United States
Environmental Protection
Agency
Air and Radiation
(6205J)
EPA430-F-00-005
September 2000
www.epa.gov
Contrails
Factsheet
Summary
This 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 are
line-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 and
military aircraft.
What are contrails?
Contrails are line-shaped clouds or "condensation trails," composed of ice particles, that
are 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, con-
trails 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.
How are aircraft emissions linked to
contrail formation?
Aircraft 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 forma-
tion are necessary for contrail formation. Initial contrail particles, however, can either be already
present in the atmosphere or formed in the exhaust gas. All other engine emissions are consid-
ered nonessential to contrail formation.
-'-This fact sheet focuses on contrails produced by aircraft engine exhaust. However, the term "contrail" is also used to
refer to the short trails sometimes briefly appearing over aircraft wings or engine propellers, especially under mild, humid
conditions. 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.
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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 engines
of a large commercial aircraft. Typically, contrails become visible within roughly a
wingspan distance behind the aircraft. (Photo: Masako Imai, Cloud Castle/Photo
Sky Japan.)
How do contrails form?
For 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 high
enough (or, equivalently the air temperature becomes low
enough) for liquid water condensation to occur. The level of
humidity reached depends on the amount of water present in
the surrounding air, the temperature of the surrounding air, and
the 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.
If sufficient humidity occurs in the exhaust plume, water con-
denses 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 a
contrail (See Figure 1). Thus, the surrounding atmosphere's
conditions 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 temperature
and humidity conditions are known.
After the initial formation of ice, a contrail evolves in one of two
ways, 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 com-
pletely mixed into the surrounding atmosphere. The resulting
line-shaped contrail will extend only a short distance behind
the aircraft (See Figure 2).
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 from
the surrounding atmosphere. The resulting line-shaped contrail
extends for large distances behind an aircraft (See Figures 2 and
3). 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 pas-
sage of aircraft, differences in wind speed along the flight track,
and possibly through effects of solar heating.
What are the ingredients of jet
fuel, and are they important to
contrail formation?
All 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 as
a by-product of combustion. Contrails would not form behind
aircraft engines without the water vapor by-product present
in exhaust.
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 con-
trails 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.)
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Figure 3. Persistent contrails and contrails evolving and spreading into cirrus clouds.
Here, the humidity of the atmosphere is high, and the contrail ice particles continue to
grow by taking up water from the surrounding atmosphere. These contrails extend for
large distances and may last for hours. On other days when atmospheric humidity is
lower, the same aircraft passages might have left few or even no contrails. (Photo: L.
Chang, Office of Atmospheric Programs, U.S. EPA.)
A common impurity in jet
fuel is sulfur (-0.05% by
weight), which contributes
to the formation of small
particles containing vari-
ous sulfur species. These
particles 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. Enough particles are present
in the surrounding atmosphere, however, that particles from the
engine are not required for contrail formation. There are no lead
or ethylene dibromide additives in jet fuel. Additives currently
used in jet fuels are all organic compounds that may also con-
tain a small fraction of sulfur or nitrogen.
Why are persistent contrails of
interest to scientists?
Persistent 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 the
atmosphere without the passage of an aircraft. Secondly, persist-
ent 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 this
more extensive cloudiness would have occurred without the
passage of an aircraft. Not enough is known about how natural
clouds form in the atmosphere to answer this question.
Changes in cloudiness are important because clouds help con-
trol the temperature of the Earth's atmosphere. Changes in
cloudiness resulting from human activities are important
because they might contribute to long-term changes in the
Earth's climate. Many other human activities also have the
potential 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' pos-
sible climate effects are one component of aviation's expected
overall climate effect.
Another key component is
carbon dioxide (CO2)
emissions from the com-
bustion of jet fuel.
Increases in CO2 and other
"greenhouse gases" are
expected to warm the
lower atmosphere and
Earth's surface. Aviation's
overall potential for influ-
encing climate was recently assessed to be approximately 3.5
percent of the potential from all human activities (See Box 1).
Persistent line-shaped contrails are estimated to cover, on aver-
age, about 0.1 percent of the Earth's surface (Sausen et al.,
1998; see Figure 4). The estimate uses:
• meteorological analysis of atmospheric humidity to specify the
global cover of air masses that are sufficiently humid (low
enough atmospheric temperature) for persistent contrails to
form
• data from 1992 reported aircraft operations to specify when
and where aircraft fly
• an estimated average for aircraft engine characteristics that
affect contrail formation
• satellite images of certain regions of the Earth in which con-
trail cover can be accurately measured (See Figure 5)
The highest percentages of cover occur in regions with the high-
est volume of air traffic, namely over Europe and the United
0-
30°S-
180°W 150°W 120°W 90°W 60°W
30°W 0 30°E
Longitude
60°E 90°E 120°E 150°E 180°E
0.0
0.1
0.2
0.5
3.0
5.0
10.0
1.0 1.5 2.0
Area Cover (%)
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 Sausen
et al., 1998, Figure 3, left panel.)
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States (See Figure 4). This estimate of contrail cloudiness cover
does not include extensive cirrus cloudiness that often evolves
from persistent line-shaped contrails. Some evidence suggests
that this additional cirrus cloudiness might actually exceed that
of line-shaped cloudiness.
How is contrail coverage
expected to change in the
future?
Contrail cover is expected to change in the future if
changes occur in key factors that affect contrail forma-
tion and evolution. These key factors include aircraft
engine technologies that affect emissions and conditions in the
exhaust plume; amounts and locations of air traffic; and back-
ground 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 the
frequency and geographical cover of contrails. Changes in air
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 a
computer to recognize and measure individual contrails over geographical regions
of known size. Photograph from the National Oceanic and Atmospheric
Administration (NOAA)-12 AVHRR satellite and processed by DLR (adapted from
Mannstein et al., 1999). (Reproduced with permission of DLR.)
traffic might also affect persistent contrail formation. It is cur-
rently estimated that regions of the atmosphere with sufficient
humidity to support the formation of persistent contrails cover
about 16 percent of the Earth's surface. If air traffic in these
regions increases in the future, persistent line-shaped contrail
BOX 1
UNEP
Scientific Assessment of the Global
Atmospheric Effects of Aviation
WMO/OMM
The Intergovernmental Panel on Climate Change
(IPCC) was established by the World Meteorological
Organisation (WMO) and the United Nations
Environment Programme (UNEP) in 1988 to assess the
science, technology, and socioeconomic information
needed to understand the risk of human-induced cli-
mate change. The 1999 IPCC report, "Aviation and the
Global Atmosphere," (see References) describes current
knowledge regarding aircraft effects on the global
atmosphere. The report was compiled by more than
100 authors from 18 countries. Technical experts from
the aviation industry, including airlines and airframe
and engine manufacturers, worked with atmospheric
scientists in creating this report.
The report considers all gases and particles emitted by
aircraft into the upper atmosphere. It also examines the
role these gases and particles play in modifying the
atmosphere's chemical properties and initiating the for-
mation of contrails and cirrus clouds. Chapter 3 of the
IPCC report provides detailed information about con-
trail formation, occurrence, and persistence. The report
also considers how potential changes in aircraft technol-
ogy; air transport operations; and the institutional,
regulatory, and economic framework might affect emis-
sions in the future. It does not address the effects of
engine emissions on local air quality near the surface or
potential human health effects of engine emissions. The
report notes that significant scientific uncertainty is
associated with aviation's predicted influence on cli-
mate. A report summary is available from the IPCC
Web site at .
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cover there will also increase. Overall, based on analysis of cur-
rent 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.
Are persistent contrails harmful
to the public?
Persistent contrails pose no direct threat to public health.
All contrails are line-shaped clouds composed of ice
particles. These ice particles evaporate when local
atmospheric conditions become dry enough (low enough rela-
tive humidity). The ice particles in contrails do not reach the
Earth's surface because they fall slowly and conditions in the
lower atmosphere cause ice particles to evaporate.
Contrail cloudiness might contribute to human-induced climate
change. Climate change may have important impacts on public
health and environmental protection.
Do authorities regulate aircraft
emissions?
In the United States, some aspects of aviation emissions are
regulated through the efforts of several government agencies.
The U.S. Environmental Protection Agency (EPA), under the
Clean Air Act (CAA) of 1970, has established commercial air-
craft engine exhaust emissions standards for certain emittants
associated with ground-level air pollution. Jet engine exhaust
contains, among other emittants, oxides of nitrogen (NOX) and
hydrocarbons that contribute to ozone formation. Jet aircraft are
one of many sources of these pollutants. Ozone is a prime
ingredient of smog in and near cities and other areas of the
country. While EPA establishes emissions standards for aircraft,
the Federal Aviation Administration (FAA) of the U.S.
Department of Transportation (DOT) administers and enforces
these 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.
BOX 2
U.S. Environmental Regulatory Framework for Aircraft Engine Emissions
The Clean Air Act (CAA) directs the U.S. Environ-
mental Protection Agency (EPA) to establish aircraft and
aircraft engine emissions standards for any air pollutant
that could reasonably endanger public health and wel-
fare. In 1997, EPA aligned U.S. emissions standards (40
CFR Part 87) with engine emissions standards and rec-
ommended practices (SARPs) prescribed by the
International Civil Aviation Organization (ICAO), a
United Nations agency established in 1944 that devel-
ops SARPs using the technical support of member states
and the aviation community. The United States is an
active member of ICAO's Committee on Aviation
Environmental Protection, which is responsible for fur-
ther development of engine emissions standards. In
establishing U.S. emissions standards, EPA must consult
with the Department of Transportation (DOT) to ensure
such regulations' effective dates permit the development
of requisite technology, giving appropriate consideration
to compliance cost. It must also consult with DOT con-
cerning aircraft safety before promulgating emissions
standards.
Under the CAA, DOT is responsible for enforcing stan-
dards established by EPA. DOT delegated enforcement
responsibility to the Federal Aviation Administration
(FAA). FAA has issued regulations administering and
enforcing the emissions standards that apply to civil air-
planes powered by gas turbine engines. FAA ensures
compliance with these regulations by reviewing and
approving 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 and
Air Quality at .
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For further information
References
Further scientific information about the effects of aircraft
on the upper atmosphere can be found in the 1999 IPCC
report, "Aviation and the Global Atmosphere" (see
References). Information about aircraft and aircraft engine
emissions regulations can be found at EPAs aviation emissions
Web site, . Information about
military aircraft and military space launch activities, and their
atmospheric and environmental effects, can be found at
. For
additional copies or further information on this fact sheet,
contact the EPA Stratospheric Protection Hotline at
800 296-1996.
Note: Some images or photos in this fact sheet were provided courtesy
of other institutions or parties and may he protected hy copyright.
Permissions regarding those photos or images need to he obtained
from the indicated source.
Intergovernmental Panel on Climate Change (IPCC), 1999.
Aviation and the Global Atmosphere. J.E. Penner, D.H. Lister,
DJ. Griggs, D.J. Dokken, and M. McFarland, editors. Cambridge
University Press. 373 pp.
Sausen, 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 R Wendling, 1999. Operational
detection of contrails from NOAA-AVHRR data. Int. J. Remote
Sensing, 20, 1641-1660.
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