ObserveCirrocumulus
Cc · High · 3 October 2026 · By
Cirrocumulus: the mackerel sky and the limits of the folklore
Cirrocumulus is a thin, high sheet of very small white grains at 6000 to 10000 metres, made mostly of ice crystals with some supercooled water. When it covers a large area in ripples it is called a mackerel sky, and the folklore around it is older than any meteorological service. Some of that folklore is good; some of it is not.
Cirrocumulus
Cirrocumulus
- Band
- High
- Altitude
- 6 km – 10 km
- Code
- Cc
- Seasonal window
- Cirrocumulus is visible year-round and is most common in the warm half of the year when the upper troposphere is moist and shallow convection or gravity waves are frequent. (Apr–Sep)
- Published
- 3 October 2026
- By
Weather clue
Cirrocumulus that thickens through the morning and is followed by a milky veil means rain within 24 hours. Patchy cirrocumulus in a clear sky with steady pressure and no thickening means fair weather.
How to observe it
- Use the edge test as well as the thumbnail test — cirrocumulus edges dissolve into the blue with no rim.
- Log element size and coverage in oktas at the same time each day for two weeks; the trend is more informative than any single observation.
- Look for iridescence within about 30° of the sun; it confirms the elements are tiny uniform droplets and therefore cirrocumulus rather than altocumulus.
- Check whether the layer is where a contrail spread out — if so, you are watching cirrus homogenitus becoming indistinguishable from natural cloud.
The three-way test in the high and middle sky
Cirrocumulus is the cloud most often confused with altocumulus, and the thumbnail test settles it. There is a second, more reliable test available in the field: look at the edges of the elements.
| Cloud | Element size at arm's length | Shading | Edges |
|---|---|---|---|
| Cirrocumulus | Fingertip, or a grain of rice | None at all | Soft, almost dissolving into the blue |
| Altocumulus | Thumbnail | Slight shading underneath | Distinct, often with a visible rim |
| Stratocumulus | Whole hand or fist | Clear shading, dark undersides | Sharp and rounded |
What a mackerel sky actually forecasts
The saying 'mackerel sky, not twenty-four hours dry' is a genuinely useful rule in the mid-latitudes, but its reliability depends on which way the sky is going.
Cirrocumulus that is increasing in coverage and thickening into cirrostratus behind it is a real frontal precursor and the saying holds: rain in 12 to 24 hours. Cirrocumulus that is decreasing, patchy, and surrounded by clear blue is usually the remains of a decaying cirrus layer or the edge of a system passing to the north, and the saying fails completely.
- Reliable version — cirrocumulus spreading and thickening, cirrus streaming in behind, pressure falling.
- Unreliable version — scattered cirrocumulus patches in an otherwise clear sky with pressure steady.
- Most reliable of all — cirrocumulus giving way to a milky cirrostratus veil with a 22° halo.
Why the grains are so small and so regular
The regular ripple pattern of a mackerel sky comes from gravity waves or from weak convection in a shallow, conditionally unstable layer. In both cases, air is rising in narrow bands and sinking in between, so the cloud only forms where the air is rising.
Because the layer is thin and the temperature is far below freezing, the droplets that do form stay small — they have very little time and very little water to grow with. The result is a field of tiny elements with a wavelength set by the wave, often 500 metres to 2 kilometres apart. That regularity is why mackerel skies look so artificial.
Ice crystal optics in cirrocumulus
Because cirrocumulus is mostly ice, it can produce the same family of halos as cirrus, though usually smaller and fainter. The most distinctive are the cirrocumulus-only effects, which are worth hunting for.
- Iridescence. Bright, soap-bubble-like patches of colour in a thin cirrocumulus layer near the sun, from diffraction by uniformly small droplets.
- Corona. A small, coloured disc around the sun or moon, again from diffraction — the smaller the ring, the smaller the droplets.
- Parhelia in a cirrocumulus deck. Occasionally visible when the deck contains horizontally oriented plates.
- Contrail seeding. Cirrocumulus is often the layer where contrails spread out and become indistinguishable from natural cloud within 20-30 minutes.
Rules of thumb
- Cirrocumulus elements are fingertip-sized, show no shading, and sit above 6000 m.
- A mackerel sky only forecasts rain when it is thickening and being followed by cirrostratus.
- The regular ripples come from gravity waves or shallow convection, with wavelengths of 0.5-2 km.
- Cirrocumulus is the layer where contrails most often spread out into natural-looking cloud.
| The other cloud | Comparison — Cirrocumulus |
|---|---|
| 高积云 · Altocumulus | Altocumulus elements are thumbnail-sized at arm's length and show shading underneath; cirrocumulus is fingertip-sized with no shading at all. |
| 絮卷云 · Cirrus floccus | Cirrus floccus tufts have visible trails of falling ice below them; cirrocumulus grains do not have trails. |
The two or three genera this one is most often confused with, and the single feature that separates them.
Observation questions
Why does a mackerel sky sometimes come to nothing?
Because rippled cirrocumulus only forecasts rain when it is part of an advancing frontal shield. Patchy cirrocumulus in an otherwise clear sky is often the dissolving remains of an old cirrus layer or the edge of a system passing to the north, and it will simply fade.
What makes the pattern so regular?
Gravity waves or shallow convection in a thin, conditionally unstable layer. Air rises in narrow bands and sinks in between, so cloud only forms in the rising bands. The wavelength, typically 500 m to 2 km, is set by the wave and is remarkably stable over large areas.