Contrails (also Vapour Trails)
Contrails (condensation trails, also called vapour trails) are line-shaped clouds produced by aircraft engine exhaust or by changes in air pressure in regions where the ambient air is cold enough and moist enough for condensation or deposition to occur.
How Contrails Form
Engine exhaust contrails form when hot, moist exhaust gases mix with cold, low-humidity ambient air at high altitude, causing water vapour to condense and freeze on soot particles from the engine. Aerodynamic contrails form when the pressure drop over wing surfaces briefly lowers temperature enough to cause condensation.
Contrail Persistence and Forecasting
- Non-persistent contrails: Evaporate quickly — the air is relatively dry at that altitude
- Persistent contrails: Last for minutes to hours — the air is near saturation at altitude; may spread into cirrus sheets
- Aviation significance: Persistent contrails indicate high humidity at cruise altitude, which may signal an approaching weather system
Military and Strategic Considerations
Military operations use contrail forecasting to predict whether aircraft will be visible from the ground. In instrument training, recognizing contrail patterns can provide information about upper-level humidity and atmospheric conditions at cruise altitude.
Related Terms
See also: Atmospheric Stability, Jet Stream.