ObserveCirrus homogenitus (contrail)
ci hom · Special · 18 October 2026 · By
Contrails: reading the sky for aviation weather clues
A contrail is a cirrus cloud that would not exist without an aircraft. The World Meteorological Organization formalised this by giving them the name cirrus homogenitus — 'man-made cirrus'. They are worth observing carefully, because whether a contrail persists or vanishes within seconds is one of the best free measurements of upper-level humidity available to anyone on the ground.
Cirrus homogenitus
Cirrus homogenitus (contrail)
- Band
- Special
- Altitude
- 8 km – 12 km
- Code
- ci hom
- Seasonal window
- Contrails are visible year-round but persist most often in winter and early spring, when the upper troposphere over mid-latitudes is coldest and ice supersaturation is most frequent. (Jan–Dec)
- Published
- 18 October 2026
- By
Weather clue
Contrails that start persisting and spreading mean the upper troposphere is moistening. If that is followed by a milky veil and a 22° halo, rain is 12-24 hours out.
How to observe it
- Time each contrail from the aircraft's passage to the moment the line is no longer distinguishable from the background.
- Classify it as short (under 2 min), persistent (over 2 min) or spreading (widening visibly).
- Log the classification at 09:00, 12:00 and 15:00 every day for a week; the change is a genuine forecast signal.
- Watch whether a contrail deck is followed by cirrostratus and a halo within 24 hours.
Two kinds of contrail, two different meanings
Not all contrails behave the same way, and the difference is diagnostically useful.
| Type | Behaviour | What it means |
|---|---|---|
| Short-lived | Vanishes within seconds to 2 minutes behind the aircraft | Ambient air is dry; no ice supersaturation |
| Persistent, non-spreading | Stays as a narrow line for many minutes, does not widen | Air is near ice saturation; humidity is marginal |
| Persistent and spreading | Widens, becomes diffuse, joins others into a deck | Air is ice-supersaturated; this is where cirrus homogenitus comes from |
| Contrail cirrus deck | A continuous thin sheet formed entirely from old contrails | The upper troposphere is very moist; often a frontal precursor |
Why they form at all
Jet engines emit water vapour and small particles, and the exhaust is hot. As the exhaust plume mixes with ambient air at −40 °C to −60 °C, it cools very rapidly, and the water vapour condenses onto the emitted particles and then freezes.
Whether the contrail survives depends entirely on the humidity of the ambient air. If the air is dry, the ice crystals sublime within seconds. If the air is supersaturated with respect to ice — which is common in the upper troposphere at temperatures below −40 °C — the crystals keep growing and the contrail persists. Ice supersaturation is invisible from the ground, and a persistent contrail is one of the few ways to detect it.
Using contrails as a weather instrument
This is the practical part, and it is a genuinely useful forecasting skill.
Contrails that persist and spread mean moist air aloft. In the 12 to 24 hours before a warm front, the upper troposphere moistens from the west, and the first sign of it may be contrails that suddenly start persisting.
Contrails that stay short mean dry air aloft, which usually means a stable, settled pattern for at least another day.
A sudden change from short to persistent contrails on the same day is a genuine signal worth logging, because it means the upper-level moisture field is changing over your location.
- Record the time, the aircraft's approximate heading, and how long the contrail lasted.
- Note whether it widened; widening is the signature of ice supersaturation.
- If several contrails merge into a layer, observe whether that layer later thickens into cirrostratus.
- Do not confuse a contrail with cirrus: contrails are geometrically straight over their whole length and have a visible origin point.
The climate question, in numbers
Persistent contrails and the cirrus they generate are estimated to contribute a net warming effect on the climate that is comparable in magnitude to, and by some estimates larger than, the CO₂ emitted by the same aircraft. The reason is that the thin cirrus they form traps outgoing infrared radiation from the surface while reflecting relatively little incoming sunlight.
The effect is highly concentrated: because contrail formation requires cold, ice-supersaturated air, a small fraction of flights — those crossing particular regions at particular times — are responsible for a large share of the effect. This is why there is active research into rerouting flights by a few hundred metres to avoid the most sensitive layers.
Rules of thumb
- Contrails are formally named cirrus homogenitus — man-made cirrus.
- Persistent, spreading contrails indicate ice-supersaturated air aloft, which is invisible from the ground.
- A change from short-lived to persistent contrails often precedes a warm front by 12-24 hours.
- Contrail cirrus has a net warming effect on the climate that is highly concentrated in a small fraction of flights.
| The other cloud | Comparison — Cirrus homogenitus (contrail) |
|---|---|
| 天然卷云 · Natural cirrus | Natural cirrus is never geometrically straight over long distances and has no visible origin point; a contrail is a dead-straight line that begins at a specific spot. |
| 旧航迹形成的人为卷云 · Cirrus homogenitus from an old outbreak | Once a contrail deck has spread it can be indistinguishable from natural cirrus; the only clue is that it appeared on a day when contrails were spreading and it formed in a straight band. |
The two or three genera this one is most often confused with, and the single feature that separates them.
Observation questions
What makes the difference between a short and a persistent contrail?
Only the humidity of the air the aircraft is flying through. Dry air sublimates the ice crystals within seconds; air that is supersaturated with respect to ice lets them keep growing. Ice supersaturation is invisible from the ground, which is why contrails are a useful measurement.
Can contrails change the weather?
They do not change the daily weather, but persistent contrails form cirrus that traps outgoing infrared radiation from the surface. The estimated net warming effect of contrail cirrus is comparable to the CO2 emitted by the same flights, and it is highly concentrated in a small fraction of them.