Contrail

Aviaticus cloud
Engine exhaust contrails forming behind an Airbus A340
Genus	Cirrus (curl of hair), cirrocumulus, or cirrostratus
Altitude	7,500 to 12,000 m (25,000 to 40,000 ft)
Classification	Family A (High-level)
Appearance	long bands
Precipitation cloud?	No

Contrails (/ˈkɒntreɪlz/; short for "condensation trails") are line-shaped clouds produced by aircraft engine exhaust, typically at aircraft cruise 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 compounds (0.05% by weight in jet fuel) provide some of the particles that can serve as sites for water droplet growth in the exhaust and, if water droplets form, they might 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 directly resulting from human activity, are collectively named homogenitus.^[3]

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 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. Persistent spreading contrails are suspected to have an effect on global climate.^[4][5]

Condensation from engine exhaust

Contrails from a Qantas Boeing 747-400 at 11,000 m (36,000 ft)

The main products of hydrocarbon fuel combustion are carbon dioxide and water vapor. At high altitudes this water vapor emerges into a cold environment, and the local increase in water vapor can raise the relative humidity of the air past saturation point. The vapor then condenses into tiny water droplets which freeze if the temperature is low enough. These millions of tiny water droplets and/or ice crystals form the contrails. The time taken for the vapor to cool enough to condense accounts for the contrail forming some 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.^[6]

A 2013-2014 study jointly hosted by NASA, 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 (hyprocessed esters and fatty acids) biofuel produced from camelina.^[7]

Condensation from decreases in pressure

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 are 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 landing of the Space Shuttle.

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.

During high-thrust settings the fan blades at the intake of a turbofan engine reach transonic speeds, causing a sudden drop in air pressure. This creates the condensation fog (inside the intake) which is often observed by air travelers during takeoff. For more information see the Prandtl-Glauert singularity effect.

C-27J Spartan with propeller tip vortex condensation

The tips of rotating surfaces (such as propellers and rotors) sometimes produce visible contrails.^[8]

Contrails and climate

Contrails, by affecting the Earth's radiation balance, act as a radiative forcing. Studies have found that contrails trap outgoing longwave radiation emitted by the Earth and atmosphere (positive radiative forcing) at a greater rate than they reflect incoming solar radiation (negative radiative forcing). NASA conducted a great deal of detailed research on atmospheric and climatological effects of contrails, including effects on ozone, ice crystal formation, and particle composition, during the Atmospheric Effects of Aviation Project (AEAP).^[9] Global radiative forcing has been calculated from the reanalysis data, climatological models and radiative transfer codes. It is estimated to amount to 0.012 W/m² (watts per square meter) for 2005, with an uncertainty range of 0.005 to 0.026 W/m², and with a low level of scientific understanding.^[10] Therefore, the overall net effect of contrails is positive, i.e. a warming effect.^[11] However, the effect varies daily and annually, and overall the magnitude of the forcing is not well known: globally (for 1992 air traffic conditions), values range from 3.5 mW/m² to 17 mW/m². Other studies have determined that night flights are mostly responsible for the warming effect: while accounting for only 25% of daily air traffic, they contribute 60 to 80% of contrail radiative forcing. Similarly, winter flights account for only 22% of annual air traffic, but contribute half of the annual mean radiative forcing.^[12]

A 2015 study found that artificial cloudiness caused by contrail "outbreaks" reduces the difference between daytime and nighttime temperatures. The former are decreased and the latter are increased, in comparison to temperatures the day before and the day after such outbreaks.^[13] On days with outbreaks the day/night temperature difference was diminished by about 6 °F in the U.S. South and 5 °F in the Midwest.^[14]

September 11, 2001, climate impact study

The grounding of planes for three days in the United States after September 11, 2001, provided a rare opportunity for scientists to study the effects of contrails on climate forcing. Measurements showed that without contrails, the local diurnal temperature range (difference of day and night temperatures) was about 1 °C (1.8 °F) higher than immediately before;^[15] however, it has also been suggested that this was due to unusually clear weather during the period.^[16]

Condensation trails have been suspected of causing "regional-scale surface temperature" changes for some time.^[17][18] Researcher David J. Travis, an atmospheric scientist at the University of Wisconsin-Whitewater, wrote in the science journal Nature that the effect of the change in aircraft contrail formation during the three days after the September 11 attacks was observed in surface temperature change, measured across over 4,000 reporting stations in the continental United States.^[17] His research documented an "anomalous increase in the average diurnal temperature change".^[17] The diurnal temperature range (DTR) is the difference in the day's highs and lows at any weather reporting station.^[19] Travis observed a 1.8 °C (3.24 °F) departure from the two adjacent three-day periods to September 11–14.^[17] This increase was the largest recorded in 30 years, more than "2 standard deviations away from the mean DTR".^[17]

USAAF 8th Air Force B-17s and their contrails

The September 2001 air closures are deeply unusual in the modern world, but similar effects have provisionally been identified from World War II records,^[20][21] when flying was more tightly controlled. A 2011 study of climate records in the vicinity of large groups of airbases found a case where contrails appeared to induce a statistically significant change in local climate, with a temperature variance around 0.8 °C, suggesting that examination of historic weather data could help study these effects.^[22]

Head-on contrails

A contrail from an airplane flying towards the observer can appear to be generated by an object moving vertically.^[23][24] 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,^[25] and its explanation as a contrail^{[23][24][26][27]} took more than 24 hours to become accepted by U.S. media and military institutions.^[28]

Distrails

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

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 causes existing water droplets to evaporate, leaving a clear wake through an otherwise cloudy sky.^[29]

Clouds form when invisible water vapor ( H₂O in gas phase) condenses into microscopic water droplets ( H₂O in liquid phase) or into microscopic ice crystals ( H₂O in solid phase). This may happen when air with a high proportion of gaseous water cools. A distrail forms when the heat of engine exhaust evaporates the liquid water droplets in a cloud, turning them back into invisible, gaseous water vapor.

Gallery

• 
  B-17 bombers over Europe, 1944
• 
  Airliner contrails, some new, some old, dispersed by wind shear
• 
  The image on the left shows the sky above Würzburg, Germany without contrails after the grounding of air traffic in 2010 and the image on the right shows the sky with regular air traffic on a day when conditions were right for contrails to form.
• 
  Iridescent contrails from a Boeing 747. This effect happens when water molecules interact with the aircraft and the sun shines through them.
• 
  MODIS tracking of contrails generated by air traffic over the southeastern United States on 29 January 2004
• 
  Multiple contrails above Nova Scotia
• 
  An Airbus A340 of Lufthansa produces contrails.
• 
  A contrail over southwest Virginia
• 
  Two USAF F-15s approaching Soviet MiG-29s
• 
  A contrail casting a shadow onto a lower cloud layer

See also

References

External links

Wikimedia Commons has media related to Contrails.

Look up contrail in Wiktionary, the free dictionary.

• Contrails.nl: Pictures of contrails and Aviation Cirrus (-Smog), from 1995 on
• Abstract of article in Nature announcing results of contrail temperature change study
• Clouds Caused By Aircraft Exhaust May Warm The U.S. Climate
• NASA Astronomy Picture of the Day: Contrails over the USA (13 October 2004)
• Contrails over the USA
• Reference site for debunking weird stories about contrails
• Contrail simulator (Java applet) – interactively shows how temperature and humidity of the surrounding air affect contrail formation and characteristics
• Contrails: What's Left Behind Is Bad News. Nick Onkow (4 March 2006
• NASA Contrail Education
• What is a contrail and how does it form?, National Weather Service
• "Why Planes Make Vapor Trails". Popular Science. Bonnier Corporation. 142 (3): 55. March 1943. ISSN 0161-7370.