When airplanes create trails of soot and moisture, water in the atmosphere condenses on the particles and freezes, leaving behind the familiar streaks known as condensation trails—or contrails. Contrails are so frequently the target of conspiracy theories that it might seem as though the word is a portmanteau of “conspiracy trails.” And although contrails do not contain harmful chemicals, these bands of condensation can, in fact, affect the atmosphere, with some reports suggesting that they account for more than half of aircrafts’ warming effect on the climate.
Most of these traces of air travel vanish within minutes of a plane’s passage. To have any effect on even local climate conditions, the air must be cold and humid enough for the contrails to last on the order of hours so that they can spread into a thin blanket of high-altitude ice crystals that captures some of Earth’s outgoing heat.
Though contrails are most recognizable when they pull a fresh veil across a clear sky, it’s within preexisting cirrus clouds that relevant climate conditions are most common. Exactly what percentage of condensation deposits form within clouds and what that means for their effects on the climate, though, have long been uncertain.
“We didn’t expect that.”
Now, new research in Nature Communications aims to elucidate scientists’ understanding of how contrails embedded within high-altitude cirrus clouds affect the climate.
Along with his team, Andreas Petzold, an atmospheric scientist at the research institution Forschungszentrum Jülich, examined 7 years of temperature and humidity data collected by sensors aboard passenger aircraft that together covered a combined 17 million kilometers (10.6 million miles) of flights. They combined these data with satellite-based weather observations to determine how often the conditions for long-lived contrails are met both inside and outside of extant clouds.
Though Petzold expected that the majority of contrails would form in regions preseeded with clouds, he didn’t anticipate the scale. “The fraction was so huge,” he said. “We didn’t expect that.”
In the flight corridors of the Northern Hemisphere over eastern North America, the North Atlantic, and western Europe, where the bulk of data were collected, roughly 90% of long-lived contrails formed within preexisting clouds. Many climate models, however, assume that the atmospheric imprints of aircraft are stamped on clear skies.
The net climate effect of a contrail changes depending on the thickness of the cloud in which it forms. Thicker cirrus clouds can buffer the warming that contrails might contribute and can even lead to local cooling. But when contrails appear in thin clouds (many so thin that the eye can’t see them), the force of their warming can become even more significant than if they had formed in clear skies.
The new findings mean that the relationships between contrails and the climate is more complex than previously realized. “We need to get a quantification of the effects from model studies,” Petzold said, “because we’ve shown that this is such a big fraction, but we do not know how they impact the whole picture.”
Cirrus Streaks
During the day, the Sun’s heat can make a cirrus cloud thickened by a contrail more reflective, creating a local cooling effect. But at night, this contrail thickening traps heat and increases local warming.
In another study published just a few weeks after Petzold’s, a Leipzig University research group studied contrails’ climate effects by examining more than 40,000 contrails that planes streaked through cirrus clouds over a 6-year span.
They found that on average, embedded contrails contributed just 5 milliwatts per square meter of warming across the planet—a measurement of the amount of change in radiative force occurring at any given moment in time. That’s a paltry sum compared to the 3,320 milliwatts per square meter of warming caused by greenhouse gases emitted over the industrial era, as estimated by the Intergovernmental Panel on Climate Change.
“Cirrus are quite important for climate in general.”
Though neither study overturns scientists’ understanding of the way contrails form, said Michael Diamond, a cloud physicist at Florida State University, “one of the really big advances here is just how much high-quality data they’re bringing to bear.”
The data collected by Petzold’s team could help inform future studies of the internal microphysics of cirrus clouds, which is important not only for a better understanding of the climate consequences of aviation but also because “cirrus are quite important for climate in general,” Diamond said. Cirrus is the only cloud type that traps more heat than it reflects, so understanding whether cirrus clouds will become more or less frequent as climate change progresses is a key question to answer.
Results like Petzold’s can also help inform the work that many in and around the aviation industry are doing to improve forecasting so that aviators can follow flight paths that limit the potential for long-lived contrails to form. And though finding ways to eliminate aviation emissions entirely through sustainable fuel sources or battery-powered planes is essential, decreasing the formation of the most impactful forms of long-lived contrails would make a meaningful difference in reducing the near-term warming caused by air travel.
—Syris Valentine (@shapersyris.bsky.social), Science Writer
Citation: Valentine, S. (2026), New insights into the foggy role of contrails within clouds,
Eos, 107, https://doi.org/10.1029/2026EO260024. Published on 12 January 2026.
Text © 2026. The authors.
CC BY-NC-ND 3.0Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
