For years, efforts to make air travel more climate friendly have focused on avoiding the clear, ice-supersaturated layers where contrails are known to form and persist.

A new study from researchers at Forschungszentrum Jülich, the University of Cologne, the University of Wuppertal, and Johannes Gutenberg University Mainz (JGU) reshapes that picture.

More than four out of five long-lasting contrails don’t arise in blue, empty skies at all. They form within ice clouds that already exist, most often in natural cirrus clouds.

That finding doesn’t just tidy up a scientific loose end. It suggests that airlines and regulators should start factoring in real-time cloud cover when plotting climate-optimized routes.

Tracking contrails across the Atlantic

The researchers stitched together a uniquely rich, real-world dataset consisting of temperature and humidity measurements collected by commercial aircraft crossing the North Atlantic between 2014 and 2021.

Those jets are part of In-service Aircraft for a Global Observing System (IAGOS), a European research infrastructure co-coordinated by Forschungszentrum Jülich that equips airliners to gather atmospheric data continuously during scheduled flights.

With that backbone, the team could systematically identify and quantify the atmospheric setups under which persistent contrails formed – such as clean skies, barely visible ice haze, or clearly visible cirrus.

The headline result is stark: more than 80 percent of persistent contrails occurred within preexisting clouds, predominantly natural cirrus.

The climate cost of contrails

Contrails start as exhaust-driven ice crystals at cruising altitudes around 5 to 7 miles (8 to 12 kilometers). In dry air, they fade quickly. In cold, humid air, they can spread and morph into contrail-cirrus clouds that persist for hours.

These high ice clouds often act like a thin thermal blanket. These thin clouds let sunlight through but trap infrared heat, making their overall effect on climate mostly warming.

Only when they become very dense, muting direct sunlight, does their reflectivity dominate and tip toward cooling.

This context makes the new findings consequential. If most long-lived contrails form inside natural cirrus, their net climate effect depends on how they modify a cloud field that was already shaping the radiation balance.

Under clear or very thin cirrus conditions, added contrails generally amplify warming. Inside dense cirrus, the balance can flip. Extra ice can reflect more sunlight than the cloud system absorbs as heat, nudging the net effect toward slight cooling.

Smarter routes through cloudy skies

Much of the operational focus to date has been on steering aircraft away from regions primed for contrails in otherwise cloud-free skies.

The new study argues for a more nuanced filter. Routes should be evaluated against the backdrop of current ice cloud structures, not just humidity fields.

“Our results show that we need to take a more differentiated view of the climate impact of contrails in the future,” said Dr. Andreas Petzold, lead scientist 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.”

That’s a practical, testable proposition. Satellite cloud products and high-resolution forecasts already feed into airline and air-traffic tools.

Integrating them explicitly into contrail-avoidance logic could refine when and where reroutes meaningfully reduce net warming, and when they don’t.

Testing contrails in cloudy layers

Beyond the observational analysis, the team ran radiative forcing calculations to estimate how contrails embedded in different cloud scenes perturb the energy budget.

The broad takeaway is that contrails introduced into thick cirrus often change very little.

“Our analysis shows that contrails in thick cirrus clouds actually have hardly any effect,” said Peter Spichtinger, a professor at JGU.

“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.”

That caveat matters. Stacked cloud layers, evolving microphysics, and day-night cycles can all influence radiative impacts.

The new dataset provides the observational scaffolding to test those effects more rigorously, but it doesn’t resolve every edge case.

Turning data into flight strategy

If contrail mitigation is going to be used alongside CO2 reductions, the industry needs decision rules that map atmospheric scenes to expected net climate effects, with uncertainty bars.

This study moves that goalpost closer. First, it shows that most persistent contrails are an “in-cloud” phenomenon rather than a “blue-sky” one.

Second, it suggests that blindly avoiding all contrail-prone regions could waste fuel and time. Such detours don’t always guarantee climate benefits, especially when aircraft are already flying through dense cirrus.

Third, it underscores the value of blended data streams. In situ profiles such as IAGOS, satellite cloud masks and optical thickness data, and weather models that resolve ice-supersaturation layers.

Tracking clouds in future flights

Two immediate priorities emerge. One is to pair contrail detection – from satellites and ground networks – with coincident cloud-property retrievals.

This combination can reveal how embedded contrails evolve and how often they change a scene’s net forcing sign.

The other priority is piloting cloud-aware routing in limited airspace to verify that predicted climate benefits materialize without compromising safety or causing large CO2 penalties.

Over the North Atlantic, where IAGOS coverage is strong and traffic flows are manageable, both are feasible.

The study reframes a simple narrative – contrails warm most in clear skies – into a more realistic one: most persistent contrails occur within clouds that already influence the climate signal.

Acting on that insight means planning flights based on the sky we actually have, not the sky we imagine.

The full study is published in the journal Nature Communications.

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