{{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's surface. They 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 at high altitudes cause the trails' formation.

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 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 clouds in ice 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 "metal oxide particles, high-temperature water vapor condensation particles, and other byproducts of 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 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 |date=26 May 2021 |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 747-438 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 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.<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> 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 {{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>

==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}}

The tips of rotating surfaces (such as propellers and 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== {{See also|Environmental impact of aviation}} Contrails' contribution to climate change might be similar to {{chem|CO|2}} emissions from aviation, but this depends on the time horizon chosen. 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 of around 0.03–0.06 watts per square meter (W/m²).<ref>{{Cite web |title=Contrails, Aviation, and Climate Change |url=https://www.rff.org/publications/issue-briefs/contrails-aviation-and-climate-change/ |access-date=2025-12-29 |website=Resources for the Future |language=en-US}}</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>{{Cite web |title=CLIMATE ACTION: ADDRESSING CONTRAILS |url=https://atag.org/media/wnqc3v41/fact-sheet_20_contrails_june-2024.pdf}}</ref> Most of the contrail heating is over Europe, North America, and the North Atlantic, whereas flights in other regions such as East Asia are often through warmer air so less likely to form contrails.<ref name=":2" />

The effect of COVID-19 reductions in flight traffic decreased contrails. The unique timing of such reductions, which were maximum in Northern Hemisphere summer, when the largest contrail cooling occurs, means that warming reductions due to fewer contrails in the spring and fall were offset by cooling reductions due to fewer contrails in summer, giving no significant annual averaged effective radiative forcing (ERF) from contrail changes in 2020. Despite no net significant global ERF, there are some land regions that cooled significantly and up to −0.2 K from what would have been expected with baseline aviation contrails. These reductions occurred in the same regions as large contrail temperature changes in the subtropical Northern Hemisphere.<ref>{{Cite journal |last1=Gettelman |first1=Andrew |last2=Chen |first2=Chieh-Chieh |last3=Bardeen |first3=Charles G. |date=2021-06-18 |title=The climate impact of COVID-19-induced contrail changes |url=https://acp.copernicus.org/articles/21/9405/2021/ |journal=Atmospheric Chemistry and Physics |language=English |volume=21 |issue=12 |pages=9405–9416 |doi=10.5194/acp-21-9405-2021 |doi-access=free |bibcode=2021ACP....21.9405G |issn=1680-7316}}</ref>

Despite the extra fuel required it is thought that rerouting (probably a slight height change) a small proportion of flights would significantly reduce overall heating, and cleaner fuels (such as sustainable aviation fuel) and newer engines might also help.<ref name=":2">{{Cite web |title=Contrails and aviation's other hidden emissions |url=https://www.transportenvironment.org/topics/planes/contrails |access-date=2025-12-29 |website=T&E |language=en}}</ref> Further research may be needed,<ref>{{Cite web |title=Airplane contrails may not be the climate villain once feared |url=https://www.science.org/content/article/airplane-contrails-may-not-be-climate-villain-once-feared |access-date=2025-12-29 |website=www.science.org |language=en}}</ref><ref>{{Cite web |title=We can drastically cut aviation's climate impact. |url=https://contrails.org/ |access-date=2025-12-29 |website=contrails.org |language=en}}</ref> such as how to estimate the systematic effects of rerouting<ref name=":1">{{Cite web |title=Contrail Avoidance Strategies |url=https://ifatca.org/wp-content/uploads/WP-2025-159.pdf}}</ref> and how much air traffic controllers' workload would increase.<ref>{{Cite web |date=2025-05-12 |title=From research to operations: MUAC is pioneering ATM Condensation Trail (CONTRAIL) avoidance measures {{!}} EUROCONTROL |url=https://www.eurocontrol.int/article/research-operations-muac-pioneering-atm-condensation-trail-contrail-avoidance-measures |access-date=2025-12-29 |website=www.eurocontrol.int |language=en}}</ref> Projects include ContrailNet from Eurocontrol,<ref>{{cite web |date=7 November 2023 |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 |access-date=12 May 2024 |website=www.eurocontrol.int |language=en}}</ref> Reviate,<ref>{{cite web |title=Reviate - Contrail avoidance for the climate |url=https://contrails.org/ |access-date=12 May 2024 |website=contrails.org}}</ref> and the Ciconia project,<ref>{{cite web |last1=Andrews |first1=Siân |date=13 December 2023 |title=Leading the Way in Contrail Avoidance |url=https://nats.aero/blog/2023/12/leading-the-way-in-contrail-avoidance/ |access-date=12 May 2024 |website=NATS Blog}}</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/ |access-date=12 May 2024 |website=Project Contrails: Preventing Contrails with AI - Google Research |language=en}}</ref> Others such as Satavia, which is part of GE, say that atmospheric regions at risk of contrails are known well enough already, and that it could be done now very cheaply, simply by modifying flight plans.<ref>{{Cite web |last=Durant |first=Adam |title=Contrail prevention through flight plan modification |url=https://www.euspa.europa.eu/sites/default/files/4_2_satavia_decisionx_nz_2022_euspa.pdf}}</ref>

Since 2025 the EU has required the warming from contrails to be monitored.<ref name=":1" /> European efforts to 2027 are coordinated by the Aviation Non-{{chem|CO|2}} Expert Network,<ref>{{Cite web |date=2020-11-25 |title=ANCEN (NONCO2) - Aviation Non-CO2 Experts Network (ANCEN) {{!}} EASA |url=https://www.easa.europa.eu/en/research-projects/ancen-nonco2 |access-date=2025-12-29 |website=www.easa.europa.eu |language=en}}</ref> but in 2025 the National Academies of Sciences, Engineering, and Medicine criticised research in the USA as "fragmented".<ref>{{Cite web |title=National Academies Call for Coordinated U.S. Contrail Climate Research Plan {{!}} AIN |url=https://www.ainonline.com/aviation-news/aerospace/2025-05-08/national-academies-call-us-contrail-research-plan |access-date=2025-12-29 |website=Aviation International News |language=en}}</ref> {{As of|2025}} most contrails cannot be seen by geostationary satellites,<ref>{{Cite journal |last1=Euchenhofer |first1=Marlene V. |last2=Prashanth |first2=Prakash |last3=Parke |first3=Sydney A. |last4=Eastham |first4=Sebastian D. |last5=Waitz |first5=Ian A. |date=2025 |title=Contrail Observation Limitations Using Geostationary Satellites |url=https://onlinelibrary.wiley.com/doi/abs/10.1029/2025GL118386 |journal=Geophysical Research Letters |language=en |volume=52 |issue=24 |article-number=e2025GL118386 |doi=10.1029/2025GL118386 |issn=1944-8007}}</ref> but they can be complemented with sun synchronous satellites and ground observations.<ref>{{Cite journal |last1=Gourgue |first1=Nicolas |last2=Boucher |first2=Olivier |last3=Barthès |first3=Laurent |date=2025-04-01 |title=A dataset of annotated ground-based images for the development of contrail detection algorithms |journal=Data in Brief |volume=59 |article-number=111364 |doi=10.1016/j.dib.2025.111364 |pmid=40027242 |pmc=11870224 |bibcode=2025DIB....5911364G |issn=2352-3409}}</ref> Satellite and ground based lidar can also help detect contrails, and visible ground observation can help show which flight created a contrail.<ref>{{Cite web |title=Contrails Research at Eurocontrol |url=https://www.eurocontrol.int/sites/default/files/2025-11/eurocontrol-showcase-summit-contrail-net.pdf}}</ref>

In November 2025 a joint study of several German research institutes showed that the climate impacts of contrails cannot yet be sufficiently estimated to justify strategies of rerouting flights.<ref name="o828">{{cite journal | last1=Petzold | first1=Andreas | last2=Khan | first2=Neelam F. | last3=Li | first3=Yun | last4=Spichtinger | first4=Peter | last5=Rohs | first5=Susanne | last6=Crewell | first6=Susanne | last7=Wahner | first7=Andreas | last8=Krämer | first8=Martina | title=Most long-lived contrails form within cirrus clouds with uncertain climate impact | journal=Nature Communications | volume=16 | issue=1 | date=2025-11-03 | issn=2041-1723 | pmid=41184289 | pmc=12583729 | doi=10.1038/s41467-025-65532-2 | url=https://www.nature.com/articles/s41467-025-65532-2 | access-date=2026-02-18 | page=}}</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 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=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/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== 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:{{Citation needed|date=December 2025}}

==See also==

* Chemtrail conspiracy theory * Cloud chamber – particle detector that works on similar principle * Fallstreak hole * Ship tracks * Skywriting * Space jellyfish * Twomey effect

==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