United States
Environmental Protection
Agency

1EPA

Air and Radiation(6205J)

EPA430-F-00-005
September 2000
www.epa.gov

Aircraft Contrails
FactsheetSummary
his fact sheet describes the formation, occurrence, and effects of “condensation trails”
or “contrails.” It was developed by scientific and regulatory experts at the Environmental
Protection Agency (EPA), the Federal Aviation Administration (FAA), the National
Aeronautics and Space Administration (NASA), and the National Oceanic and Atmospheric
Administration (NOAA) in response to public inquiries regarding aircraft contrails. Contrails areline-shaped clouds sometimes produced by aircraft engine exhaust, typically at aircraft cruise
altitudes several miles above the Earth’s surface. The combination of water vapor in aircraft
engine exhaust and the low ambient temperatures that often exists at these high altitudes allows
the formation of contrails. Contrails are composed primarily of water (in the form of ice crystals)
and do not pose health risks to humans. They do affect the cloudiness of the Earth’s atmosphere,
however, and therefore might affect atmospheric temperature and climate. The
basic processes of contrail formation described in this fact sheet apply to both civil andmilitary aircraft.

T

What are contrails?
ontrails are line-shaped clouds or “condensation trails,” composed of ice particles, thatare visible behind jet aircraft engines, typically at cruise altitudes in the upper atmos­
phere1. Contrails have been a normal effect of jet aviation since its earliest days.
Depending on the temperature and the amount of moisture in the air at the aircraft altitude, con­trails evaporate quickly (if the humidity is low) or persist and grow (if the humidity is high). Jet
engine exhaust provides only a small portion of the water that forms ice in persistent contrails.
Persistent contrails are mainly composed of water naturally present along the aircraft flight path.

CHow are aircraft emissions linked tocontrail formation?
ircraft engines emit water vapor, carbon dioxide (CO2), small amounts of nitrogen oxides(NOx), hydrocarbons, carbon monoxide, sulfur gases, and soot and metal particles
formed by the high-temperature combustion of jet fuel during flight. Of these emittants,
only water vapor is necessary for contrail formation. Sulfur gases are also of potential interest
because they lead to the formation of small particles. Particles suitable for water droplet forma­tion are necessary for contrail formation. Initial contrail particles, however, can either be alreadypresent in the atmosphere or formed in the exhaust gas. All other engine emissions are consid­
ered nonessential to contrail formation.

A

1This fact sheet focuses on contrails produced by aircraft engine exhaust. However, the term “contrail” is also used torefer to the short trails sometimes briefly appearing over aircraft wings or engine propellers, especially under mild, humidconditions. These contrails consist entirely of atmospheric water that condenses as a result of local reductions in pressure
due to the movement of the wing or propeller.

2 Printed on paper that contains at least 30 percent postconsumer fiber.

If the humidity is high (greater than that needed for ice conden­
sation to occur), the contrail will be persistent. Newly formed
ice particles will continue to grow in size by taking water fromthe surrounding atmosphere. The resulting line-shaped contrail
extends for large distances behind an aircraft (See Figures 2 and3). Persistent contrails can last for hours while growing to sev­
eral kilometers in width and 200 to 400 meters in height.
Contrails spread because of air turbulence created by the pas­sage of aircraft, differences in wind speed along the flight track,and possibly through effects of solar heating.

Figure 1. Contrails forming behind the engines of a Lufthansa Airbus A310-330
cruising at an altitude of 35,100 ft (10.7 km) as seen from research aircraft.
(Photo:German Aerospace Center (Deutsches Zentrum fur Luft- und Raumfahrt(DLR)), Oberpfaffenhofen, Germany.) Inset: Contrails forming behind the enginesof a large commercial aircraft. Typically, contrails become visible within roughly awingspan distance behind the aircraft. (Photo: Masako Imai, Cloud Castle/PhotoSky Japan.)

What are the ingredients of jet
fuel, and are they important tocontrail formation?

How do contrails form?
or a contrail to form, suitable conditions must occur
immediately behind a jet engine in the expanding engine
exhaust plume. A contrail will form if, as exhaust gases
cool and mix with surrounding air, the humidity becomes highenough (or, equivalently, the air temperature becomes lowenough) for liquid water condensation to occur. The level ofhumidity reached depends on the amount of water present in
the surrounding air, the temperature of the surrounding air, andthe amount of water and heat emitted in the exhaust.
Atmospheric temperature and humidity at any given location
undergo natural daily and seasonal variations and hence, are
not always suitable for the formation of contrails.

Fll jet fuel is a hydrocarbon mixture containing small
amounts of impurities and additives. All aircraft jet
fuel is analyzed for strict impurity limits before use.
The hydrocarbon content of jet fuel produces water vapor asa by-product of combustion. Contrails would not form behindaircraft engines without the water vapor by-product present
in exhaust.

AIf sufficient humidity occurs in the exhaust plume, water con­denses on particles to form liquid droplets. As the exhaust air
cools due to mixing with the cold local air, the newly formed
droplets rapidly freeze and form ice particles that make up acontrail (See Figure 1). Thus, the surrounding atmosphere’sconditions determine to a large extent whether or not a contrail
will form after an aircraft’s passage. Because the basic processes
are very well understood, contrail formation for a given aircraft
flight can be accurately predicted if atmospheric temperatureand humidity conditions are known.
After the initial formation of ice, a contrail evolves in one of twoways, again depending on the surrounding atmosphere’s humid­
ity. If the humidity is low (below the conditions for ice conden­
sation to occur), the contrail will be short-lived. Newly formed
ice particles will quickly evaporate as exhaust gases are com­pletely mixed into the surrounding atmosphere. The resulting
line-shaped contrail will extend only a short distance behind
the aircraft (See Figure 2).

Figure 2. Photograph of two contrail types. The contrail extending across the image is an
evolving persistent contrail. Shown just above it is a short-lived contrail. Short-lived con­trails evaporate soon after being formed due to low atmospheric humidity conditions.
The persistent contrail shown here was formed at a lower altitude where higher humidity
was present Inset: Another example of a short-lived contrail. (Photos: J. Holecek, NOAA
Aeronomy Laboratory, Boulder, CO.)02

A common impurity in jetoverall climate effect.
fuel is sulfur (~0.05% byAnother key component isweight), which contributes
carbon dioxide (CO2)to the formation of small
emissions from the com­
particles containing vari­bustion of jet fuel.
ous sulfur species. These
Increases in CO2 and otherparticles can serve as sites Figure 3. Persistent contrails and contrails evolving and spreading into cirrus clouds.
“greenhouse gases” areHere, the humidity of the atmosphere is high, and the contrail ice particles continue tofor water droplet growth
expected to warm thegrow by taking up water from the surrounding atmosphere. These contrails extend forlarge
distancesandmaylastforhours.
Onotherdays
when
atmospherichumidity
isin the exhaust and, if
lower atmosphere andlower, the same aircraft passages might have left few or even no contrails. (Photo: L.
water droplets form, theyEarth’s surface. Aviation’s
Chang, Office of Atmospheric Programs, U.S. EPA.)
might freeze to form ice
overall potential for influ­
particles that compose a contrail. Enough particles are present
encing climate was recently assessed to be approximately 3.5
in the surrounding atmosphere, however, that particles from thepercent of the potential from all human activities (See Box 1).
engine are not required for contrail formation. There are no lead
Persistent line-shaped contrails are estimated to cover, on aver­or ethylene dibromide additives in jet fuel. Additives currently
age, about 0.1 percent of the Earth’s surface (Sausen et al.,used in jet fuels are all organic compounds that may also con­1998; see Figure 4). The estimate uses:
tain a small fraction of sulfur or nitrogen.
• meteorological analysis of atmospheric humidity to specify the
global cover of air masses that are sufficiently humid (lowenough atmospheric temperature) for persistent contrails toform

Why are persistent contrails ofinterest to scientists?

• data from 1992 reported aircraft operations to specify whenand where aircraft fly

ersistent contrails are of interest to scientists because
they increase the cloudiness of the atmosphere. The
increase happens in two ways. First, persistent contrails
are line-shaped clouds that would not have formed in theatmosphere without the passage of an aircraft. Secondly, persist­ent contrails often evolve and spread into extensive cirrus cloud
cover that is indistinguishable from naturally occurring cloudi­
ness (See Figure 3). At present, it is unknown how much of thismore extensive cloudiness would have occurred without thepassage of an aircraft. Not enough is known about how naturalclouds form in the atmosphere to answer this question.

P• an estimated average for aircraft engine characteristics thataffect contrail formation• satellite images of certain regions of the Earth in which con­trail cover can be accurately measured (See Figure 5)
The highest percentages of cover occur in regions with the high­est volume of air traffic, namely over Europe and the United

Changes in cloudiness are important because clouds help con­trol the temperature of the Earth’s atmosphere. Changes incloudiness resulting from human activities are importantbecause they might contribute to long-term changes in theEarth’s climate. Many other human activities also have thepotential of contributing to climate change. Our climate
involves important parameters such as air temperature, weather
patterns, and rainfall. Changes in climate may have important
impacts on natural resources and human health. Contrails’ pos­sible climate effects are one component of aviation’s expected

Figure 4. Estimated global persistent contrail coverage (in percent area cover) for the
1992 worldwide aviation fleet. The global mean cover is 0.1 percent. See text for
description of how this estimate was made. (Reproduced with permission from Sausenet al., 1998, Figure 3, left panel.)03

States (See Figure 4). This estimate of contrail cloudiness coverdoes not include extensive cirrus cloudiness that often evolves
from persistent line-shaped contrails. Some evidence suggeststhat this additional cirrus cloudiness might actually exceed thatof line-shaped cloudiness.

How is contrail coverage
expected to change in thefuture?
Figure 5. Satellite photograph showing an example of contrails covering central
Europe on May 4, 1995. The average cover in a photograph is estimated by using acomputer to recognize and measure individual contrails over geographical regionsof known size. Photograph from the National Oceanic and Atmospheric
Administration (NOAA)-12 AVHRR satellite and processed by DLR (adapted fromMannstein et al., 1999). (Reproduced with permission of DLR.)

ontrail cover is expected to change in the future ifchanges occur in key factors that affect contrail forma­tion and evolution. These key factors include aircraft
engine technologies that affect emissions and conditions in theexhaust plume; amounts and locations of air traffic; and back­ground atmospheric humidity conditions. Changes in engine
fuel efficiency, for example, might change the amount of heat
and water emitted in the exhaust plume, thereby affecting thefrequency and geographical cover of contrails. Changes in airCtraffic might also affect persistent contrail formation. It is cur­rently estimated that regions of the atmosphere with sufficienthumidity to support the formation of persistent contrails coverabout 16 percent of the Earth’s surface. If air traffic in theseregions increases in the future, persistent line-shaped contrail

BOX 1

Scientific Assessment of the Global
Atmospheric Effects of Aviation
The Intergovernmental Panel on Climate Change(IPCC) was established by the World MeteorologicalOrganisation (WMO) and the United Nations
Environment Programme (UNEP) in 1988 to assess thescience, technology, and socioeconomic informationneeded to understand the risk of human-induced cli­mate change. The 1999 IPCC report, “Aviation and theGlobal Atmosphere,” (see References) describes current
knowledge regarding aircraft effects on the globalatmosphere. The report was compiled by more than
100 authors from 18 countries. Technical experts fromthe aviation industry, including airlines and airframeand engine manufacturers, worked with atmosphericscientists in creating this report.
The report considers all gases and particles emitted byaircraft into the upper atmosphere. It also examines therole these gases and particles play in modifying theatmosphere’s chemical properties and initiating the for­mation of contrails and cirrus clouds. Chapter 3 of theIPCC report provides detailed information about con­trail formation, occurrence, and persistence. The report
also considers how potential changes in aircraft technol­ogy; air transport operations; and the institutional,regulatory, and economic framework might affect emis­
sions in the future. It does not address the effects ofengine emissions on local air quality near the surface orpotential human health effects of engine emissions. The
report notes that significant scientific uncertainty is
associated with aviation’s predicted influence on cli­mate. A report summary is available from the IPCC
Web site at <www.ipcc.ch>.

04

cover there will also increase. Overall, based on analysis of cur­rent meteorological data and on assumptions about future air
traffic growth and technological advances, persistent contrail
cover is expected to increase between now and the year 2050.

Do authorities regulate aircraftemissions?
n the United States, some aspects of aviation emissions areregulated through the efforts of several government agencies.
The U.S. Environmental Protection Agency (EPA), under theClean Air Act (CAA) of 1970, has established commercial air­craft engine exhaust emissions standards for certain emittants
associated with ground-level air pollution. Jet engine exhaustcontains, among other emittants, oxides of nitrogen (NOx) andhydrocarbons that contribute to ozone formation. Jet aircraft areone of many sources of these pollutants. Ozone is a prime
ingredient of smog in and near cities and other areas of thecountry. While EPA establishes emissions standards for aircraft,
the Federal Aviation Administration (FAA) of the U.S.
Department of Transportation (DOT) administers and enforcesthese standards. This domestic framework for regulating aircraft
engine emissions is more fully described in Box 2. Currently,
there are no regulations addressing contrails and their atmos­
pheric effects.

IAre persistent contrails harmfulto the public?
ersistent contrails pose no direct threat to public health.
All contrails are line-shaped clouds composed of iceparticles. These ice particles evaporate when local
atmospheric conditions become dry enough (low enough rela­tive humidity). The ice particles in contrails do not reach theEarth’s surface because they fall slowly and conditions in thelower atmosphere cause ice particles to evaporate.

P

Contrail cloudiness might contribute to human-induced climatechange. Climate change may have important impacts on publichealth and environmental protection.

BOX 2U.S. Environmental Regulatory Framework for Aircraft Engine Emissions
The Clean Air Act (CAA) directs the U.S. Environ­mental Protection Agency (EPA) to establish aircraft andaircraft engine emissions standards for any air pollutant
that could reasonably endanger public health and wel­fare. In 1997, EPA aligned U.S. emissions standards (40CFR Part 87) with engine emissions standards and rec­ommended practices (SARPs) prescribed by theInternational Civil Aviation Organization (ICAO), aUnited Nations agency established in 1944 that devel­
ops SARPs using the technical support of member states
and the aviation community. The United States is anactive member of ICAO's Committee on Aviation
Environmental Protection, which is responsible for fur­ther development of engine emissions standards. Inestablishing U.S. emissions standards, EPA must consult
with the Department of Transportation (DOT) to ensure
such regulations' effective dates permit the developmentof requisite technology, giving appropriate consideration
to compliance cost. It must also consult with DOT con­cerning aircraft safety before promulgating emissions
standards.
Under the CAA, DOT is responsible for enforcing stan­dards established by EPA. DOT delegated enforcement
responsibility to the Federal Aviation Administration(FAA). FAA has issued regulations administering andenforcing the emissions standards that apply to civil air­planes powered by gas turbine engines. FAA ensurescompliance with these regulations by reviewing andapproving certification test plans, procedures, test
reports, and engine emissions certification levels. For
more information on aircraft emissions or to access
EPA's or FAA's aircraft regulations, visit the Aviation
Emissions Website of EPA's Office of Transportation andAir Quality at <www.epa.gov/otaq/aviation.htm>.

05For further information

Referencesurther scientific information about the effects of aircrafton the upper atmosphere can be found in the 1999 IPCC
report, “Aviation and the Global Atmosphere” (seeReferences). Information about aircraft and aircraft engine
emissions regulations can be found at EPA’s aviation emissions
Web site, <www.epa.gov/otaq/aviation.htm>. Information about
military aircraft and military space launch activities, and theiratmospheric and environmental effects, can be found at<http://xre604.brooks.af.mil/safmiq/esoh_issues.htm>. For
additional copies or further information on this fact sheet,contact the EPA Stratospheric Protection Hotline at
800 296-1996.

Intergovernmental Panel on Climate Change (IPCC), 1999.
Aviation and the Global Atmosphere. J.E. Penner, D.H. Lister,D.J. Griggs, D.J. Dokken, and M. McFarland, editors. Cambridge
University Press. 373 pp.

FSausen, R., K. Gierens, M. Ponater, and U. Schumann, 1998. A
diagnostic study of the global distribution of contrails. Part I:
Present day climate. Theoretical and Applied Climatology 61:
127-141.
Mannstein, H., R. Meyer, and P. Wendling, 1999. Operational
detection of contrails from NOAA-AVHRR data. Int. J. Remote
Sensing, 20, 1641-1660.

Note: Some images or photos in this fact sheet were provided courtesyof other institutions or parties and may be protected by copyright.
Permissions regarding those photos or images need to be obtained
from the indicated source.

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