ObserveCavum (fallstreak hole)
cav · Special · 15 October 2026 · By
Cavum: the hole punched in a cloud by an aircraft
A fallstreak hole is a circular or elliptical gap in a thin mid-level cloud, often with a delicate trail of ice crystals hanging below it. It is one of the very few cloud features that is almost certainly man-made, and it is a remarkably good demonstration of a physics process that also governs natural precipitation.
Cavum
Cavum (fallstreak hole)
- Band
- Special
- Altitude
- 2 km – 8 km
- Code
- cav
- Seasonal window
- Fallstreak holes occur year-round but are most often noticed in winter and early spring, when altocumulus layers with supercooled water are most common at mid-latitudes. (Jan–Dec)
- Published
- 15 October 2026
- By
Weather clue
A fallstreak hole means the cloud layer overhead is composed of supercooled water and is therefore unstable to any disturbance. If the layer later thickens into a frontal sequence, the precipitation from it may begin as snow even if the surface is above freezing.
How to observe it
- Measure the hole's apparent diameter against a fixed landmark and repeat the measurement ten minutes later.
- Look for a faint straight line running through two or more holes; that is the flight path.
- Check whether the layer is altocumulus or cirrocumulus first — cavum only forms in layers of supercooled water.
- Photograph it immediately; most fallstreak holes are gone or have lost their shape within 20-30 minutes.
The mechanism, step by step
The cloud has to be the right kind: an altocumulus or cirrocumulus layer containing supercooled water droplets at a temperature between about 0 °C and −15 °C. Water in this state is liquid but below freezing, and it is unstable — it will freeze if given a nucleus or a strong enough disturbance.
An aircraft passing through the layer causes a sudden pressure drop and cooling behind its wingtips and propellers, which is enough to freeze a few droplets. Those ice crystals then grow by collecting the surrounding supercooled water, and because the ice is heavier than the droplets it falls out of the layer as a visible trail.
As the crystals fall and collect water, the droplets around them are consumed, and the cloud in that patch evaporates. The hole therefore grows outwards from the initial freezing point, which is why fallstreak holes are almost perfectly circular and can reach several kilometres across within 30 minutes.
What you actually see
From below, a mature fallstreak hole has three components, and recognising them confirms the diagnosis.
| Component | Appearance | Cause |
|---|---|---|
| The hole | Sharp-edged circular or elliptical gap | Supercooled droplets evaporated after being collected |
| The trail | A narrow streak of fibrous ice hanging below the hole | Ice crystals falling out of the layer |
| The rim | The cloud edge around the hole, often brighter than the rest | Remaining droplets at the edge, back-lit |
| The mid-cloud line | Sometimes a faint straight line through the hole | The aircraft's flight path |
Why this matters for real clouds
Supercooled water at −10 °C is not an exotic laboratory condition; it is normal in mid-level and high clouds and in the tops of winter storms. The same collection process that makes a fallstreak hole also makes natural precipitation: ice crystals in a cold cloud grow by collecting supercooled droplets until they are heavy enough to fall. This is called the Bergeron process, and it is how most mid-latitude rain and snow begins.
The difference is only in what starts it. In natural clouds it is a dust particle, a biological particle or a rare spontaneous freezing event. In a fallstreak hole it is an aircraft. The cloud cannot tell the difference.
How to observe and time one
Fallstreak holes are ephemeral and they change fast, so a useful observation has to include timing.
Note the diameter when you first see it and again ten minutes later. A hole that grows from 200 m to 2 km in ten minutes is expanding at about 1.5 m/s, which is typical. Also note whether there is more than one hole: aircraft routes are straight lines, so multiple holes along a line is a strong confirmation of an aircraft origin rather than natural localised freezing.
Rules of thumb
- Fallstreak holes need a layer of supercooled water between about 0 °C and −15 °C.
- An aircraft's wingtip vortices trigger freezing, and the hole then grows outward as the crystals collect the droplets.
- The same collection process — the Bergeron process — is how most mid-latitude rain and snow begins naturally.
- Multiple holes along a straight line confirm an aircraft origin.
| The other cloud | Comparison — Cavum (fallstreak hole) |
|---|---|
| 云单体之间的天然缝隙 · A genuine gap between cloud elements | A natural gap has irregular, cloud-shaped edges following the element pattern; a fallstreak hole is a clean circle or ellipse cutting across the element pattern. |
| 航迹云的阴影 · Contrail shadow | A contrail casts a linear shadow or leaves a straight line, not a circular hole with an ice trail beneath it. |
The two or three genera this one is most often confused with, and the single feature that separates them.
Observation questions
Why are fallstreak holes circular?
Because the process spreads outwards at a roughly constant rate in all directions from the initial freezing point. The hole grows as the ice crystals fall and collect the surrounding supercooled droplets, and there is no preferred direction for that, so the result is a circle.
Do I need to see an aircraft to confirm it?
No, and usually you cannot. The stronger evidence is the shape of the layer and the trails: a clean circular hole in a thin altocumulus deck with a fibrous ice trail hanging beneath it is diagnostic. Multiple holes in a straight line is even better evidence.