Storm Station 247

The Field Book/Vol. III, The Sky/Cloud classification/FB-SKY-011

Cirrocumulus

Cirrocumulus is a thin high sheet or patch of small white grains or ripples, without shading, each element less than one degree wide as seen from the ground. It holds supercooled droplets as well as ice, and often turns to cirrus or cirrostratus as they freeze.

Plate FB-SKY-011The generaRevision 1, 2026-09-24Status draftAlso called Cc, mackerel sky, cirrocumulus stratiformis, cirrocumulus lenticularis, fallstreak hole, cavum, high cloud

A painted section of cirrocumulus over a prairie, heights to scale from the ground to 46,000 feet (14 kilometres): three rows of small white grains of ice near 25,000 feet (7.6 kilometres) in the high étage, a hole in the sheet with ice falling in its middle, and an observer on the ground with two lines of sight to one grain, each grain under one degree wide; in a round window, the sheet as seen from the ground, grains in rows around a round hole of blue sky.
Fig. A Cirrocumulus in section. A thin sheet of small grains of ice in rows, a hole in it, and the sheet as seen from the ground.MaximizeThe drawing, to scale

What it is

Cirrocumulus is a thin high sheet or patch of cloud made of very small grains, ripples or rounded elements, white and without shading, in rows or scattered. It stands in the high étage, from 16,400 to 43,000 ft (5 to 13 km) in the middle latitudes, with cirrus and cirrostratus, Cloud classification FB-SKY-001. Its grains are the smallest elements of any cloud: seen more than 30 degrees above the horizon, most are less than one degree wide.

The Atlas lists four species: stratiformis, a wide sheet; lenticularis, lens shaped patches over and downwind of hills; castellanus, with small turrets; and floccus, tufts with ragged bases. Two varieties, undulatus and lacunosus, describe waves in the sheet and a net of holes. Three features, virga, mamma and cavum, can come with it. Fig. B draws each one. A sky of cirrocumulus in rows is one of the skies called a mackerel sky.

What it is not

It is not altocumulus, Altocumulus FB-SKY-013. The difference is the size of the elements and their shading: altocumulus elements are one to five degrees wide, often grey on one side, and stand lower, in the middle étage. When the two are hard to tell apart the Atlas settles it by that width alone.

It is not cirrus, Cirrus FB-SKY-010, which is filaments, and not cirrostratus, Cirrostratus FB-SKY-012, which is an even veil with no grains at all.

Lookalikes

Altocumulus
Elements 1° to 5° wide, often shaded on one side, and lower, in the middle étage.
Cirrus
Filaments and hooks, not grains.
Cirrostratus
An even veil with no grains; it makes the halo.

The machine

Cirrocumulus forms where a thin layer high in the troposphere is lifted to saturation and stirred from within, so that it breaks into small cells or rows. Waves in the air flowing over mountains, or a change of wind with height across the layer, can set the rows. The cloud holds supercooled droplets, liquid water far below freezing, as well as ice.

Those droplets are why cirrocumulus seldom lasts. As they freeze, the grains turn fibrous and the patch becomes cirrus or cirrostratus, often within minutes. Where one part of a sheet freezes first, its ice grows at the droplets' expense and falls, and a round hole opens in the cloud with streaks of ice hanging in its middle: cavum, the fallstreak hole, drawn in Fig. A. An aircraft climbing through such a sheet can start the same freezing along its path.

Thin cirrocumulus in front of the sun or moon can make a corona or iridescence, rings and fringes of colour from light bent round its small, even droplets, Corona, iridescence and glory FB-SKY-052.

Small paintings of cirrocumulus, one for every name the International Cloud Atlas lists for it, each labelled with its full name and its rank: the species stratiformis, lenticularis, castellanus and floccus; the varieties undulatus and lacunosus; and the supplementary features virga, mamma and cavum; with the count of each rank beneath, a hyphen where the Atlas gives none.
Fig. B The cirrocumulus inventory. Every name the Atlas lists for cirrocumulus, each in a small painting.MaximizeThe drawing, to scale

The drawings

Each figure drawn as an engineering sheet, its parts numbered, to print at 11 by 17.

Section of cirrocumulus from the ground to 46,000 feet (14 kilometres): a thin sheet of small grains of ice in rows near 25,000 feet (7.6 kilometres), in the high étage; a round hole in the sheet with ice falling in its middle; and an observer at the ground for whom each grain spans less than one degree.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-011-AA THIN SHEET OF SMALL GRAINS IN THE HIGH ÉTAGE, ONE HOLE WITH ICE FALLING IN IT; HEIGHTS TYPICAL OF THE MIDDLE LATITUDESFIG. A CIRROCUMULUS IN SECTION01020304002468101214kft(km)LOWMIDDLEHIGHEACH GRAIN UNDER 1°WIDE, SEEN FROM THE GROUNDKEYIce crystals1234TITLECirrocumulus, a section to height scaleVOL. III THE SKY · THE GENERATYPESECTIONSCALEHEIGHT TO SCALE, WIDTH NTSREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-24IDFB-SKY-011-ADRAWN AS linework on paperSOURCES WMO, AMS, NWS
Fig. A, the drawing A section to height scale. A thin sheet of small grains in the high étage, one hole with ice falling in it; heights typical of the middle latitudes HEIGHT TO SCALE, WIDTH NTSMaximizeThe sheet, SVG, 11 by 17The painting
  1. Small grains in rows
  2. A hole with ice falling in its middle: cavum
  3. The observer: each grain under 1° wide
  4. The high étage
Inventory of cirrocumulus as the International Cloud Atlas lists it, each name with a small drafted sketch and one line on what it is: the species stratiformis, lenticularis, castellanus, floccus; the varieties undulatus, lacunosus; the supplementary features virga, mamma, cavum; the accessory clouds a hyphen.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-011-BEVERY SPECIES, VARIETY, FEATURE AND ACCESSORY CLOUD THE INTERNATIONAL CLOUD ATLAS LISTS FOR CIRROCUMULUS; SKETCHES NOT TO SCALEFIG. B THE CIRROCUMULUS INVENTORY1SPECIES4, AT MOST ONEcirrocumulus stratiformisSpread in a wide sheet or layercirrocumulus lenticularisLens or almond shapes with sharp outlines,over and downwind of hillscirrocumulus castellanusTurrets rising from a common base, likebattlementscirrocumulus floccusSmall tufts with ragged bases, often trailingfall streaks2VARIETIES2, ANY THAT APPLYcirrocumulus undulatusWaves in the sheet or its elementscirrocumulus lacunosusA net of round holes, often with fringed edges3SUPPLEMENTARY FEATURES3, ANY THAT APPLYcirrocumulus virgaFall streaks that evaporate before they reachthe groundcirrocumulus mammaPouches hanging from the undersidecirrocumulus cavumA round hole with fall streaks hanging in itsmiddle4ACCESSORY CLOUDS-TITLECirrocumulus, its inventory in the AtlasVOL. III THE SKY · THE GENERATYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-24IDFB-SKY-011-BDRAWN AS linework on paperSOURCES WMO, AMS, NWS
Fig. B, the drawing Its inventory in the Atlas. Every species, variety, feature and accessory cloud the International Cloud Atlas lists for cirrocumulus; sketches not to scale NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17The painting
  1. Species, at most one
  2. Varieties, any that apply
  3. Supplementary features, any that apply
  4. Accessory clouds, any that apply

Ingredients

  • Moist air lifted high in the troposphere, often in a wave
  • A thin layer stirred from within, which breaks the sheet into small cells or ripples
  • Droplets cooled below freezing that have not yet frozen

Scales

time
minutes to an hour; its droplets freeze quickly
horizontal
a patch of a few miles to a sheet of tens of miles
vertical
a few hundred feet deep, in the high étage
orlanski
meso

Signatures

surface
added to the sky report from sight at staffed stations
satellite
thin and high; hard to tell from other high cloud in the images
radar
no echo

The numbers

QuantityValue, and the kind of number it is
AbbreviationCcStandard, International Cloud Atlas
ÉtageHigh: 16,400 to 43,000 ft (5 to 13 km) in the middle latitudesStandard, International Cloud Atlas
Made ofIce crystals, often with supercooled water droplets that soon freezeStandard, International Cloud Atlas
Element sizeMost elements less than 1° wide, seen more than 30° above the horizonStandard, International Cloud Atlas
Species4: stratiformis, lenticularis, castellanus, floccusStandard, International Cloud Atlas
Varieties2: undulatus, lacunosusStandard, International Cloud Atlas
Supplementary features3: virga, mamma, cavumStandard, International Cloud Atlas
Accessory clouds-Standard, International Cloud Atlas
Airport ceilometerCloud bases to 12,000 ft (3.7 km) overheadStandard, Automated Surface Observing System (ASOS) User's Guide

How the station sees it

An airport station measures cloud overhead with a laser ceilometer to 12,000 ft (3.7 km). Cirrocumulus stands well above that, so the station's sky report does not carry it; staffed stations add it from sight. From a satellite it is hard to separate from other thin high cloud, and it is usually known by eye from the ground.

How it is warned

Cirrocumulus is not warned and brings no weather to the ground. Its supercooled droplets matter to aircraft: cloud of liquid water below freezing can ice an airframe that flies through it, and icing is forecast for pilots rather than warned to the public. The cloud's shape is its message to a reader on the ground: moist, stirred air high up, often ahead of a change.

See also

Sources

  1. World Meteorological Organization. International Cloud Atlas.
  2. American Meteorological Society. Glossary of Meteorology.
  3. National Weather Service. Automated Surface Observing System (ASOS) User's Guide.

Definition after the International Cloud Atlas. Plate FB-SKY-011, revision 1, 2026-09-24. The number is permanent; cite it.