Learn how cold air, engine exhaust and humidity create aircraft vapour trails, why some vanish quickly and why others persist and spread.
Aircraft vapour trails, or contrails, form when hot, water-rich engine exhaust mixes with very cold air at altitude. The water vapour condenses and freezes around tiny particles into ice crystals. If the surrounding air is dry, the trail vanishes; if it is ice-supersaturated, it can persist and spread.
How does a jet engine make a contrail?
A jet engine makes a contrail by adding water vapour and microscopic particles to air cold enough for the mixed exhaust plume to become saturated. The visible white material is frozen water, not water vapour itself; gaseous water vapour is invisible.
- Combustion produces water: burning hydrocarbon fuel with oxygen creates water vapour and carbon dioxide. The exhaust also contains small particles that can act as condensation nuclei.
- The exhaust mixes with cold air: the hot plume cools rapidly as it leaves the engine and mixes with the atmosphere.
- Moisture condenses: if the mixture reaches saturation, microscopic water droplets form around particles in the plume.
- The droplets freeze: at normal contrail temperatures, they quickly become the ice crystals that make the trail visible.
Contrails are most common near airliner cruising levels, where temperatures are often around −40°C or colder. There is no fixed contrail altitude: the limiting temperature changes with atmospheric pressure, humidity and engine efficiency. These factors are formalised by the Schmidt–Appleman criterion.
Why do some vapour trails disappear while others spread?
A contrail's lifetime is controlled mainly by the relative humidity with respect to ice at flight level. The exhaust can supply enough extra moisture to create the initial trail, but the surrounding atmosphere decides what happens after the plume has diluted.
| Atmospheric condition | What happens to the contrail | Reason |
|---|---|---|
| Dry with respect to ice | Disappears within seconds or minutes | Ice crystals sublimate directly back into invisible water vapour |
| Near ice saturation | Remains narrow for longer | The crystals lose moisture slowly |
| Ice-supersaturated | Persists, grows and spreads | Ice crystals collect additional atmospheric moisture |
A persistent contrail may widen under wind shear and develop into a sheet of artificial cirrus cloud. Humidity measured at the ground says little about this process because the relevant layer may be several kilometres above it.
Why does a contrail suddenly start and stop?
A broken contrail usually shows that the aircraft is crossing alternating layers of suitable and unsuitable air. Small changes in temperature or ice-relative humidity can move conditions across the formation threshold, making a trail appear or disappear while engine power remains steady.
Wind shear can also distort an older trail or create apparent gaps. A start-stop pattern does not normally mean the pilots are switching an engine or another system on and off.
Are wingtip vapour trails the same as exhaust contrails?
No: wing and wingtip condensation clouds are produced by a local pressure drop rather than primarily by engine exhaust. Air cools as its pressure falls over a lifting wing or inside a strong wingtip vortex; in sufficiently humid conditions, that cooling causes moisture to condense.
These aerodynamic trails are often seen during take-off, landing or hard manoeuvring, and they usually vanish quickly. The wake vortices still exist when no cloud is visible—the condensation merely reveals part of their structure. At high altitude, exhaust contrails can also be rolled up and repositioned by the wake, so the number of visible lines does not always match the number of engines.
Are aircraft vapour trails smoke or dumped fuel?
Normal white contrails are neither smoke nor dumped fuel. They consist mainly of ice crystals formed from engine-produced and atmospheric water. An engine can emit soot, particularly at high power or in older designs, but a dark exhaust plume is visually and chemically different from a white contrail.
Fuel jettison is a separate, uncommon procedure available only on suitably equipped aircraft. Atomised fuel generally evaporates and does not explain the long white trails routinely seen behind cruising airliners.
Do persistent contrails affect the climate?
Persistent contrails can develop into contrail cirrus, which both reflects incoming sunlight and traps outgoing infrared heat. The balance varies with time, location and existing cloud, but averaged globally, the heat-trapping effect is understood to outweigh the cooling effect, producing a net warming influence.
How are vapour trails represented in flight simulators?
Flight simulators approximate contrails in different ways. Some effects respond to atmospheric temperature and humidity, while others appear above a preset altitude or use fixed emitters attached to engines and wingtips.
In our downloads library, an FS2004 example of three-dimensional high-altitude contrails demonstrates the classic exhaust effect. A separate Boeing 737 package combining engine and wingtip trails shows why simulator add-ons may group two physically different forms of condensation under the same visual-effects label.