View source for Contrail

{{short description|Long, thin artificial clouds that sometimes form behind aircraft}}
{{hatnote group|
{{redirect|Vapor trail|other uses|Vapor Trail (disambiguation)}}
{{Distinguish|Chemtrail conspiracy theory}}
{{Other uses}}
}}
{{Use dmy dates|date=September 2019}}
{{Infobox Cloud
|name           = Contrails
|image location = File:Jet-contrails-tokyosky-japan-2018.webm
|image name     = A jet forming contrails in a blue sky
|abbreviation   =
|symbol         =
|genus          = Cirrus (''curl of hair''), cirrocumulus, or cirrostratus
|species        =
|variety        =
|altitude_m     = 7,500 to 12,000
|altitude_ft    = 25,000 to 40,000
|level          = high
|appearance     = Long bands
|precipitation  = No
| thickness      =
| Ice content    =
}}
'''Contrails''' ({{IPAc-en|ˈ|k|ɒ|n|t|r|eɪ|l|z}}; short for "condensation trails") or vapour trails are line-shaped [[clouds]] produced by [[aircraft]] engine exhaust or changes in [[air pressure]], typically at aircraft cruising altitudes several kilometres/miles above the [[Earth|Earth's]] surface. They are composed primarily of [[water]], in the form of [[Ice crystal|ice crystals]]. The combination of [[water vapor]] in aircraft engine exhaust and the low ambient temperatures at high altitudes causes the trails' formation. 

[[Chemical impurity|Impurities]] in the engine exhaust from the fuel, including [[soot]] and [[sulfur]] compounds (0.05% by weight in jet fuel) provide some of the particles that serve as [[cloud condensation nuclei]] for [[water droplet]] growth in the exhaust. If water droplets form, they can freeze to form ice particles that compose a contrail.<ref name="FAA.GOV">{{cite web |url=https://www.faa.gov/sites/faa.gov/files/regulations_policies/policy_guidance/envir_policy/contrails.pdf |archive-url=https://web.archive.org/web/20060928114935/http://www2.faa.gov/regulations_policies/policy_guidance/envir_policy/media/contrails.pdf |archive-date=2006-09-28 |url-status=live |title=Aircraft Contrails Factsheet |publisher=FAA.Gov |access-date=10 September 2023}}</ref> Their formation can also be triggered by changes in air pressure in [[wingtip vortices]], or in the air over the entire wing surface.<ref>{{cite encyclopedia |title=vapour trail |url=https://www.britannica.com/EBchecked/topic/623212/vapour-trail |encyclopedia=Encyclopædia Britannica |publisher=Encyclopædia Britannica Inc. |access-date=17 April 2012}}</ref> Contrails, and other clouds caused directly by human activity, are called homogenitus.<ref>{{cite news|last1=Sutherland|first1=Scott|title=Cloud Atlas joins the digital age while adding a dozen new cloud types|url=https://www.theweathernetwork.com/en/news/science/earth-science/international-cloud-atlas-digital-update-adds-11-new-cloud-types|access-date=13 August 2025|work=The Weather Network|agency=Pelmorex Media|date=March 23, 2017}}</ref>

The vapor trails produced by [[Rocket|rockets]] are referred to as "missile contrails"<ref>{{Cite web |date=2013-10-17 |title=The Russian Missile Contrail You May Have Missed During the Shutdown |url=https://earthobservatory.nasa.gov/blogs/earthmatters/2013/10/17/the-missile-contrail-you-may-have-missed-during-the-shutdown/ |archive-url=https://web.archive.org/web/20250217051509/https://earthobservatory.nasa.gov/blogs/earthmatters/2013/10/17/the-missile-contrail-you-may-have-missed-during-the-shutdown/ |archive-date=17 Feb 2025 |access-date=2025-02-28 |website=earthobservatory.nasa.gov |publisher=[[NASA Earth Observatory]] |language=en}}</ref> or "rocket contrails." The water vapor and aerosol produced by rockets promote the "formation of [[Ice cloud|ice clouds]] in ice [[Supersaturation|supersaturated]] layers of the atmosphere."<ref name=":0">{{Cite journal |last1=Li |first1=Chenshuo |last2=Fu |first2=Debin |last3=Wei |first3=Tianyu |date=2025-02-21 |title=Random walk dispersion model for missile contrail particles in cross-airspace environments |url=https://www.sciencedirect.com/science/article/pii/S2214914725000546 |journal=Defence Technology |volume=49 |pages=307–320 |doi=10.1016/j.dt.2025.02.015 |issn=2214-9147}}</ref><ref>{{Cite journal |last1=Voigt |first1=Ch. |last2=Schumann |first2=U. |last3=Graf |first3=K. |editor-first1=M. |editor-first2=L. |editor-first3=S. |editor-first4=L. |editor-first5=O. |editor-last1=Calabro |editor-last2=Deluca |editor-last3=Frolov |editor-last4=Galfetti |editor-last5=Haidn |date=2016-07-01 |title=Contrail formation in the tropopause region caused by emissions from an Ariane 5 rocket |url=https://ui.adsabs.harvard.edu/abs/2016EUCAS...8..183V/abstract |journal=EUCASS Proceedings Series |volume=8 |pages=183–196 |doi=10.1051/eucass/201608183 |bibcode=2016EUCAS...8..183V |isbn=978-5-94588-191-4 }}</ref> Missile contrail clouds mainly comprise "[[Oxide|metal oxide]] particles, high-temperature water vapor condensation particles, and other byproducts of [[Rocket engine|engine combustion]]."<ref name=":0" />

Depending on the temperature and humidity at the altitude where the contrails form, they may be visible for only a few seconds or minutes, or may persist for hours and spread to be several kilometres/miles wide, eventually resembling natural [[cirrus cloud|cirrus]] or [[altocumulus]] clouds.<ref name="FAA.GOV"/> Persistent contrails are of particular interest to scientists because they increase the cloudiness of the atmosphere.<ref name="FAA.GOV"/> The resulting cloud forms are formally described as homomutatus,<ref name="ICA2017">{{cite news|last1=Sutherland|first1=Scott|title=Cloud Atlas joins the digital age while adding 11 new cloud types|url=https://www.theweathernetwork.com/en/news/science/earth-science/international-cloud-atlas-digital-update-adds-11-new-cloud-types|access-date=13 August 2025|work=The Weather Network|agency=Pelmorex Media|date=March 23, 2017}}</ref> and may resemble cirrus, cirrocumulus, or cirrostratus, and are sometimes called cirrus aviaticus.<ref>{{cite web |title=Cirrus Aviaticus – Cirrus – Names of Clouds |url=http://namesofclouds.com/cirrus/cirrus-aviaticus.html |website=namesofclouds.com |access-date=13 October 2021}}</ref> Some persistent spreading contrails contribute to [[climate change]].<ref>{{Cite web |last=Timperley |first=Jocelyn |title=The fastest ways aviation could cut emissions |url=https://www.bbc.com/future/article/20210525-how-aviation-is-reducing-its-climate-emissions |access-date=2021-06-11 |website=www.bbc.com |language=en}}</ref>

==Condensation trails as a result of engine exhaust==
[[File:Qantas Boeing 747-400 VH-OJU over Starbeyevo Kustov.jpg|thumb|Contrails of a [[Boeing 747-400|Boeing 747-438]] from [[Qantas]] at {{cvt|11000|m|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 compound|inorganic]] gases, [[polycyclic aromatic hydrocarbons]], [[Oxide|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.<ref>{{Cite journal |title = Biological and health effects of exposure to kerosene-based jet fuels and performance additives|year = 2003|doi = 10.1080/10937400306473|s2cid = 30595016|last1 = Ritchie|first1 = Glenn|last2 = Still|first2 = Kenneth|last3 = Rossi Iii|first3 = John|last4 = Bekkedal|first4 = Marni|last5 = Bobb|first5 = Andrew|last6 = Arfsten|first6 = Darryl|journal = Journal of Toxicology and Environmental Health, Part B|volume = 6|issue = 4|pages = 357–451|pmid = 12775519| bibcode=2003JTEHB...6..357R }}</ref> (Micron-sized metallic particles resulting from engine wear have also been detected.{{cn|reason=this may be true but the quantities would be extremely small and vanishingly less than the water and co2: let's see the exact amounts detected|date=May 2024}}) 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 [[dew point|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 {{convert|8000|m|ft|abbr=on}}, where the air temperature is below {{convert|-36.5|C|0|lk=on}}. They can also form closer to the ground when the air is cold and moist.<ref>{{cite web|url=http://science-edu.larc.nasa.gov/contrail-edu/faq.php|title=Contrail Education – FAQ|work=nasa.gov|archive-url=https://web.archive.org/web/20160408184845/http://science-edu.larc.nasa.gov/contrail-edu/faq.php|archive-date=8 April 2016}}</ref>

A 2013–2014 study jointly supported by NASA, the German aerospace center DLR, and Canada's National Research Council NRC, determined that [[biofuel]]s 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 [[Douglas DC-8|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]].<ref>{{cite news |url= http://aviationweek.com/technology/week-technology-march-20-24-2017 |at= Paper published in ''Nature'', Rich Moore & Hans Schlager, authors |work= Aviation Week & Space Technology |title= The Week in Technology |date= 20–24 March 2017 |url-access= subscription}}</ref><ref>{{cite news |url= https://www.ainonline.com/aviation-news/business-aviation/2017-12-24/biofuels-could-reduce-contrail-formation-research-finds |title= Biofuels Could Reduce Contrail Formation, Research Finds |author= Sean Broderick |date= 24 December 2017 |access-date=13 October 2021}}</ref><ref>{{cite journal |title= Biofuel blending reduces particle emissions from aircraft engines at cruise conditions |author= Richard H. Moore|display-authors=et al |journal= Nature |volume= 543 |issue= 7645 |pages= 411–415 |date= 15 March 2017|doi= 10.1038/nature21420 |pmid= 28300096 |pmc= 8025803|bibcode= 2017Natur.543..411M |s2cid= 4447403|url= https://elib.dlr.de/112943/1/Moore_et_al_Nature_2017.pdf |archive-url=https://web.archive.org/web/20190427124937/https://elib.dlr.de/112943/1/Moore_et_al_Nature_2017.pdf |archive-date=2019-04-27 |url-status=live}}</ref>

==Condensation from decreases in pressure==
[[File:Propeller tip vortices being generated by P-40N1 Warhawk VH-ZOC at Temora.jpg|thumb|A vintage [[P-40 Warhawk]] with propeller tip vortex condensation]]
{{Main|Wingtip vortices}}

As a wing generates [[Lift (force)|lift]], it causes a [[vortex]] to form at the wingtip, and at the tip of the [[Flap (aircraft)|flap]] when deployed (wingtips and flap boundaries represent 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 [[Space Shuttle]] landings.{{citation needed|date=July 2025}}

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.{{citation needed|date=July 2025}}

At high-thrust settings the fan blades at the intake of a [[Jet engine#Turbofan|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 [[Propeller (aeronautics)|propellers]] and [[Helicopter rotor|rotors]]) sometimes produce visible contrails.<ref>{{cite web |title=Photos from the field |url=http://www.verticalmag.com/digital_issue/2014/v13i2/files/2.html |website=Vertical Magazine |access-date=8 July 2014 |page=39 |date=April–May 2014 |archive-date=16 July 2014 |archive-url=https://web.archive.org/web/20140716081046/http://www.verticalmag.com/digital_issue/2014/v13i2/files/2.html }}</ref>

In firearms, a vapor trail is sometimes observed when firing under rare conditions, due to condensation induced by changes in air pressure around the bullet.<ref name="snipercountry">{{cite web |title=Vapor trail and Bullet trace |url=https://www.snipercountry.com/bullet-trail/ |website=Sniper Country |access-date=13 October 2021 |date=9 August 2018}}</ref><ref>{{cite web |title=Vapor Trail vs Bullet Trace |url=https://www.youtube.com/watch?v=dRWTzTB_8Tg |website=YouTube | date=18 October 2017 |access-date=13 October 2021 |language=en}}</ref> A vapor trail from a bullet is observable from any direction.<ref name="snipercountry"/> Vapor trail should not be confused with [[bullet trace]], a refractive effect due to changes in air pressure as the bullet travels, which is a much more common phenomenon (and is usually only observable directly from behind the shooter).<ref name="snipercountry"/><ref>{{cite web |last1=Norseman |first1=Dave the |title=Language Lessons: TRACE |url=https://www.breachbangclear.com/language-lessons-trace/ |website=Breach Bang Clear |access-date=13 October 2021 |date=15 June 2017}}</ref>

==Impacts on climate==
[[Image:Sfc.contrail.1.26.01.JPG|thumb|[[NASA]] photograph showing aircraft contrails and natural clouds]]
It is considered that the largest contribution of aviation to climate change comes from contrails.<ref>{{cite web |author=KATIE CAMERO |title=Aviation's dirty secret: Airplane contrails are a surprisingly potent cause of global warming Warming effect of thin, white clouds will triple by 2050 |url=https://www.science.org/content/article/aviation-s-dirty-secret-airplane-contrails-are-surprisingly-potent-cause-global-warming |website=www.science.org |access-date=10 May 2024 |date=28 June 2019}}</ref>  
In general, aircraft contrails trap [[outgoing longwave radiation]] emitted by the Earth and atmosphere more than they reflect incoming [[solar radiation]], resulting in a net increase in [[radiative forcing]]. In 1992, this warming effect was estimated between 3.5&nbsp;mW/m<sup>2</sup> and 17&nbsp;mW/m<sup>2</sup>.<ref>{{cite journal|last=Ponater|first=M.|display-authors= etal |year=2005|title=On contrail climate sensitivity|journal=[[Geophysical Research Letters]]|volume=32|issue=10|pages=L10706|article-number=2005GL022580 |doi=10.1029/2005GL022580|bibcode=2005GeoRL..3210706P|doi-access=free}}</ref>
In 2009, its 2005 value was estimated at 12&nbsp;mW/m<sup>2</sup>, based on the [[Atmospheric reanalysis|reanalysis]] data, [[climate model]]s, and [[Atmospheric radiative transfer codes|radiative transfer codes]]; with an uncertainty range of 5 to 26&nbsp;mW/m<sup>2</sup>, and with a low level of scientific understanding.<ref>{{cite journal|last=Lee|first=D. S.|display-authors= etal|year=2009 |title=Aviation and global climate change in the 21st century|journal=[[Atmos. Environ.]]|volume=43|issue=22|pages=3520–3537|doi=10.1016/j.atmosenv.2009.04.024|pmid=32362760|pmc=7185790|bibcode=2009AtmEn..43.3520L|url=http://elib.dlr.de/59761/1/lee.pdf |archive-url=https://web.archive.org/web/20160716195614/http://elib.dlr.de/59761/1/lee.pdf |archive-date=2016-07-16 |url-status=live}}</ref> 
[[File:Bomber stream.jpg|thumb|left|USAAF 8th Air Force B-17s and their contrails]]
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&nbsp;mW/m<sup>2</sup> by 2050 without intervention.<ref>{{cite news |url=https://www.newscientist.com/article/2207886-it-turns-out-planes-are-even-worse-for-the-climate-than-we-thought/ |website=New Scientist |title=It turns out planes are even worse for the climate than we thought |date= 27 June 2019 |author=Michael Le Page |access-date=13 October 2021}}</ref><ref>{{cite journal |journal=Atmospheric Chemistry and Physics |title=Contrail cirrus radiative forcing for future air traffic |year=2019 |url=https://acp.copernicus.org/articles/19/8163/2019/ |last1=Bock |first1=Lisa |last2=Burkhardt |first2=Ulrike |volume=19 |issue=12 |page=8163 |doi=10.5194/acp-19-8163-2019 |bibcode=2019ACP....19.8163B |doi-access=free}}</ref> For comparison, the total radiative forcing from human activities amounted to 2.72 W/m<sup>2</sup> (with a range between 1.96 and 3.48W/m<sup>2</sup>) in 2019, and the increase from 2011 to 2019 alone amounted to 0.34W/m<sup>2</sup>.<ref name="IPCC_WGI_SPM">IPCC, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM.pdf Summary for Policymakers]. In: [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J. B. R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 3–32, {{doi|10.1017/9781009157896.001}}.</ref> Contrail effects differ a lot depending on when they are formed, as they decrease the daytime temperature and increase the nighttime temperature, reducing their difference.<ref>{{citation |author1= Bernhardt, J. |author2= Carleton, A. M. |date= 14 March 2015 |title= The impacts of long-lived jet contrail 'outbreaks' on surface station diurnal temperature range |journal= Journal of International Climatology |volume= 35 |issue= 15 |pages= 4529–4538 |doi= 10.1002/joc.4303 |bibcode= 2015IJCli..35.4529B |s2cid= 128789946 |url= https://rmets.onlinelibrary.wiley.com/doi/epdf/10.1002/joc.4303}}</ref> In 2006, it was estimated that [[Red-eye flight|night flights]] contribute 60 to 80% of contrail radiative forcing while accounting for 25% of daily air traffic, and winter flights contribute half of the annual mean radiative forcing while accounting for 22% of annual air traffic.<ref>{{cite journal |last=Stuber|first=Nicola | display-authors= etal |date= 15 June 2006 |title=The importance of the diurnal and annual cycle of air traffic for contrail radiative forcing |journal=[[Nature (journal)|Nature]] |volume=441 |issue=7095 |pages=864–7 |doi=10.1038/nature04877 |pmid=16778887 |bibcode=2006Natur.441..864S |s2cid=4348401 |url=https://www.nature.com/articles/nature04877}}</ref>

Starting from the 1990s, it was suggested that contrails during daytime have a strong cooling effect, and when combined with the warming from night-time flights, this would lead to a substantial [[diurnal temperature variation]] (the difference in the day's highs and lows at a fixed station).<ref>{{citation |author= Perkins, Sid. |title= September's Science: Shutdown of airlines aided contrail studies |journal= Science News |date= 11 May 2002 |publisher= Science News Online |url= https://www.sciencenews.org/article/septembers-science-shutdown-airlines-aided-contrail-studies  |access-date=13 October 2021}}</ref> When [[Closings and cancellations following the September 11 attacks|no commercial aircraft flew]] across the USA following the [[September 11 attacks]], the [[diurnal temperature variation]] was widened by {{cvt|1.1|C-change|F-change}}.<ref name=Travis2002Aug>{{cite journal |author= Travis, D. J. |author2= A. Carleton |author3= R. G. Lauritsen |date= August 2002 |title=Contrails reduce daily temperature range |journal= Nature |volume= 418 |issue= 6898 |page= 601 |doi= 10.1038/418601a |pmid= 12167846 |bibcode= 2002Natur.418..601T |s2cid= 4425866 |doi-access= free }}</ref> Measured across 4,000 [[weather station]]s in the continental United States, this increase was the largest recorded in 30 years.<ref name=Travis2002Aug/> Without contrails, the local diurnal temperature range was {{convert|1|°C-change|°F-change|abbr=on}} higher than immediately before.<ref>{{cite journal|last=Travis|first=D. J.|author2=A. M. Carleton|author3=R. G. Lauritsen|date=March 2004|title=Regional Variations in U.S. Diurnal Temperature Range for the 11–14 September 2001 Aircraft Groundings: Evidence of Jet Contrail Influence on Climate|journal=J. Clim.|volume=17|issue=5|page=1123|doi=10.1175/1520-0442(2004)017<1123:RVIUDT>2.0.CO;2|bibcode=2004JCli...17.1123T|url= https://journals.ametsoc.org/view/journals/clim/17/5/1520-0442_2004_017_1123_rviudt_2.0.co_2.xml}}</ref> In the southern US, the difference was diminished by about {{cvt|6|F-change|C-change|order=flip}}, and by {{cvt|5|F-change|C-change|order=flip}} in the US midwest.<ref>{{citation |url= https://www.sciencedaily.com/releases/2015/06/150618122236.htm |title= Jet contrails affect surface temperatures |work= [[Science Daily]] |date= 18 June 2015 |access-date=13 October 2021}}</ref><ref name=contrails>{{cite journal|title=Contrails reduce daily temperature range|first1=David J.|last1=Travis|last2=Carleton|first2=Andrew M.|last3=Lauritsen|first3=Ryan G.|journal=[[Nature (journal)|Nature]]|page=601|volume=418|year=2002|url=http://facstaff.uww.edu/travisd/pdf/jetcontrailsrecentresearch.pdf|doi=10.1038/418601a|pmid=12167846|issue=6898|bibcode=2002Natur.418..601T|s2cid=4425866|archive-url=https://web.archive.org/web/20060503192714/http://facstaff.uww.edu/travisd/pdf/jetcontrailsrecentresearch.pdf|archive-date=3 May 2006}}</ref> However, follow-up studies found that a natural change in cloud cover can more than explain these findings.<ref>{{cite journal|last1=Kalkstein|last2=Balling Jr.|year=2004|title=Impact of unusually clear weather on United States daily temperature range following 9/11/2001|journal=Climate Research|volume=26|page=1|doi=10.3354/cr026001|bibcode=2004ClRes..26....1K|url=http://www.int-res.com/abstracts/cr/v26/n1/p1-4/|doi-access=free}}</ref> The authors of a 2008 study wrote, "The variations in high cloud cover, including contrails and contrail-induced cirrus clouds, contribute weakly to the changes in the diurnal temperature range, which is governed primarily by lower altitude clouds, winds, and humidity."<ref>{{cite journal|year=2008|doi=10.1029/2008GL036108|title=Do contrails significantly reduce daily temperature range?|journal=Geophysical Research Letters|volume=35|issue=23|pages=L23815|bibcode=2008GeoRL..3523815H|last1=Hong|first1=Gang|last2=Yang|first2=Ping|last3=Minnis|first3=Patrick|last4=Hu|first4=Yong X.|last5=North|first5=Gerald|doi-access=free}}</ref> 
[[File:Ashcloud.png|thumb|The sky above [[Würzburg]] without contrails after [[Air travel disruption after the 2010 Eyjafjallajökull eruption|air travel disruption in 2010]] (left) and with regular air traffic and the right conditions (right)]]
In 2011, a study of British meteorological records taken during [[World War II]] identified one event where the temperature was {{convert|0.8|°C-change|°F-change|abbr=on}} higher than the day's average near [[airbase]]s used by [[USAAF]] [[strategic bomber]]s after they flew in a formation. However, its authors cautioned that this was a single event, making it difficult to draw firm conclusions from it.<ref>{{cite web|url=http://www.scientificamerican.com/article/contrails-aviation-affects-climate/|title=World War II Bomber Contrails Show How Aviation Affects Climate|first=Umair |last=Irfan |work=scientificamerican.com (ClimateWire) |date=7 July 2011 |access-date=13 October 2021}}</ref><ref>{{cite web|url=http://www.livescience.com/14944-wwii-bombing-raids-contrails-weather-climate.html|title=WWII Bombing Raids Altered English Weather |work=livescience.com |last=Parry|first=Wynne |date=7 July 2011 |access-date=13 October 2021}}</ref><ref>{{Cite journal|last1=Ryan|first1=A. C.|display-authors= etal |title=World War II contrails: A case study of aviation-induced cloudiness|journal=International Journal of Climatology|year=2012|volume=32|issue=11|pages=1745–1753|doi=10.1002/joc.2392|bibcode=2012IJCli..32.1745R|s2cid=129296874 |doi-access=free}}</ref> Then, the global response to the [[2020 coronavirus pandemic]] led to a reduction in global air traffic of nearly 70% relative to 2019. Thus, it provided an extended opportunity to study the impact of contrails on regional and global temperature. Multiple studies found "no significant response of diurnal surface air temperature range" as the result of contrail changes, and either "no net significant global ERF" (effective [[radiative forcing]]) or a very small warming effect.<ref>{{cite journal|date=29 September 2021|title=An Observational Constraint on Aviation-Induced Cirrus From the COVID-19-Induced Flight Disruption|journal=Geophysical Research Letters|volume=48|issue=20|article-number=e2021GL095882|last1=Digby|first1=Ruth A. R.|last2=Gillett|first2=Nathan P.|last3=Monahan|first3=Adam H.|last4=Cole|first4=Jason N. S. |doi=10.1029/2021GL095882 |pmid=34924638 |pmc=8667656 |bibcode=2021GeoRL..4895882D |doi-access=free}}</ref><ref>{{cite journal|date=18 June 2021|url=https://acp.copernicus.org/articles/21/9405/2021/ |doi=10.5194/acp-21-9405-2021|title=The climate impact of COVID-19-induced contrail changes|journal=Atmospheric Chemistry and Physics|volume=21|pages=9405–9416|last1=Gettelman|first1=Andrew|last2=Chen|first2=Chieh-Chieh|last3=Bardeen|first3=Charles G.|issue=12 |bibcode=2021ACP....21.9405G |doi-access=free}}</ref><ref>{{cite journal |last1=Zhu |first1=Jialei |last2=Penner |first2=Joyce E. |last3=Garnier |first3=Anne |last4=Boucher |first4=Olivier |last5=Gao |first5=Meng |last6=Song |first6=Lei |last7=Deng |first7=Junjun |last8=Liu |first8=Cong-qiang |last9=Fu |first9=Pingqing |date=18 March 2022 | title=Decreased Aviation Leads to Increased Ice Crystal Number and a Positive Radiative Effect in Cirrus Clouds |journal=AGU Advances | volume=3 |issue=2 |page=ee2020GL089788 |doi=10.1029/2021AV000546 |bibcode=2022AGUA....300546Z |doi-access=free |hdl=2027.42/172020 |hdl-access=free }}</ref> 

An EU project launched in 2020 aims to assess the feasibility of minimising contrail effects by the operational choices in making flight plans.<ref>{{cite web |title=A unique opportunity to accelerate development {{!}} EUROCONTROL |url=https://www.eurocontrol.int/article/unique-opportunity-accelerate-development |website=www.eurocontrol.int |access-date=10 May 2024 |language=en |date=16 November 2020}}</ref> Other similar projects include ContrailNet from Eurocontrol,<ref>{{cite web |title=EUROCONTROL launches ContrailNet - the new network to create a common repository of contrail observation data {{!}} EUROCONTROL |url=https://www.eurocontrol.int/news/eurocontrol-launches-contrailnet-new-network-create-common-repository-contrail-observation |website=www.eurocontrol.int |access-date=12 May 2024 |language=en |date=7 November 2023}}</ref>    Reviate,<ref>{{cite web |title=Reviate - Contrail avoidance for the climate |url=https://contrails.org/ |website=contrails.org |access-date=12 May 2024}}</ref> and the Ciconia project,<ref>{{cite web |last1=Andrews |first1=Siân |title=Leading the Way in Contrail Avoidance |url=https://nats.aero/blog/2023/12/leading-the-way-in-contrail-avoidance/ |website=NATS Blog |access-date=12 May 2024 |date=13 December 2023}}</ref> as well as Google's 'project contrails'.<ref>{{cite web |title=Project Contrails: Preventing Contrails with AI - Google Research |url=https://sites.research.google/contrails/ |website=Project Contrails: Preventing Contrails with AI - Google Research |access-date=12 May 2024 |language=en}}</ref>

==Head-on contrails==
A contrail from an airplane flying towards the observer can appear to be generated by an object moving vertically.<ref name="NS">{{cite magazine|first=Maggie|last=McKee|title=Mystery 'missile' likely a jet contrail, says expert|date=9 November 2010|magazine=[[New Scientist]]|url=https://www.newscientist.com/article/dn19704-mystery-missile-likely-a-jet-contrail-says-expert.html|access-date=10 November 2010|archive-url=https://web.archive.org/web/20101110210639/http://www.newscientist.com/article/dn19704-mystery-missile-likely-a-jet-contrail-says-expert.html|archive-date=10 November 2010|url-status=live}}</ref><ref name=cont>{{cite web|last=West|first=Mick|title=A Problem of Perspective – New Year's Eve Contrail|date=10 November 2010|url=http://contrailscience.com/a-problem-of-perspective-in-the-oc-new-years-eve-contrail/|access-date=10 November 2010|archive-url=https://web.archive.org/web/20101112192207/http://contrailscience.com/a-problem-of-perspective-in-the-oc-new-years-eve-contrail/|archive-date=12 November 2010|url-status=live}}</ref> On 8 November 2010 in the US state of [[California]], a [[2010 California contrail incident|contrail of this type]] gained media attention as a "mystery missile" that could not be explained by U.S. military and aviation authorities,<ref>{{cite news|title=Pentagon Can't Explain "Missile" off California|date=9 November 2010|publisher=[[CBS]]|url=https://www.cbsnews.com/news/pentagon-cant-explain-missile-off-california/|access-date=10 November 2010|archive-url=https://web.archive.org/web/20101110212004/http://www.cbsnews.com/stories/2010/11/09/national/main7037857.shtml|archive-date=10 November 2010|url-status=live}}</ref> and its explanation as a contrail<ref name="NS"/><ref name=cont/><ref>{{cite web|last=Pike|first=John E.|title=Mystery Missile Madness|publisher=GlobalSecurity.org|date=November 2010|url=http://www.globalsecurity.org/org/news/2010/101110-contrail.htm|access-date=11 November 2010}}</ref><ref>{{cite web|last=Bahneman|first=Liem|title=It was US Airways flight 808|date=9 November 2010|url=http://blog.bahneman.com/content/it-was-us-airways-flight-808|access-date=10 November 2010|archive-url=https://web.archive.org/web/20101113060703/http://blog.bahneman.com/content/it-was-us-airways-flight-808|archive-date=13 November 2010|url-status=live}}</ref> took more than 24 hours to become accepted by U.S. media and military institutions.<ref>{{cite news |title=Pentagon: 'Mystery missile' was probably airplane |date=10 November 2010 |publisher=[[Mercury News]]/[[Associated Press|AP]] |url=http://www.mercurynews.com/news/ci_16574898 |access-date=11 November 2010 |archive-url=https://web.archive.org/web/20120112232749/http://www.mercurynews.com/news/ci_16574898 |archive-date=12 January 2012 }}</ref>

==Distrails==
[[File:10sec old Distrail in Hong Kong.jpg|thumb|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.<ref>{{cite web |title=Distrail on Earth Science Picture of the Day |url=http://epod.usra.edu/archive/epodviewer.php3?oid=110784 |website=epod.usra.edu |access-date=11 January 2008 |archive-date=16 October 2002 |archive-url=https://web.archive.org/web/20021016164705/http://epod.usra.edu/archive/epodviewer.php3?oid=110784 }}</ref> An early satellite observation of distrails that most likely were elongated, aircraft-induced [[fallstreak holes]] appeared in Corfidi and Brandli (1986).<ref>{{cite journal |last=Corfidi|first=Stephen |author2=Brandli, Hank |title=GOES views aircraft distrails |journal=National Weather Digest |date=May 1986 |volume=11 |pages=37–39 |url=http://nwafiles.nwas.org/digest/papers/1986/Vol11-Issue2-May1986/Pg37-Corfidi.pdf |archive-url=https://web.archive.org/web/20170421061015/http://nwafiles.nwas.org/digest/papers/1986/Vol11-Issue2-May1986/Pg37-Corfidi.pdf |archive-date=2017-04-21 |url-status=live  |access-date=13 October 2021}}</ref>

Clouds form when invisible water vapor condenses into microscopic water droplets or into microscopic ice crystals. 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==
{{Div col|colwidth=25em}}
* [[Chemtrail conspiracy theory]]
* [[Cirrus cloud]]
* [[Cloud chamber]] – particle detector that works on similar principle
* [[Environmental impact of aviation]]
* [[Fallstreak hole]]
* [[Global dimming]]
* [[Ship tracks]]
* [[Skywriting]]
* [[Space jellyfish]]
* [[Twomey effect]]
{{Div col end}}
{{clear}}

==References==
{{Reflist}}

==External links==
{{Commons category|Contrails}}
{{Wiktionary}}
* [https://web.archive.org/web/20070202122112/http://asd-www.larc.nasa.gov/GLOBE/ Contrail Education] (archived) | NASA
* [http://www.contrails.nl/index.htm Contrails.nl: Contrails and AviationSmog] {{Webarchive|url=https://web.archive.org/web/20110220031116/http://www.contrails.nl/index.htm |date=20 February 2011 }} | Galleys of contrails and aviation smog
* [http://contrailscience.com/ Contral Science] | Reference site for debunking weird stories about contrails
* {{Skeptoid | id=4027 | number=27 | title= Chemtrails: Real or Not?| date= 15 February 2007}}

{{Cirrus cloud types}}
{{Cloud types}}

{{Authority control}}

[[Category:Aviation meteorology]]
[[Category:Cloud types]]
[[Category:Cirrus]]
[[Category:Cloud and fog physics]]
[[Category:Climate forcing]]
[[Category:Articles containing video clips]]