What causes aircraft vapour trails, and why do they form?
What causes aircraft vapour trails? Learn how contrails form, why some vanish or spread, and how they differ from wingtip clouds, smoke and fuel.
Aircraft vapour trails, or contrails, form when hot, moist engine exhaust mixes with very cold upper air. The mixture becomes saturated, water condenses on tiny particles and rapidly freezes into visible ice crystals. Dry air causes the ice to sublimate, so the trail vanishes quickly; ice-supersaturated air lets it persist, widen and become contrail cirrus.
In our Aviation & Real-World Flying coverage, we distinguish these high-altitude exhaust contrails from the brief condensation sometimes seen over wings and inside wake vortices. They may look similar, but the physical triggers are different.
What is a vapour trail?
A vapour trail is a visible condensation cloud produced by an aircraft; behind a cruising airliner, it is usually an exhaust contrail composed mainly of ice crystals. The word contrail is a contraction of condensation trail.
Vapour trail is the British spelling, while vapor trail is the US spelling. Despite the name, gaseous water vapour is invisible. The white line we see consists of countless microscopic ice crystals scattering sunlight.
Jets are the most familiar source, but suitably cold conditions can also allow piston- and turboprop-powered aircraft to form exhaust contrails. The aircraft type alone does not decide whether a trail appears.
How are aircraft vapour trails formed?
An exhaust contrail forms when an engine adds enough water and particles to cold air for the rapidly cooling plume to become saturated. Readers wanting the machinery behind that exhaust can see our explanation of combustion and jet exhaust.
- Combustion produces water vapour: Hydrocarbon fuel reacts with oxygen to produce water vapour and carbon dioxide. The exhaust also carries tiny particles, including soot, which can provide surfaces on which droplets form.
- The plume mixes with cold air: Hot exhaust expands and mixes rapidly with the atmosphere behind the aircraft. Its temperature falls while its relative humidity rises.
- The mixture reaches saturation: If the atmospheric temperature, pressure and humidity are suitable, microscopic water droplets condense around exhaust or ambient particles.
- The droplets freeze: At normal contrail-forming temperatures, the droplets freeze almost immediately. These ice crystals create the visible white exhaust trail.
- The wake reshapes the trail: Turbulence and the aircraft's trailing vortices pull the young contrail into the wake. Separate engine trails may merge, twist or appear as fewer lines than the aircraft has engines.
The exhaust can create a short-lived saturated plume even when the surrounding atmosphere is not itself saturated. Formation is therefore a different question from persistence: the engine helps initiate a contrail, but the ambient air decides how long it survives.
At what altitude do plane vapour trails form?
There is no fixed contrail altitude; sufficiently low temperature is more important than height alone. Contrails are common near airliner cruising levels, where temperatures are often around −40°C or colder, but they can form lower in exceptionally cold air or fail to form at cruise when the threshold is not met.
The formal test is called the Schmidt–Appleman criterion. It accounts for atmospheric pressure and humidity, the amount of water released by the fuel, and the relationship between exhaust heat and engine efficiency. Other conditions being equal, a more efficient engine can permit contrail formation at a slightly warmer ambient temperature because less waste heat accompanies the water in its exhaust.
Why do some vapour trails disappear while others spread?
A contrail's lifetime depends mainly on the surrounding relative humidity with respect to ice. Initial formation does not guarantee that the ice crystals will remain after the exhaust plume has diluted.
| Air at flight level | What the contrail does | Why |
|---|---|---|
| Dry with respect to ice | Vanishes within seconds or minutes | The crystals sublimate directly from ice into invisible water vapour |
| Close to ice saturation | Stays narrow for longer | The crystals gain or lose moisture slowly |
| Ice-supersaturated | Persists, grows and spreads | The crystals collect additional water from the surrounding atmosphere |
Wind shear can stretch a persistent trail sideways and distort it until it resembles natural cirrus. Several trails may overlap and form a broader sheet known as contrail cirrus. Persistence is normal atmospheric physics; it is not evidence that the aircraft is dumping fuel.
Why does a vapour trail suddenly start and stop?
A start-stop contrail usually means the aircraft is passing through alternating layers of suitable and unsuitable air. A change of only a few hundred feet can cross a sharp boundary in temperature or ice-relative humidity.
Changes in engine thrust can also affect a marginal contrail, particularly during a climb, but steady engines routinely produce broken trails when the atmosphere is layered. Wind can later break up or displace an older trail, creating apparent gaps from the ground.
Are wingtip vapour trails the same as exhaust contrails?
No. Wing and wingtip condensation is caused by a local pressure drop, whereas an exhaust contrail is initiated by water and particles released during combustion.
Air cools as pressure falls over a lifting surface or within a strong vortex. In humid conditions that brief cooling can produce visible droplets over the wing, behind the flaps or around the wingtips. These clouds are common during take-off, landing and high-load manoeuvres, and they normally disappear as soon as the pressure recovers.
The vortex remains present when no cloud is visible; condensation merely reveals part of it. Our guide to visible wing condensation and wake-vortex avoidance explains why pilots treat an invisible wake just as seriously.
At altitude, wake vortices can roll engine contrails into two broad trailing lines. This is why counting visible trails is not a reliable way to identify how many engines an aircraft has.
Are exhaust trails smoke or dumped fuel?
A normal white aircraft contrail is neither smoke nor dumped fuel. It is an ice cloud, although the exhaust that helps create it also contains invisible gases and small combustion particles.
A dark grey or black exhaust trail can contain visible soot, particularly from an older engine, an engine operating at high power or a fault producing smoke. Heat shimmer may also reveal hot exhaust without any white condensation. These are distinct from a bright, persistent contrail.
Fuel jettison is a separate and uncommon procedure available only on aircraft equipped for it. The fuel leaves designated outlets as a spray and normally disperses as vapour or fine droplets; it does not explain the routine white lines behind cruising aircraft.
What can a contrail tell us about the weather?
A short contrail confirms that conditions in the exhaust plume were cold enough for ice to form, while a persistent, spreading trail indicates an ice-saturated or ice-supersaturated layer aloft. Its movement and deformation can also reveal wind direction and shear at flight level.
It does not provide a simple forecast for conditions at the surface. One mistake we see often is comparing a contrail with ground-level humidity: the relevant air mass may be several kilometres above the observer and have completely different temperature and moisture values.
Do persistent vapour trails affect the climate?
Persistent contrails can develop into contrail cirrus that reflects some incoming sunlight but also traps outgoing infrared heat. The balance changes with location, time of day, season and existing cloud, but scientific assessments find a net warming influence when contrail effects are averaged globally.
The climate concern is chiefly persistent contrail cirrus over many flights, not every brief white line. A short contrail that quickly sublimates has neither the lifetime nor the coverage of a spreading ice cloud.
How are vapour trails represented in flight simulators?
Flight simulators model vapour trails with anything from fixed visual emitters to systems influenced by atmospheric temperature and humidity. The label may cover exhaust contrails, wingtip condensation and flap vortices even though those effects have different real-world causes.
For weather-dependent effects, inspect conditions at the aircraft's altitude rather than the humidity shown for the departure airport. Our explanation of how simulators reproduce real-world weather layers covers the limitations of imported temperature, humidity, wind and cloud data.
- No contrail at cruise: The upper air may be too warm or dry, the visual effect may be disabled, or that aircraft model may not define one. Test in a colder, humid upper-air layer before editing aircraft files.
- A trail appears on the runway or in every weather condition: It is probably a fixed emitter rather than a formation model. Check for duplicate or permanently enabled effects supplied with the aircraft or an add-on.
- Trails come from the wingtips or flaps: These are intended to represent aerodynamic condensation. If they remain visible in dry air and during unloaded cruise, the implementation is decorative rather than weather-driven.
- The visible lines do not match the engine count: The effect attachment points may be wrong, but merged trails and wake roll-up can also produce this appearance realistically.
Older simulators commonly rely on attached effects rather than atmospheric calculations. For example, this FS2004 Boeing 737 package combining engine and wingtip effects illustrates how two physically different types of condensation may be grouped together by a simulator add-on.