Yes: the white trails that remain behind aircraft are, in meteorological terms, very narrow clouds caused by human activity. They are called contrails, short for condensation trails, and consist mainly of ice crystals. But they do not all develop in the same way: a trail may disappear within seconds or minutes, or it may persist, broaden in the wind and become visually difficult to distinguish from a natural cirrus cloud.
The key is not the weather experienced on the ground, but the atmosphere at cruising altitude. The air may be dry and warm for us, yet very cold and sufficiently humid at 8–12 kilometres above the ground, where many commercial aircraft fly. Those conditions determine whether a trail evaporates or becomes a persistent cloud.
For more information on climate change, see also Earth after 100 years without humans: what would remain, what would collapse and what would not recover, which provides complementary details on this subject.
What are aircraft’s white trails and how do they form?
A jet engine releases hot gases that include water vapour and fine particles. When this plume suddenly mixes with the extremely cold air at high altitude, the vapour condenses onto particles and freezes very quickly. The visible result is the stream of ice crystals behind the aircraft.
The Federal Aviation Administration (FAA) explains that these trails form in cold, humid conditions and that ice crystals can form around particles in engine exhaust as well as particles already present in the atmosphere. They are therefore not simply “steam” floating in the air like that above a cup: at cruising altitude, the relevant process is rapid freezing.
One important qualification is useful: aircraft exhaust initiates the trail, but for a persistent contrail, most of the water that fuels crystal growth comes from moisture already present in the atmosphere through which the aircraft is flying. This explains why the same type of aircraft may leave a long-lasting trail on one day and almost nothing visible on another.
Why some trails vanish and others spread: the difference is in the upper air
Two conditions must be distinguished. The first is initial formation: the air at the aircraft’s altitude must be sufficiently cold, and the mixture of exhaust and ambient air must allow condensation and freezing. The second is persistence: once they have formed, the crystals must find enough water in vapour form around them to avoid sublimating—that is, changing directly back into vapour.
Meteorologists call this second environment “ice-supersaturated”. The term does not mean the air is necessarily full of visible droplets; it means that, at that temperature, there is enough water vapour for ice to remain or grow. Recent research regards this supersaturation as the main condition for contrails to persist, and forecasting it remains difficult precisely because humidity in the upper troposphere is hard to measure and model with high precision.
| What you see in the sky | What it suggests about the atmosphere at altitude | What may happen next |
|---|---|---|
| A thin trail that breaks up or disappears quickly | After formation, the air is too dry for the ice to last long. | The trail evaporates within seconds or minutes. |
| A long trail with still relatively well-defined edges | Cold air that is sufficiently humid to sustain ice crystals. | It may last from minutes to hours. |
| Trails that widen, break up and form a whitish veil | Wind and humidity favour the spread of a persistent contrail. | It may develop into aviation-induced cirrus, which is visually difficult to distinguish from natural cirrus. |
From the ground, you cannot determine the exact humidity at aircraft altitude simply by looking at the sky. Nor is a weather forecast for your locality, by itself, a direct test of conditions at an altitude of 10 kilometres. It may show whether surface weather is changing, but it cannot confirm the cause of every trail you see.
Why crossed trails or a “grid” can appear in the sky
A grid-like appearance does not require a mysterious explanation. Air routes intersect, aircraft fly in different directions, and wind can shift or bend the trails after the aircraft has passed. If the favourable layer of air is extensive and air traffic is heavy, several persistent contrails can overlap.
A trail may appear broken because the aircraft passes through layers of air with different properties. Even along a route of several hundred kilometres, humidity and temperature can change enough for the trail to appear, disappear and reappear. The US Environmental Protection Agency states that trails may be straight, curved, spiralled, fragmented or crossed, depending on atmospheric conditions, wind and aircraft routes.
Contrails and climate: why CO2 is not the only factor
Aviation warms the climate through carbon dioxide emissions, which accumulate and act over the long term. But flying also has non-CO2 effects: nitrogen oxides, particles, water vapour at altitude and, especially in this discussion, changes in cloud cover through contrails and aviation-induced cirrus.
A thin ice cloud can have two opposing effects: during the day, it reflects some sunlight back into space, which tends to cool, but it also retains some of the infrared radiation emitted by the Earth, which tends to warm. At night, the cooling component of reflected sunlight is no longer present. Overall, persistent contrails are estimated to have a warming effect in aggregate, but the precise magnitude depends on the time of day, season, location, cloud thickness, crystal properties and pre-existing clouds.
The European Union Aviation Safety Agency (EASA) stresses that non-CO2 effects accounted for more than half of aviation’s historical net warming effect between 1940 and 2018, but also that uncertainty around these effects is much greater than for CO2. This is the balanced position: the link to warming is real, but the climate impact of a particular trail photographed from a city in Romania cannot be inferred.
What aviation is trying to do to reduce persistent trails
Research does not start from the premise that all flights must avoid every trail. The most climatically relevant are persistent trails that spread, and these occur only in certain atmospheric areas. One of the approaches under study is therefore to forecast those layers and make limited changes to the route or altitude of certain flights, only where the climate benefit justifies the extra fuel use and safety is not affected.
The FAA explicitly refers to research into predicting, avoiding or reducing the formation of persistent contrails. In Europe, EASA states that aircraft operators must monitor and report non-CO2 effects annually, beginning with emissions produced on 1 January 2025, under the applicable European requirements. Fuels with fewer particles that form ice nuclei are also being investigated, as are improvements to weather models.
What a trail in the sky does not actually show
A persistent trail does not prove that an aircraft is “spraying” a secret substance, nor is it, by itself, evidence about weather at ground level or a local climate event. The verified physical explanation is the formation of ice crystals from aircraft exhaust and water vapour available in the cold air at altitude. The EPA specifies that the term “chemtrails” is often incorrectly used as a synonym for contrails.
In short: white trails are flight-induced clouds, but their behaviour depends on an atmosphere we cannot see directly. A short trail indicates that the ice did not find conditions in which it could persist. A widening trail shows that the aircraft passed through a cold, humid layer favourable to persistent cirrus—the very type of artificial cloud that researchers now include in assessments of aviation’s climate effect.
Frequently asked questions
Are all white trails behind aircraft contrails?
Most long white trails observed at cruising altitude are contrails. However, an aircraft can also produce other local visible effects, and exact identification from a single photograph is not always possible.
Why do not all aircraft leave the same trail?
Because temperature, humidity and wind vary from one atmospheric layer to another. Even the same aircraft may leave no trail in one area and a persistent one a few minutes later.
Does a clear day on the ground rule out persistent contrails?
No. Conditions at ground level can be very different from those at an altitude of 8–12 kilometres. Persistence depends mainly on humidity and temperature in the upper atmosphere.
Do contrails always warm the climate?
Not every individual contrail has the same effect at all times. Ice clouds can reflect sunlight during the day and retain infrared radiation. Aggregate assessments indicate a net warming effect for persistent contrails, with significant uncertainty over its magnitude.
Sources consulted
- Federal Aviation Administration — Contrails
- US Environmental Protection Agency — Information on Contrails from Aircraft
- European Union Aviation Safety Agency — Climate Change and Aviation Environmental Impacts
- Atmospheric Chemistry and Physics — How well can persistent contrails be predicted? An update
