Contrails (/ˈkɒntreɪlz/; 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 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 causes 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.^[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 caused directly by human activity, are called homogenitus.^[3]

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.^[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]

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.^{[citation needed]}) 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]

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 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.

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 condensation induced by 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, 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).^[12][14]

It is considered that the largest contribution of aviation to climate change comes from contrails.^[15] 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 mW/m² and 17 mW/m².^[16] In 2009, its 2005 value was estimated at 12 mW/m², based on the reanalysis data, climate models, and radiative transfer codes; with an uncertainty range of 5 to 26 mW/m², and with a low level of scientific understanding.^[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 intervention.^[18][19] For comparison, the total radiative forcing from human activities amounted to 2.72 W/m² (with a range between 1.96 and 3.48W/m²) in 2019, and the increase from 2011 to 2019 alone amounted to 0.34W/m².^[20] 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.^[21] In 2006, it was estimated that 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.^[22]

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).^[23] 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).^[24] Measured across 4,000 weather stations in the continental United States, this increase was the largest recorded in 30 years.^[24] Without contrails, the local diurnal temperature range was 1 °C (1.8 °F) higher than immediately before.^[25] 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.^[26][27] However, follow-up studies found that a natural change in cloud cover can more than explain these findings.^[28] 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."^[29]

In 2011, a study of British meteorological records taken during World War II identified one event where the temperature was 0.8 °C (1.4 °F) higher than the day's average near airbases used by USAAF strategic bombers 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.^[30][31][32] 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.^[33][34][35]

An EU project launched in 2020 aims to assess the feasibility of minimising contrail effects by the operational choices in making flight plans.^[36] Other similar projects include ContrailNet from Eurocontrol,^[37] Reviate,^[38] and the Ciconia project,^[39] as well as Google's 'project contrails'.^[40]

A contrail from an airplane flying towards the observer can appear to be generated by an object moving vertically.^[41][42] 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,^[43] and its explanation as a contrail^{[41][42][44][45]} took more than 24 hours to become accepted by U.S. media and military institutions.^[46]

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.^[47] An early satellite observation of distrails that most likely were elongated, aircraft-induced fallstreak holes appeared in Corfidi and Brandli (1986).^[48]

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:

Wikimedia Commons has media related to Contrails.

Look up contrail in Wiktionary, the free dictionary.

• Contrail Education (archived) | NASA
• Contrails.nl: Contrails and AviationSmog Archived 20 February 2011 at the Wayback Machine | Galleys of contrails and aviation smog
• Contral Science | Reference site for debunking weird stories about contrails
• Dunning, Brian (15 February 2007). "Skeptoid #27: Chemtrails: Real or Not?". Skeptoid.