Long-lived contrails usually form in natural ice clouds
Research team identifies common environmental conditions for the formation of contrails and provides initial insights into their impact on the climate
Johannes Gutenberg Universitaet Mainz
image:
Contrails over Jülich, embedded in very thin and therefore barely visible cirrus clouds
view moreCredit: Photo/©: Andreas Petzold
Long-lived contrails form predominantly not in cloud-free skies, but within already existing ice clouds. This is the conclusion reached by a team of scientists from Forschungszentrum Jülich, the University of Cologne, the University of Wuppertal, and Johannes Gutenberg University Mainz (JGU). Using extensive observational data, the researchers were able, for the first time, to systematically determine the atmospheric conditions under which long-lasting contrails form – whether in cloudless skies, in very thin and barely visible ice clouds, or in more clearly visible ice clouds, known as cirrus clouds. The result: more than 80 percent of all persistent contrails form within pre-existing clouds, mostly within natural cirrus clouds. The effects of this on the climate are not yet clearly understood. The study, now published in Nature Communications, provides important insights for further research – and, beyond that, strong arguments for taking cloud cover into account when planning flight routes adapted to climate considerations.
Effect of natural and man-made ice clouds on the climate
Contrails are a visible signature of daily air traffic in the sky. They form when the hot exhaust gases from aircraft engines mix with the cold air at an altitude of around ten kilometers. In dry air, most contrails dissipate quickly. In cold and humid air, however, they can persist for several hours and develop into extensive ice clouds or cirrus clouds. Cirrus clouds are thin ice clouds that occur at altitudes of about eight to twelve kilometers and often appear as fine, fibrous veils in the sky. The overall climate impact of these cirrus clouds formed from contrails is greater than that of the direct CO₂ emissions produced by air traffic.
The decisive factor for their climate impact is whether the man-made clouds form in a blue, cloudless sky or within existing natural cirrus clouds. High ice clouds, whether natural or man-made, exist at cold temperatures below -40°C. Although they often appear optically very thin, they can act like a blanket that prevents heat from escaping from the atmosphere into space, thereby contributing to the greenhouse effect. Only when the clouds are very dense and the sun is barely visible does the amount of sunlight reflected back into space become large enough to produce a cooling effect on the climate.
Accordingly, artificial clouds formed by contrails affect the climate differently depending on their environment: Under clear conditions – such as blue skies or very thin cirrus clouds – they tend to contribute to warming, because they trap some of the Earth's radiation that would otherwise escape into space, while allowing sunlight to pass through. In dense, clearly visible cirrus clouds, however, the opposite effect can occur: Contrails reflect more sunlight than they absorb heat radiation, leading to a slight cooling effect. How exactly contrails and natural cirrus clouds influence each other is still poorly understood.
"Our results show that we need to take a more differentiated view of the climate impact of contrails in the future," says Professor Andreas Petzold from the Institute of Climate and Energy Systems – Troposphere (ICE-3) at Forschungszentrum Jülich. "If most persistent contrails occur within natural clouds anyway, it might be more effective to plan climate-relevant flight routes not only according to clear skies but also with regard to existing ice cloud structures."
For the study, the research team used measurement data on temperature and water vapor collected by commercial aircraft flying over the North Atlantic between 2014 and 2021. These aircraft are part of the European research infrastructure IAGOS (In-service Aircraft for a Global Observing System, https://www.iagos.org/), which is co-coordinated by Forschungszentrum Jülich. IAGOS aircraft are equipped with instruments that continuously collect atmospheric data during scheduled operations – something unique worldwide.
Mainz contribution to the study: model calculations of radiative forcing
The data evaluation was supplemented by model calculations on radiative forcing. "Our analysis shows that contrails in thick cirrus clouds actually have hardly any effect," says Professor Peter Spichtinger from JGU, who contributed this aspect to the study. "However, additional effects in more complex scenarios – such as those arising from multiple layers of contrails and cirrus clouds on top of each other – are difficult to estimate and will be investigated in more detail in the future."
The results of the study are being incorporated into ongoing international activities of the World Meteorological Organization (WMO), the International Civil Aviation Organization (ICAO), the European Aviation Safety Agency (EASA), and the aviation industry. The goal is to develop a sustainable flight-planning strategy to reduce climate-relevant contrails by designing flight routes with climate impact in mind. IAGOS aircraft will continue to play a key role in evaluating such strategies in the future.
Journal
Nature Communications
Method of Research
Observational study
Subject of Research
Not applicable
Article Title
Most long-lived contrails form within cirrus clouds with uncertain climate impact
Article Publication Date
6-Nov-2025
Understanding clouds to improve the accuracy of climate predictions
Max Planck Institute for Dynamics and Self-Organization
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Stratocumulus clouds form large carpets in the sky covering approximately 20% of the earth’s surface. They reflect around 40% of the sun’s radiation back into space. The international research team will investigate the behavior and dynamics of these clouds with respect to climate change to improve climate and weather models.
view moreCredit: © Eberhard Bodenschatz, October 2025 over Central Europe
Clouds play an important role with respect to global climate change by affecting how Earth interacts with sunlight and heat radiation. At the height of around one kilometer, stratocumulus clouds are the most common cloud type, forming carpets in the sky, covering about 20% of Earth’s surface.
These clouds will now be investigated by an interdisciplinary research team from the Max Planck Institute for Dynamics and Self-Organization (Göttingen, Germany), the University of Gothenburg (Sweden), Delft University of Technology (Netherlands) and the Freie Universität Berlin (Germany). The team has been granted 13.7 million euros by the European Research Council (ERC) over the next six years. The goal of this ERC Synergy project TurPhyCloud – The role or Turbulence in the Physics of Clouds – is to reduce the uncertainty of weather and climate predictions by understanding the physics of stratocumulus clouds.
Turbulent processes at the stratocumulus cloud top affect their entire evolution, right down to precipitation. During the coming six years, the researchers will assess these processes using the well-established CloudKite observatory developed at MPI-DS. The observatory employs a stationary balloon that lifts state of the art instruments weighing 120 kg two kilometers high into the sky, allowing precise measurements of cloud parameters with unprecedented spatial resolution.
“In particular, we are interested in the upper part of the cloud which is mostly affected by radiation from the sun as well as evaporation and where physical dynamics are only understood to some extend”, explains Eberhard Bodenschatz, director at MPI-DS and coordinator of TurPhyCloud. In addition, a fleet of research drones from TU Delft will be used for continuous measurements of wind, temperature, humidity, and pressure in and around the clouds.
The researchers will gather atmospheric data from local stratocumulus clouds in the Baltic Sea. Using these field measurements, they will develop models for turbulent processes in stratocumulus clouds, which will be validated by further measurements. By combining numerical approaches and model calculations, the team aims to create a simulation tool that can be incorporated in current weather and climate models.
“One of the greatest challenges in climate science is predicting the behavior of clouds, which have a major impact on the rate of global warming,” summarizes Bodenschatz. “I’m convinced that understanding the physics of stratocumulus clouds will make it possible to improve current weather and climate models.”
About the ERC Synergy Grant
The Synergy Grant awarded by the European Research Council is given annually to interdisciplinary teams of 2-4 research groups who work together on complex questions that cannot be solved by a single team. The project must demonstrate high research quality as well as synergy effects making collaboration between the researchers essential. The grant is open to all fields of research and provided for six years with a maximum funding volume of 14 million euros.
In 2025, more than 700 proposals for ERC synergy grants have been submitted out of which 48 received funding.
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