P. doi:10.1029/2005GL022580. Retrieved 2008-11-21. 
  9. ^ Stuber, Nicola; Piers Forster; Gaby Rädel; Keith Shine (2006-06-15). "The importance of the diurnal and annual cycle of air traffic for contrail radiative forcing". Nature 441 (7095): 864–867. Bibcode:2006Natur.441..864S. doi:10.1038/nature04877. PMID 16778887. 
  10. ^ Travis, D.J.; A.M. Carleton and 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" (PDF). J. Clim. 17 (5): 1123–1134. Bibcode:2004JCli...17.1123T. doi:10.1175/1520-0442(2004)017<1123:RVIUDT>2.0.CO;2. ISSN 1520-0442. Retrieved 2008-11-06. 
  12. ^ a b c d e Travis, D.J.; A. Carleton and R.G. Lauritsen (August 2002). "Contrails reduce daily temperature range". Nature 418 (6898): 601. Bibcode:2002Natur.418..601T. doi:10.1038/418601a. PMID 12167846. 
  15. ^ Ryan, A. C.; MacKenzie, A. R.; Watkins, S.; Timmis, R. (2012). "World War II contrails: A case study of aviation-induced cloudiness". International Journal of Climatology 32 (11): 1745. doi:10.1002/joc.2392.  edit
  17. ^ a b West, Mick (2010-11-10). "A Problem of Perspective – New Year's Eve Contrail". Archived from the original on 2010-11-10. Retrieved 2010-11-10. 
  19. ^ Pike, John E. (November 2010). "Mystery Missile Madness". Retrieved 2010-11-11. 
  22. ^ Distrail on Earth Science Picture of the Day

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