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Contrails (; short for "condensation trails") or vapor trails are line-shaped
clouds In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may comp ...
produced by
aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines. ...
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 Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
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. Their formation can also be triggered by changes in air pressure in
wingtip vortices Wingtip vortices are circular patterns of rotating air left behind a wing as it generates lift.Clancy, L.J., ''Aerodynamics'', section 5.14 One wingtip vortex trails from the tip of each wing. Wingtip vortices are sometimes named ''trailing' ...
or in the air over the entire wing surface. Contrails, and other clouds directly resulting from human activity, are collectively named homogenitus. 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 Cirrus may refer to: Science *Cirrus (biology), any of various thin, thread-like structures on the body of an animal * Cirrus (botany), a tendril *Infrared cirrus, in astronomy, filamentary structures seen in infrared light *Cirrus cloud, a typ ...
or
altocumulus Altocumulus (From Latin ''Altus'', "high", ''cumulus'', "heaped") is a middle-altitude cloud genus that belongs mainly to the ''stratocumuliform'' physical category characterized by globular masses or rolls in layers or patches, the individual el ...
clouds. Persistent contrails are of particular interest to scientists because they increase the cloudiness of the atmosphere. The resulting cloud forms are formally described as homomutatus, and may resemble cirrus, cirrocumulus, or cirrostratus, and are sometimes called cirrus aviaticus. Some persistent spreading contrails contribute to climate change.


Condensation trails as a result of engine exhaust

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 Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature. High vapor pressure correlates with a low boiling point, which relates to the number of the sample's molecules in the surrounding air, a t ...
,
inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistr ...
gases,
polycyclic aromatic hydrocarbons A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. P ...
, oxygenated organics,
alcohols In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl () functional group bound to a saturated carbon atom. The term ''alcohol'' originally referred to the primary alcohol ethanol (ethyl alcohol), which is ...
,
ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lowe ...
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. (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 Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity depen ...
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 , where the air temperature is below . They can also form closer to the ground when the air is cold and moist. A 2013–2014 study jointly supported by NASA, the German aerospace center DLR, and Canada's National Research Council NRC, determined that
biofuel Biofuel is a fuel that is produced over a short time span from biomass, rather than by the very slow natural processes involved in the formation of fossil fuels, such as oil. According to the United States Energy Information Administration (EIA ...
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
DC-8 The Douglas DC-8 (sometimes McDonnell Douglas DC-8) is a long-range narrow-body airliner built by the American Douglas Aircraft Company. After losing the May 1954 US Air Force tanker competition to the Boeing KC-135, Douglas announced in Jul ...
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 ''Camelina'' is a genus within the flowering plant family Brassicaceae. The ''Camelina'' species, commonly known as false flax, are native to Mediterranean regions of Europe and Asia. Most species of this genus have been little studied, with the ...
.


Condensation from decreases in pressure

As a wing generates
lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobile ...
, it causes a
vortex In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...
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 Wingtip vortices are circular patterns of rotating air left behind a wing as it generates lift.Clancy, L.J., ''Aerodynamics'', section 5.14 One wingtip vortex trails from the tip of each wing. Wingtip vortices are sometimes named ''trailing' ...
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 Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program nam ...
. 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 The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the ''turbo'' portion refers to a gas turbine engine which achieves mechanical ...
reach
transonic Transonic (or transsonic) flow is air flowing around an object at a speed that generates regions of both subsonic and supersonic airflow around that object. The exact range of speeds depends on the object's critical Mach number, but 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 A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upo ...
and
rotors Rotor may refer to: Science and technology Engineering *Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator *Helicopter rotor, the rotary wing(s) of a rotorcraft ...
) sometimes produce visible contrails. In firearms, a vapor trail is sometimes observed when firing under rare conditions due to changes in air pressure around the bullet.Vapor trail and Bullet trace
''Sniper Country'' accessed 13 October 2021
A vapor trail from a bullet is observable from any direction. Vapor trail should not be confused with
bullet trace A bullet is a kinetic projectile, a component of firearm ammunition that is shot from a gun barrel. Bullets are made of a variety of materials, such as copper, lead, steel, polymer, rubber and even wax. Bullets are made in various shapes and con ...
, which is a much more common phenomenon (and is usually only observable directly from behind the shooter).


Radiative forcing

Contrails, by affecting the Earth's radiation balance, act as a
radiative forcing Radiative forcing (or climate forcing) is the change in energy flux in the atmosphere caused by Climate variability and change, natural or Human impact on the environment#Impacts on climate, anthropogenic factors of climate change as measured by ...
: they trap
outgoing longwave radiation Outgoing Long-wave Radiation (OLR) is electromagnetic radiation of wavelengths from 3–100 μm emitted from Earth and its atmosphere out to space in the form of thermal radiation. It is also referred to as up-welling long-wave radiation an ...
emitted by the Earth and atmosphere more than they reflect incoming
solar radiation Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/ ...
. In 1992, the warming effect was estimated between 3.5  mW/ m2 and 17 mW/m2. Global radiative forcing has been calculated from the reanalysis data,
climate model Numerical climate models use quantitative methods to simulate the interactions of the important drivers of climate, including atmosphere, oceans, land surface and ice. They are used for a variety of purposes from study of the dynamics of the c ...
s, 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. 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 Winter is the coldest season of the year in polar and temperate climates. It occurs after autumn and before spring. The tilt of Earth's axis causes seasons; winter occurs when a hemisphere is oriented away from the Sun. Different cultures ...
flights contribute half of the annual mean radiative forcing while accounting for 22% of annual air traffic. Contrail cirrus may be air traffic's largest radiative forcing component, larger than all accumulated from aviation, and could triple from a 2006 baseline to 160-180 mW/m2 by 2050 Without interventions. Condensation trails may cause
regional In geography, regions, otherwise referred to as zones, lands or territories, are areas that are broadly divided by physical characteristics (physical geography), human impact characteristics (human geography), and the interaction of humanity and t ...
-scale surface temperature changes for some time. 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). Bomber contrails affected climate during World War II. A hotter temperature was recorded near airbases.


Diurnal temperature variation

The
diurnal temperature variation In meteorology, diurnal temperature variation is the variation between a high air temperature and a low temperature that occurs during the same day. Temperature lag Temperature lag is an important factor in diurnal temperature variation: peak da ...
is the difference in the day's highs and lows at a fixed station. Contrails decrease the daytime temperature and increase the nighttime temperature, reducing their difference. When no commercial aircraft flew across the USA following the September 11 attacks, the
diurnal temperature variation In meteorology, diurnal temperature variation is the variation between a high air temperature and a low temperature that occurs during the same day. Temperature lag Temperature lag is an important factor in diurnal temperature variation: peak da ...
was widened by . Measured across 4,000
weather station A weather station is a facility, either on land or sea, with instruments and equipment for measuring atmospheric conditions to provide information for weather forecasts and to study the weather and climate. The measurements taken include temperat ...
s in the continental United States, this increase was the largest recorded in 30 years. Without contrails, the local diurnal temperature range was higher than immediately before. This was maybe due to unusually clear
weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the t ...
during the period. In the southern US, the difference was diminished by about , and by in the US midwest.


Head-on contrails

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


Distrails

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. An early satellite observation of distrails that most likely were elongated, aircraft-induced fallstreak holes appeared in Corfidi and Brandli (1986). Clouds form when invisible water vapor ( in gas phase) condenses into microscopic water droplets ( in liquid phase) or into microscopic ice crystals ( 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:


Gallery

File:Contrails over Nova Scotia.jpeg, Multiple contrails above
Nova Scotia Nova Scotia ( ; ; ) is one of the thirteen provinces and territories of Canada. It is one of the three Maritime provinces and one of the four Atlantic provinces. Nova Scotia is Latin for "New Scotland". Most of the population are native Engl ...
File:Contrail.jpg, Contrails formed due to two jets crossing over File:Aa oldcontrails01.jpg, Airliner contrails, some new, some old, dispersed by wind shear File:Con Trail Virginia.JPG, A contrail over southwest
Virginia Virginia, officially the Commonwealth of Virginia, is a state in the Mid-Atlantic and Southeastern regions of the United States, between the Atlantic Coast and the Appalachian Mountains. The geography and climate of the Commonwealth are ...
File:2015-11-12 13 57 13 Side-by-side contrail and distrail above the National Weather Service's Baltimore-Washington Weather Forecast Office in Sterling, Virginia.jpg, A contrail casting a shadow onto a lower cloud layer File:Airplane contrail.jpg, Airplane contrail File:Missile (6872511711).jpg, Over the
City of Culture of Galicia The City of Culture of Galicia ( gl, Cidade da Cultura de Galicia or simply ''Cidade da Cultura'') is a complex of cultural buildings in Santiago de Compostela, Province of A Coruña, Galicia, Spain, designed by a group of architects led by Pete ...
File:Distrail_2020_03_19_BA_home_IMG_5146.png, Distrail – a narrow line of clearance produced by a passing airplane, here produced in a patch of thin cirrostratus. File:Distrail splits stratocumulus cloud - remnants of contrail visible ( DSC2134-1).jpg, Distrail splits stratocumulus cloud – remnants of contrail visible (see annotations on Wiki Commons description page)
File:Iridescent contrail.jpg,
Iridescent Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to gradually change color as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, feathers, butterfl ...
contrails from a
Boeing 747 The Boeing 747 is a large, long-range wide-body aircraft, wide-body airliner designed and manufactured by Boeing Commercial Airplanes in the United States between 1968 and 2022. After introducing the Boeing 707, 707 in October 1958, Pan Am w ...
, when the sun shines through a group of similarly sized water droplets at a relatively small angle File:Vapour trails.jpg, An
Airbus A340 The Airbus A340 is a long-haul, long-range, wide-body passenger airliner that was developed and produced by Airbus. In the mid-1970s, Airbus conceived several derivatives of the Airbus A300, A300, its first airliner, and developed the A340 qu ...
of Lufthansa produces contrails File:Mig-29s intercepeted by F-15s - DF-ST-90-05759.jpg, Two USAF
F-15 The McDonnell Douglas F-15 Eagle is an American twin-engine, all-weather tactical fighter aircraft designed by McDonnell Douglas (now part of Boeing). Following reviews of proposals, the United States Air Force selected McDonnell Douglas's ...
s approaching Soviet
MiG-29 The Mikoyan MiG-29 (russian: Микоян МиГ-29; NATO reporting name: Fulcrum) is a twin-engine fighter aircraft designed in the Soviet Union. Developed by the Mikoyan design bureau as an air superiority fighter during the 1970s, the M ...
s File:BOEING 737 & SUN (13959950456).jpg, Contrails in front of the sun A340-313X.jpg, Engine exhaust contrails behind an
Airbus A340 The Airbus A340 is a long-haul, long-range, wide-body passenger airliner that was developed and produced by Airbus. In the mid-1970s, Airbus conceived several derivatives of the Airbus A300, A300, its first airliner, and developed the A340 qu ...
from
Swiss International Air Lines Swiss International Air Lines AG, colloquially known as SWISS, is the flag carrier of Switzerland, operating scheduled services in Europe and to North America, South America, Africa and Asia. Zurich Airport serves as its sole hub and Genev ...
File:340bs-97bg-b-17s.jpg,
B-17 The Boeing B-17 Flying Fortress is a four-engined heavy bomber developed in the 1930s for the United States Army Air Corps (USAAC). Relatively fast and high-flying for a bomber of its era, the B-17 was used primarily in the European Theater ...
bombers over Europe, 1944 File:Bomber stream.jpg, USAAF 8th Air Force B-17s and their contrails File:Fighter plane contrails in the sky.jpg, Fighter plane contrails in the sky during the Battle of the Philippine Sea on 19 June 1944 File:Antonov 225 passing over Birmingham - 2020-08-02 - Andy Mabbett - 04 (cropped).jpg, The unique Antonov 225 had distinctive sextuple vapor trails File:Cirrocumulus-virga-homogenitus.jpg,
Virga In meteorology, a virga, also called a dry storm, is an observable streak or shaft of precipitation falling from a cloud that evaporates or sublimates before reaching the ground. A shaft of precipitation that does not evaporate before re ...
falling from contrail File:Circular contrail, Cambridge City airport.jpg, circular contrail at
Cambridge Cambridge ( ) is a university city and the county town in Cambridgeshire, England. It is located on the River Cam approximately north of London. As of the 2021 United Kingdom census, the population of Cambridge was 145,700. Cambridge becam ...
City airport


See also

*
Chemtrail conspiracy theory The chemtrail conspiracy theory is the erroneous belief that long-lasting condensation trails are "chemtrails" consisting of chemical or biological agents left in the sky by high-flying aircraft, sprayed for nefarious purposes undisclosed to ...
*
Cirrus cloud Cirrus ( cloud classification symbol: Ci) is a genus of high cloud made of ice crystals. Cirrus clouds typically appear delicate and wispy with white strands. Cirrus are usually formed when warm, dry air rises, causing water vapor deposition o ...
*
Cloud chamber A cloud chamber, also known as a Wilson cloud chamber, is a particle detector used for visualizing the passage of ionizing radiation. A cloud chamber consists of a sealed environment containing a supersaturated vapour of water or alcohol. An ...
– particle detector that works on similar principle *
Environmental impact of aviation Like other emissions resulting from fossil fuel combustion, aircraft engines produce gases, noise, and particulates, raising environmental concerns over their global effects and their effects on local air quality. Jet airliners contribute t ...
* Fallstreak hole *
Global dimming Global dimming is the reduction in the amount of global direct irradiance at the Earth's surface that has been observed since systematic measurements began in the 1950s. The effect varies by location, but worldwide it has been estimated to be of ...
*
Ship tracks Ship tracks are clouds that form around the exhaust released by ships into the still ocean air. Water molecules collect around the tiny particles ( aerosols) from exhaust to form a cloud seed. More and more water accumulates on the seed until ...
*
Skywriting Skywriting is the process of using one or more small aircraft, able to expel special smoke during flight, to fly in certain patterns that create writing readable from the ground. These messages can be advertisements, general messages of celebrat ...
* Space jellyfish * Twomey effect


References


External links


Contrail Education
(archived) , NASA

, Galleys of contrails and aviation smog
Contral Science
, Reference site for debunking weird stories about contrails * {{Authority control Aviation meteorology Cloud types Cirrus Cloud and fog physics Climate forcing