How Contrails Form Behind Aeroplanes at High Altitude
Contrails are lines of ice crystals that form when water vapour from jet exhaust meets very cold air. Learn why some vanish quickly and others spread for hours.
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A contrail is a line of ice crystals that forms behind an aircraft flying high in very cold air. The name is short for "condensation trail". Water vapour in the engine exhaust cools, turns to droplets and freezes. Whether the trail vanishes in seconds or stretches across the sky for hours depends mainly on how humid the surrounding air is.
How a contrail forms
According to Wikipedia, engine exhaust is mostly water vapour and carbon dioxide. At cruising height it enters air that is extremely cold. The extra vapour raises the humidity in that small patch of air to saturation, so the vapour condenses into tiny droplets, which freeze if it is cold enough. Soot and sulphur compounds in the exhaust give the vapour something to condense on.
The trail does not start right at the engine. The exhaust takes a little time to mix with the cold air and cool, so the visible trail begins a short distance behind the aircraft. NOAA's description agrees on the basic process: vapour in the exhaust cools, condenses and freezes into ice crystals that form a visible trail, when the added vapour pushes already moist air to saturation.
How cold and how high
Contrails usually form above about 8,000 metres (26,000 feet), where the air is colder than about minus 36.5 degrees Celsius, says Wikipedia. NOAA likewise cites a cruising altitude of 26,000 feet or higher for most commercial jets in this context. Contrails can also form lower down if the air is cold and moist enough. Other sources quote slightly different temperature thresholds, so treat the numbers as approximate.
Why some trails vanish and others stay
Wikipedia says that, depending on temperature and humidity, a contrail may last for seconds or minutes, or persist for hours and spread several kilometres wide, eventually resembling natural cirrus or altocumulus cloud. NOAA's material points to the same ingredient: higher relative humidity means a contrail lasts longer, and wind affects how fast it spreads.
- Dry air around the aircraft: the ice crystals evaporate quickly and the trail is short
- Air already humid enough to hold ice: the crystals persist and can grow as they attract more moisture
- Wind: stretches and spreads a persistent trail
The crystals in a persistent trail keep collecting water vapour from the surrounding moist air, which is why they can grow and fan out. NOAA adds that the amount of supersaturation (air holding more moisture than it normally could) affects how dense or thick the trail looks.
A useful clue about the air
Because the length of a trail reflects how humid the upper air is, people watching the sky sometimes use contrails as a rough hint about the weather. The sources used here describe the physics but do not make forecasting claims, so this is an observation rather than a rule.
Do contrails affect the climate?
This is an area of active research. Wikipedia reports that persistent contrails add to cloudiness and that their net radiative forcing is roughly 0.03 to 0.06 watts per square metre, because they trap outgoing heat more than they reflect incoming sunlight. It adds that their warming effect may be similar in size to that of aviation's carbon dioxide emissions, depending on the time span used, and that they tend to cool daytime temperatures and warm those at night.
Rerouting a small share of flights around the most contrail-prone air has been suggested as a way to cut the effect. Wikipedia also mentions a German study from November 2025 which concluded that the climate impact of contrails cannot yet be estimated well enough to justify rerouting strategies. In other words, the size of the effect is still being worked out.
Quick summary
- Hot exhaust adds water vapour to very cold air
- The vapour condenses on tiny particles and freezes into ice crystals
- Dry surroundings make the trail short; humid ones let it last and spread
- Persistent contrails add cloud, and scientists are still working out how much they matter for the climate
Next time a bright line appears behind a distant aircraft, the way it behaves tells you something about the air up there at that moment, even though you cannot see the temperature or humidity directly.
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