Contrails: What are they and how do they form?

An airplane with contrails across the surface of a pink hued sun against a blue sky.
We most commonly see contrails behind planes at cruising altitude. (Image credit: Grant Faint via Getty Images)

If you live under a popular plane route, you're probably no stranger to long, thin clouds in the sky. These are contrails emitted from airplanes. 

Contrails, or condensation trails, are essentially human-made clouds; they are trails of condensed water vapor created by jet engines, according to the National Weather Service

We most commonly see them behind planes at cruising altitude, but they can also be emitted by rockets.

Related: Wow! Rare rainbow contrails caught on camera (photos) 

How do contrails form?

Contrails are created when the hot water vapor emitted by a jet engine after combustion cools and condenses in Earth's atmosphere, according to the Environmental Protection Agency (EPA). The atmosphere's temperature and humidity must be in just the right place for condensation to occur — the air must be cold with some humidity. 

Contrails most commonly form at an airplane's cruising altitude, between about 32,000 and 42,000 feet (10,000 to 13,000 meters) in the upper troposphere, per the Rocky Mountain Institute (RMI), because that's where those conditions are found. Because the atmosphere is ever-changing, conditions might not be right for contrail formation at this altitude, which is why not all airplanes create contrails during every flight. 

Contrail FAQs

rainbow contrails behind an airplane in front of a blue sky.

A series of stunning rainbow contrails were captured by amateur photographer Soumyadeep Mukherjee over Kolkata, India. (Image credit: Soumyadeep Mukherjee)

Are contrails good or bad?

Generally speaking, contrails are neither good nor bad. They are simply a cloud created by jet engines under certain atmospheric conditions. That said, research suggests that contrails contribute to atmospheric warming and cooling, according to RMI, and the warming, in particular, is a problem for the planet. 

Why are contrails bad for the environment?

Contrails that form at night or last into nighttime are the main contributors to atmospheric warming, per the Rocky Mountain Institute (RMI). Earth undergoes radiative cooling at night — heat radiates from the surface into space — but clouds like contrails can trap that heat.

Why are they called contrails?

 Contrail is a portmanteau of "condensation" and "trail." Contrails are trails of condensation created by jet engines at cruising altitude in the atmosphere. 

Why do some planes not produce contrails?

All jet engines can potentially produce contrails, but specific atmospheric conditions are required for contrail formation, per the National Weather Service. The air needs to be both cool and humid — conditions that are most commonly found at cruising altitude.

How long do contrails last?

If conditions are right, contrails can last for hours. (Image credit: Richard Newstead via Getty Images)

If a contrail forms behind a plane, it may last just seconds before dissipating, or it can last hours. It all depends on the atmospheric conditions at the time, according to the EPA; lower humidity leads to short-lived contrails, while higher humidity leads to persistent contrails. In the latter case, contrails may then be spread out by the wind, reaching a few miles wide and 650 to 1,300 feet (200 to 400 m) high. We can accurately predict whether a contrail will form based on the atmospheric temperature and humidity, the EPA says. 

Are contrails dangerous to humans?

No, contrails are not directly dangerous to humans. There is a conspiracy theory that the government uses aircraft to disperse toxic chemicals into the atmosphere for a variety of nefarious reasons, creating chemical trails, or "chemtrails," that look similar to contrails. However, there is no evidence to support this scenario.

"There's no basis to the conspiracy, and scientists do not consider it as at all credible," Steven Barrett, the H. N. Slater professor of aeronautics and astronautics at the Massachusetts Institute of Technology (MIT) and director of the MIT Laboratory for Aviation and the Environment, told

David Keith, a professor of applied physics at Harvard's School of Engineering and Applied Sciences and a professor of public policy at the Harvard Kennedy School, provides a detailed debunking of the theory on his research group's website. Keith studies climate science and technology, as well as public policy of solar geoengineering.

Environmental effects

Contrails are made from water vapor, just like clouds are. Although contrails and clouds are not outwardly dangerous to the environment, they do play a role in Earth's temperature regulation via the greenhouse effect. Per NASA, clouds — including contrails — can have both cooling and warming effects on our planet. The sun emits thermal radiation that travels toward Earth. When solar radiation reaches thick clouds in Earth's atmosphere, those clouds can reflect the radiation out into space, keeping the planet cool. But when the radiation does reach Earth's surface, the planet itself reflects solar radiation out into space at night — unless there are thin clouds like contrails in the night sky. Those clouds can trap the reflected solar radiation and warm Earth.

The issue is not the short-lived contrails but rather the long-duration ones that extend over a large area and, in effect, become cirrus clouds, which are a major culprit in trapping heat on Earth, per the National Weather Service.

Several public and private entities are currently researching ways to mitigate contrails' effect on the climate. For example, a study by NASA and DLR (the German Aerospace Center) revealed that sustainable aviation fuel can reduce contrail formation by 50% to 70%. 

Contrail Q&A with an expert

We asked Steven Barrett, a professor of aeronautics and astronautics at MIT and the director of the MIT Laboratory for Aviation and the Environment, a few frequently asked questions about contrails. Barrett researches the ways aviation can achieve zero environmental impacts. 

Steven Barrett expert Q&A contrails
Steven Barrett

 Steven Barrett is a professor of aeronautics and astronautics at MIT, an interim department head of the Aeronautics and Astronautics Department, and director of the MIT Laboratory for Aviation and the Environment. 

Can all types of airplanes create contrails?

No, typically only jet aircraft fly high enough to be in the coldest part of the atmosphere where contrails can form. It's typically coldest around 35,000 feet [11,000 m]. Below that, it gets warmer as you approach the ground, and above that, the atmosphere gets warmer due to the ozone layer absorbing UV radiation from the sun. 

What atmospheric conditions affect the formation and longevity of contrails?

It has to be cold enough and humid enough for the water emitted by burning fuel to freeze into what is essentially an ice cloud. The contrails can last hours if those conditions persist, but lifetimes vary a great deal. Much of the time, a short-lived contrail forms where it's not cold or humid enough for it to freeze. Often, you can see these short-lived contrails behind airplanes, but they're not of concern when it comes to the climate. 

Do contrails have any effect on humans? What about the climate?

They don't have a direct impact on humans but do contribute to climate change and so have an indirect effect. Contrails reflect incoming sunlight and trap outgoing heat, with a net warming effect that's similar to the warming from aviation's CO2 emissions

Additional resources

See photos of different types of contrails from the GLOBE (Global Learning and Observations to Benefit the Environment) Program. Learn how contrails affect the climate from the Yale School of the Environment. Discover how AI can mitigate contrails' impact on climate in this article from Google


Aircraft Contrails Factsheet. Environmental Protection Agency. Retrieved Aug. 18, 2023, from

Cathcart, J. and Andrew Chen. Contrail Mitigation: A Collaborative Approach in the Face of Uncertainty. Rocky Mountain Institute. Retrieved Aug. 18, 2023, from

Clouds and Contrails. National Weather Service. Retrieved Aug. 18, 2023, from 

How Do Clouds Affect Earth's Climate? NASA Climate Kids. Retrieved Aug. 18, 2023, from 

Keith, D. Chemtrails Conspiracy Theory. David Keith's Research Group. Retrieved Aug. 18, 2023, from

Weather in Action: Contrails. National Weather Service. Retrieved Aug. 18, 2023.

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Stefanie Waldek
Contributing writer contributing writer Stefanie Waldek is a self-taught space nerd and aviation geek who is passionate about all things spaceflight and astronomy. With a background in travel and design journalism, as well as a Bachelor of Arts degree from New York University, she specializes in the budding space tourism industry and Earth-based astrotourism. In her free time, you can find her watching rocket launches or looking up at the stars, wondering what is out there. Learn more about her work at

  • Jan Steinman
    Thank you for the detailed analysis.

    Of course, that won't keep the chemtrail folks from believing their irrational belief. Now Space DOT com is part of the conspiracy!

    I once worked out the molar math of converting jet fuel and atmospheric oxygen into water vapour and CO2. It turns out that most of the volume of the contrails come from atmospheric oxygen!

    The implication is that, if one were to really haul chemicals up into the air and disperse them, it would require four times the quantity of chemicals than it requires of jet fuel. This means it would need four times the lift, four times the engine power, four times the storage space, etc. And yes, all this additional capacity would also require four times the conventional jet fuel expended.

    So now, you don't need a bunch of "hard science" to dispel the notion of chemtrails. It's simple common sense that there aren't special aircraft with four times the lifting capacity out there. They would be pretty easy to spot!
  • billslugg
    This is from the OP referenced article:
    "Several public and private entities are currently researching ways to mitigate contrails' effect on the climate. For example, a study by NASA and DLR (the German Aerospace Center) revealed that sustainable aviation fuel can reduce contrail formation by 50% to 70%. "
    The impact of sustainable fuel is stated as fact.

    This is from the article it references:
    "It's too early to report any conclusive results from the tests, but a quick look at the data seems to indicate that the alternate fuel blend reduces black carbon emissions by more than 30 percent on the ground, with less obvious results in the air, including the alternate fuel's effect on contrail formation.
    We don't know if the emissions have any impact on ice particle formation. That's something that will probably be buried in the statistics, and we'll be examining that," Anderson said."
    The impact of sustainable fuel is "too early to report", "a quick look seems to indicate". And then they state their underlying model is unproven,

    Which one am I supposed to believe?
  • Pogo
    If they manage to minimize or eliminate contrails, what are the chem trail folks gonna do?
  • Jan Steinman
    Pogo said:
    If they manage to minimize or eliminate contrails, what are the chem trail folks gonna do?
    Oh, they'll find something else to fixate on, perhaps vaccines.

    Oh, wait…