Cirrocumulus is the cloud of the finest dappled skies: tiny white grains with no grey shading, arranged in banks, ripples or regular lines. Like cirrus, it lives at the high level and is made mostly of ice crystals. It is uncommon, often short-lived, and usually seen alongside cirrus or cirrostratus.
- Level: High level (5,000 to 13,000 m)
- Base height: 5,000 to 13,000 m
- Appearance: banks of tiny white grains with no grey side
- Associated weather: change possible; short-lived
How cirrocumulus forms
Cirrocumulus forms in a thin layer at the high level where moist air is stirred by small convection cells. Where a pocket of air rises a little, it cools, reaches saturation and fills with ice crystals; between two pockets the air sinks and warms, leaving a gap. The alternation of pockets and gaps produces the grid of grains.
The layer is thin. The grains sometimes hold supercooled droplets, meaning liquid water below freezing, but these freeze very quickly, so a cirrocumulus changes its look in a short time. It often arises from the breaking-up of a cirrostratus or a thickening cirrus, and it can merge back into a veil.
How to recognise cirrocumulus
The surest test is the apparent size of the grains: in cirrocumulus each element spans less than 1° of sky, about the width of your little finger at arm's length. The grains are white, with no grey part and no shading, and tightly packed. As soon as the elements look bigger, with a darker side, you are into altocumulus, at the middle level.
Cirrocumulus can form parallel ripples (the famous mackerel sky), a network of cells or regular rows, always very fine. It is thin enough to let the sun show through, barely dimmed, sometimes ringed by a coloured corona.
What weather cirrocumulus signals
A lone cirrocumulus means nothing in particular: in a high-pressure setting it vanishes quickly and fair weather returns. Together with thickening cirrus, it is among the first signs of deterioration, because it reflects moisture and instability at height. The old saying goes "mackerel sky and mares' tails make tall ships carry low sails", and it holds for cirrocumulus as for altocumulus, since both are unstable clouds.
Keep the general rule in mind: a dappled sky that spreads and clouds over late in the day usually means a change of weather within the next day; a dappled sky that breaks up and clears means fair weather.
Risks and precautions
Cirrocumulus poses no direct danger and gives no precipitation. Like all thin clouds it lets ultraviolet light through, and its brightness can dazzle if you look at the sun through it without sunglasses. It slightly spoils views of the night sky, but it is a useful indicator for anyone following the weather.
In the sky over Toronto today
Here is what the Open-Meteo forecast of 2026-11-02 08:13 gives for Toronto, averaged over the next 24 hours: 77% of the sky covered at low level, 58% at middle level and 76% at high level.
For this kind of cloud the level that matters is this one: at high level the sky is mostly covered (76% on average).
These percentages are model values on a grid: they say how much of the sky is occupied at each level, not which kind of cloud is there. They change at every update.
Frequently asked questions
What is the difference between cirrocumulus and altocumulus?
Height and grain size. Cirrocumulus is at the high level, its elements span less than 1° and have no shading. Altocumulus is at the middle level, its elements span 1° to 5° and have a grey side.
What is a mackerel sky?
A sky covered with cirrocumulus or altocumulus in tight ripples, whose pattern recalls the stripes on a mackerel's skin. It is common shortly before a change in the weather.
Does cirrocumulus mean rain is coming?
Not by itself. It reflects moisture at height. If it thickens and clouds over into a veil, rain may arrive within the day; if it breaks up, the fair weather continues.
Sources
- World Meteorological Organization (WMO), International Cloud Atlas, WMO-No. 407: definitions of the genera, levels and base heights for temperate regions. https://cloudatlas.wmo.int (copyright WMO; no text or image from the Atlas is reproduced here).
- Drawing and text on this page: original to this site, schematic.