Contrails explained: how they form, their climate impact and why some last so long

A clear autumn morning, the sky deep blue – and above it a grid of white lines. Some are barely longer than the aircraft itself, others stretch as broad bands from horizon to horizon and turn into a milky veil over the course of the morning. Same altitude, similar jets, completely different picture. Contrails are the most visible sign of air traffic and at the same time a fairly complex piece of atmospheric physics. This article explains how they form, why some vanish at once while others last for hours, and what research knows today about their climate impact.

Contrails: the short answer

A contrail is an artificial ice cloud. A jet engine burns kerosene, and in the process, besides CO₂, it produces large amounts of water vapor, plus soot particles and other tiny particles. Behind the aircraft, the hot, humid exhaust mixes with the ambient air, which at cruising altitude is usually colder than minus 40 degrees. The mixture briefly becomes supersaturated with respect to water, the vapor condenses onto the particles and freezes immediately into ice crystals. The result is what you see as a white line.

Whether the line is gone again after a few seconds or grows into a veil cloud is then no longer decided by the aircraft but by the air it flies through: If the air at flight level is dry, the crystals evaporate. If it is supersaturated with respect to ice, they keep growing.

How a contrail forms

The process takes only fractions of a second but can be broken down into three steps:

1. Combustion: Every kilogram of kerosene produces significantly more than a kilogram of water vapor – hydrogen from the fuel combines with oxygen from the air. Soot particles, sulfur compounds and volatile organic particles come on top.
2. Mixing: The exhaust leaves the engine hot and humid. As it mixes with the ice-cold ambient air, it cools abruptly. Because cold air can hold much less water vapor than warm air, the mixture briefly passes through a state in which it is supersaturated with respect to liquid water.
3. Freezing: The vapor condenses onto the particles into tiny droplets, which freeze instantly at these temperatures. Behind a large jet, an unimaginable number of ice crystals form every second.

Meteorology sums up the condition in the Schmidt-Appleman criterion, named after the German physicist Ernst Schmidt and the US meteorologist Herbert Appleman. From the pressure, temperature and humidity of the surroundings and from properties of the engine, it predicts whether a streak can form at all. An interesting side effect: The more efficiently an engine works, the cooler its exhaust is at the same water vapor content – so contrails can form behind modern engines at somewhat higher ambient temperatures than behind old ones. How modern and older engines differ visually is shown in the guide Recognizing engines.

Why at cruising altitude in particular?

Because it is cold enough there. In lower, warmer layers, the cooling during mixing is usually not enough to supersaturate the mixture. Most jets fly en route at roughly eight to twelve kilometers – exactly the range where the conditions are often met. Why airliners fly so high in the first place is explained in the article Why aircraft fly at around 10,000 meters.

Exhaust trails and aerodynamic condensation

Not every white trail on an aircraft comes from the engine. On takeoff and landing, on humid days you often see wisps of mist over the wings or spiral tubes at the flap ends. They form because the air pressure above the wing and in the core of the vortices drops sharply and the air cools in the process. This aerodynamic condensation is a completely different phenomenon, short-lived and near the ground – and a rewarding photo subject. How to catch it is described in the guide Photographing vortices and condensation over the wings.

Why some streaks stay and others don’t

This is the question asked most often at the spotter’s fence. The answer lies in a quantity you can’t see from the ground: the relative humidity over ice at flight level.

State of the air at flight level	What happens to the streak	Typical picture in the sky
Too warm or too dry for the Schmidt-Appleman criterion	No streak forms at all	The jet passes as a silver dot without a trail
Cold enough, but subsaturated with respect to ice	Crystals evaporate after seconds to a few minutes	Short white line that follows the aircraft like a tail
Saturated to supersaturated with respect to ice	Crystals stay stable or keep growing	Long streak that stays, widens and drifts
Clearly supersaturated, over a large area	Streaks spread for hours and merge	Net of streaks, later a milky veil of cirrus

Ice-supersaturated air layers are a normal part of the upper troposphere. There is actually enough moisture in them for ice clouds, but nucleation sites are missing – it is the aircraft that supplies them. These layers are often only a few hundred meters thick, but sometimes very extensive. That is why you can see two jets a few minutes apart leave completely different trails: One flies right through the humid layer, the other a few hundred meters above or below it.

From streak to cloud: contrail cirrus

If a streak stays, a second phase begins. The wind aloft changes with height – this wind shear pulls the streak sideways. The ice crystals grow, slowly sink and spread out. After an hour, the sharp line has often become a broad, fibrous band, and after a few hours a veil that is hardly distinguishable from natural cirrus clouds. Experts speak of contrail cirrus. Such formations can grow several kilometers wide and persist for a long time; on satellite images, individual streaks can be followed far longer than you can still recognize them from the ground as an aircraft track.

With four-engine aircraft you can watch how the individual exhaust plumes first run side by side, then merge in pairs and finally into one broad streak. The reason is the wingtip vortices, which catch the exhaust and carry it downward.

The opposite trail: distrails

It also works the other way around. If a jet flies through a thin cloud layer, it can leave a clear lane – a so-called dissipation trail or distrail. Heat from the exhaust and the mixing in of drier air make the cloud droplets along the flight path evaporate. Distrails are much rarer than contrails and therefore a nice find for sky watchers.

The climate impact of contrails

For a long time, CO₂ was the main yardstick when it came to flying and climate. It is now clear that air traffic also affects the climate through other pathways, and contrails are the most conspicuous of them.

Two effects against each other

A contrail influences the radiation budget in two directions:

• Cooling: In daytime, the ice crystals reflect some of the sunlight back into space before it reaches the ground.
• Warming: At the same time, they trap heat radiation emitted by the Earth’s surface and the lower atmosphere – much like a natural cirrus cloud.

In the global average, the warming part outweighs the cooling according to current knowledge. This is especially clear at night: Then there is no sunlight to reflect, and only the heat effect remains. An analysis of North Atlantic traffic concluded that night flights account for a vastly disproportionate share of the contrail effect, as do winter flights.

The numbers and their uncertainty

The most-cited overall assessment is the study by Lee et al. (2021) in the journal Atmospheric Environment. It rates the climate impact of air traffic for the year 2018 using the so-called effective radiative forcing, measured in milliwatts per square meter.

Contribution of air traffic (2018)	Effective radiative forcing	Authors’ confidence
Contrail cirrus	about 57 mW/m²	low, wide range
CO₂ from kerosene	about 34 mW/m²	high
Nitrogen oxides (net)	about 18 mW/m²	low
Total air traffic (net)	about 101 mW/m²	–

According to this assessment, air traffic accounts for about 3.5 percent of total human-caused effective radiative forcing, and about two thirds of its effect is not due to CO₂. Contrail cirrus is the largest single item.

The caveats are important, though:

• Wide range: For the contrail value, the authors give an uncertainty that is almost as large as the value itself. For CO₂ it is much smaller.
• Effectiveness at the surface: Work from the German Aerospace Center (DLR) suggests that a given radiative forcing from contrails warms the Earth’s surface less than the same forcing from CO₂.
• Short versus long: A contrail acts for hours, while CO₂ stays in the atmosphere for centuries. How to compare the two depends on the chosen time horizon – over short periods, streaks have a relatively stronger effect, while over long ones CO₂ catches up.
• Open questions: Recent studies show that many long-lived streaks form within already existing cirrus clouds. That makes the assessment more complicated than long assumed.

In short: That contrails make a relevant contribution to the climate impact of flying is scientific consensus. Exactly how large it is is still being researched.

Can contrails be avoided?

Because the effect is so strongly tied to certain weather conditions, there are two promising approaches.

Flying around humid layers

A study by Teoh and colleagues (2020) on the airspace over Japan found that only about 2 percent of flights caused about 80 percent of the contrail effect. Small altitude changes on a fraction of flights could therefore avoid a large part of the effect. The catch: A different flight level often costs some extra fuel and thus CO₂. The trade-off is only worthwhile if the forecast of the humid layers is reliable.

That is exactly what was tested in practice. Google Research, American Airlines and Breakthrough Energy published a trial in 2023: Over half a year, on 70 test flights, pilots deliberately avoided flight levels where streaks were likely, based on an AI-assisted forecast. Satellite analyses showed about 54 percent fewer contrails, and the flights concerned burned about 2 percent more fuel. Trials with air traffic control are also under way in Europe; the role that organizations such as Eurocontrol or DFS (Germany’s air navigation service provider) play is explained in the linked articles.

For spotters, that means: If you see on a flight tracker map that a jet changes flight level for no apparent reason, this can be a reason too – besides turbulence, conflicting traffic or wind. More on altitude changes due to rough air can be found under Turbulence explained.

Burning cleaner

The second lever lies in the fuel. Fewer soot particles in the exhaust mean fewer nucleation sites – the streak then consists of fewer but larger ice crystals, is optically thinner and dissolves faster. In the ECLIF3 measurement campaign by DLR, Airbus and Rolls-Royce, an Airbus A350-900 with Rolls-Royce engines flew on sustainable aviation fuel, and a research aircraft followed it through the streak. Significantly fewer ice crystals were measured compared with conventional kerosene, with a reduction of a good half reported. The main causes are considered to be the lower content of aromatics and sulfur.

Watching contrails: what spotters get out of them

For plane spotters, contrails are more than a climate topic. They reveal aircraft you would otherwise miss, and they are a photo subject in their own right.

• Determine the type: Two or four exhaust plumes can often be told apart with binoculars or a telephoto lens before you recognize the aircraft itself. A four-engine jet like the Boeing 747 initially draws four separate lines.
• Match the flight: Note the time and direction of flight and compare with a flight tracker app. How that works cleanly is described in Matching sightings with ADS-B; an overview of the tools is provided by the comparison of flight tracker apps.
• Read routes: Noticeably curved streaks over central Europe often follow the big North Atlantic routes. Why aircraft seem to fly arcs is explained in Great-circle flight routes.
• Photograph: The technique for sharp pictures at ten kilometers distance – focal length, exposure, focus – is covered in detail in the guide Photographing aircraft at cruising altitude.

Myths about contrails

“Long streaks mean more exhaust.” No. The length and persistence depend almost entirely on the air, not on the amount of exhaust. The same jet can leave no streak at all one day and a miles-long one the next.

“Contrails are chemtrails.” The claim that long-lived streaks are secretly sprayed chemicals is a conspiracy theory without evidence. Atmospheric researchers have repeatedly examined and rejected it. The differing behavior of the streaks is fully explained by temperature and humidity at flight level.

“Contrails are smoke.” They consist essentially of ice crystals. Soot is involved, but only as a tiny seed on which the water freezes.

“Modern aircraft no longer make streaks.” Quite the opposite: More efficient engines emit cooler exhaust and can therefore form streaks in somewhat warmer ambient air. Less soot can, however, make the streaks thinner and shorter-lived.

Conclusion

Contrails form when humid engine exhaust freezes in the ice-cold air at cruising altitude. Whether they vanish at once or last for hours is decided by the humidity at flight level: In ice-supersaturated air they grow into veil clouds, the contrail cirrus. According to current knowledge, these clouds contribute a large part of the climate impact of air traffic, especially at night and in winter. The exact size is still uncertain, though. Targeted altitude changes and cleaner fuels are considered the most important countermeasures, and both are currently being tested in practice. For you as a spotter, every streak is also a clue: to the weather up there, to the aircraft type – and to a subject worth shooting with a long focal length.