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United States Department of TransportationUnited States Department of Transportation

Contrails 101

What is a contrail?

aircraft condensation trailsA contrail is an aircraft condensation trail that appears as line-shaped clouds in the sky.

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What are contrails made of?

They are composed of ice particles that form in the exhaust of an aircraft when flying in a narrow range of altitudes in the upper atmosphere, several miles above the ground.

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Can I see them?

They are easily identifiable in the sky behind a jet-engine aircraft at the head of the trail. Contrails can form within a few wingspans of the aircraft behind the exhaust and can dissipate within a few aircraft lengths from the engine exhaust because of the lack of available water vapor in the atmosphere.

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How are they formed?

The formation of contrail ice particles starts at microscopic scales when extremely small soot or other particles form in the aircraft combustion exhaust through a process in which a few tens to hundreds of molecules come together and grow to larger sizes by sticking together. Further growth may occur by condensation of water vapor present in exhaust gases on the particle surfaces made up of compounds such as sulfates and nitrates. Subsequent growth of particles happens due to ambient relative humidity that exceeds 100%.

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Where are they formed?

They are easily identifiable in the sky behind a jet-engine aircraft at the head of the trail. Contrails can form within a few wingspans of the aircraft behind the exhaust and can dissipate within a short distance from the engine exhaust because of the lack of available water vapor in the atmosphere.

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How long do they last in the sky?

Thermodynamics is the controlling factor for contrail formation, described by an equation that is a function of atmospheric temperature and pressure, and among other parameters, water content in the exhaust and the atmosphere. If the relative humidity is over 100%, they can persist for long periods of time, typically a few minutes to hours, thus covering much of the sky in a narrow path. Frequently, because of mixing due to turbulence in the upper atmosphere, the jets broaden vertically and horizontally. The horizontal broadening can at times cover a large portion of the sky depending on the amount of water vapor available in the atmosphere at cruise altitudes, resulting in contrail-induced cirrus clouds. These can persist for much longer times — typically several hours, similar to ordinary cirrus clouds that are also composed of ice particles.

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Are they dangerous to humans?

Absolutely not. This question is often asked based on the misunderstanding of the so-called ‘chemtrails’ which presumably represent trails consisting of harmful chemicals. Contrails are a type of cirrus clouds consisting of predominantly ice particles. A major portion of the water to form these particles comes from the atmosphere itself, and a small portion is from the engine exhaust. The engine exhaust contains products of combustion of aviation fuel that are like the exhaust composition of automobile exhaust. Since these combustion products occur high above the ground, their impact is far less than that of automobiles. FAA, with other agencies, is constantly studying the climate and surface air quality impacts of aviation exhaust, both present and into the future when aviation emissions are expected to increase.

25,000 to 50,000 Feet Typical Altitude Range at Which Aircraft Fly 50,000 ft 40,000 ft Mt. Everest 29,028 ft 25,000 ft Mt. McKinley 20,032 ft Rocky Mountains 14,439 ft

When you look up, it is hard to judge the height of contrails that you may see in the sky above. Airplanes are flying between 25,000 feet and 50,000 feet above the earth's surface. This illustration captures the height of contrails relative to some of the earth's highest mountains. Contrails formed by airplanes have no ill effects on man. Scientific research has shown that contrails do not deposit onto the earth's surface.

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Do contrails affect climate?

Ice particles in short-lived contrails will quickly evaporate leaving behind the original nucleus whose size will be very small and, thus, will not significantly interact with solar or thermal radiation. However, if there is a substantial amount of ambient moisture at a low temperature, further growth will ensue, resulting in the formation and growth of contrail particles to sizes like that of particles in natural cirrus clouds, resulting in reflection of sunlight. Also, because of their large sizes, the contrail particles interact strongly with the longer-wavelength thermal infrared radiation. The net effect of contrails is therefore typically one of warming. With the projected increase in air-traffic, the contrail coverage is expected to increase with a corresponding increase in warming.

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What are the ingredients of jet fuel, and are they necessary for the formation of contrails?

Jet fuels are a mixture of hydrocarbons with some impurities and additives. Upon combustion, jet fuel produces water vapor along with oxides of carbon, oxides of sulfur, and oxides of nitrogen. Sulfates and nitrates act as nuclei for subsequent cloud droplet growth which ultimately form in contrail particles.

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Why are persistent contrails of interest to scientists?

Contrails, like the natural cirrus clouds, generally warm the earth. Persistent contrails increase this warming effect over time. The highest percentages of cover occur in regions with the highest volume of air traffic, namely over Europe and the United States.

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How is the contrail coverage expected to change in the future?

Both aircraft emissions and atmospheric conditions will change in the future. Emissions are determined by changes in aircraft engine technologies and amounts and locations of air traffic. Atmospheric conditions are affected by changes in atmospheric humidity. It is currently estimated that regions of the atmosphere with sufficient humidity to support the formation of contrails cover 16 percent of the Earth's surface. An increase in air traffic in these regions will increase the contrail cover.

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Glossary of Aviation Terms

The air surrounding and bound to earth.
A cloud in the form of thin, white feather-like clouds in patches or narrow bands; has a fibrous and/or silky sheen; large ice crystals often trail downward a considerable vertical distance in fibrous, slanted, or irregularly curved wisps called mares' tails.
A condensation trail, a cirrus like trail of water vapor.
The percentage of water vapor in the air.
Disturbance in the atmosphere causing gusts of varying strengths.

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