Storm Station 247

The Field Book/Vol. III, The Sky/Cloud classification/FB-SKY-013

Altocumulus

Altocumulus is a middle cloud of rounded masses, rolls or cloudlets in patches or sheets, white or grey and usually shaded, each element one to five degrees wide as seen from the ground. It is made mostly of water droplets, often well below freezing.

Plate FB-SKY-013The generaRevision 1, 2026-09-24Status draftAlso called Ac, altocumulus castellanus, altocumulus lenticularis, lenticular cloud, mackerel sky, middle cloud

A painted section of altocumulus over a prairie, heights to scale from the ground to 26,000 feet (8 kilometres): a row of rounded grey and white cloudlets with bases near 10,500 feet (3.2 kilometres) and tops near 12,000 feet (3.7 kilometres), blue sky between them, virga trailing from three of them; the middle étage bracketed from 6,600 to 23,000 feet (2 to 7 kilometres); and an observer on the ground for whom each cloudlet spans one to five degrees.
Fig. A Altocumulus in section. Rounded cloudlets in a row with sky between them, virga hanging from three.MaximizeThe drawing, to scale

What it is

Altocumulus is a middle cloud of rounded masses, rolls or cloudlets, white or grey, in patches or sheets, usually with shading on one side. It stands in the middle étage, from 6,600 to 23,000 ft (2 to 7 km) in the middle latitudes, Cloud classification FB-SKY-001. Seen more than 30 degrees above the horizon, most of its elements are one to five degrees wide. It is made mostly of water droplets, often far below freezing.

It is the most varied of the genera. The Atlas lists five species: stratiformis, a wide sheet; lenticularis, smooth lens shapes over and downwind of hills; castellanus, turrets rising from a common base; floccus, tufts with ragged bases; and volutus, a long tube that seems to roll. Seven varieties and five features can apply, from translucidus, thin enough to show the sun, to asperitas, a rough, chaotic underside. Fig. B draws every one.

What it is not

It is not cirrocumulus, Cirrocumulus FB-SKY-011, whose grains are under one degree wide and never shaded. It is not stratocumulus, Stratocumulus FB-SKY-016, whose elements are over five degrees wide and stand in the low étage. The width of the elements seen from the ground is the Atlas's own test, and it is drawn in Fig. A.

It is not altostratus, Altostratus FB-SKY-014, which is an even sheet without cloudlets.

Lookalikes

Cirrocumulus
Elements less than 1° wide, never shaded, and higher.
Stratocumulus
Elements more than 5° wide, and lower.
Altostratus
An even sheet with no cloudlets.

The machine

Altocumulus forms where a layer of moist air at middle levels is lifted or cooled to saturation and then stirred from within, so that it breaks into cells. Cooling at the top of the layer by radiation to space, or a change of wind speed with height across it, can do the stirring; the cells line up in rows where the wind sets them.

Its droplets are often supercooled. Some freeze, and the ice grows and falls as streaks, virga, that evaporate before they reach the ground: Fig. A draws three.

Two of its species say more than their shapes. Lenticularis is the lens cloud of mountain waves. Air flowing over a ridge rises and sinks in a train of waves downwind; the cloud forms at each crest and evaporates at each trough, so it stands still while the wind blows through it, Orographic clouds FB-SKY-035. Castellanus, turrets rising from a middle level base, shows air unstable at middle levels. On a summer morning it is a sign that thunderstorms may follow later in the day.

Small paintings of altocumulus, 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, floccus and volutus; the varieties translucidus, perlucidus, opacus, duplicatus, undulatus, radiatus and lacunosus; and the supplementary features virga, mamma, fluctus, asperitas and cavum; with the count of each rank beneath, a hyphen where the Atlas gives none.
Fig. B The altocumulus inventory. Every name the Atlas lists for altocumulus, 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 altocumulus from the ground to 26,000 feet (8 kilometres): a row of rounded cloudlets of water droplets with bases near 10,500 feet (3.2 kilometres) and tops near 12,000 feet (3.7 kilometres), in the middle étage, 6,600 to 23,000 feet (2 to 7 kilometres), with sky between them and virga hanging from three; and an observer for whom each cloudlet spans one to five degrees.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-013-AROUNDED CLOUDLETS OF WATER IN THE MIDDLE ÉTAGE, SKY BETWEEN THEM, VIRGA BENEATH THREE; HEIGHTS TYPICAL OF THE MIDDLE LATITUDESFIG. A ALTOCUMULUS IN SECTION051015202502468kft(km)LOWMIDDLEHIGHEACH CLOUDLET 1° TO 5° WIDE,SEEN FROM THE GROUNDKEYWater droplets, often below freezing12345TITLEAltocumulus, a section to height scaleVOL. III THE SKY · THE GENERATYPESECTIONSCALEHEIGHT TO SCALE, WIDTH NTSREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-24IDFB-SKY-013-ADRAWN AS linework on paperSOURCES WMO, AMS, NWS
Fig. A, the drawing A section to height scale. Rounded cloudlets of water in the middle étage, sky between them, virga beneath three; heights typical of the middle latitudes HEIGHT TO SCALE, WIDTH NTSMaximizeThe sheet, SVG, 11 by 17The painting
  1. Rounded cloudlets of water droplets
  2. Sky seen between them: perlucidus
  3. Virga
  4. The observer: each cloudlet 1° to 5° wide
  5. The middle étage
Inventory of altocumulus 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, volutus; the varieties translucidus, perlucidus, opacus, duplicatus, undulatus, radiatus, lacunosus; the supplementary features virga, mamma, fluctus, asperitas, cavum; the accessory clouds a hyphen.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-013-BEVERY SPECIES, VARIETY, FEATURE AND ACCESSORY CLOUD THE INTERNATIONAL CLOUD ATLAS LISTS FOR ALTOCUMULUS; SKETCHES NOT TO SCALEFIG. B THE ALTOCUMULUS INVENTORY1SPECIES5, AT MOST ONEaltocumulus stratiformisSpread in a wide sheet or layeraltocumulus lenticularisLens or almond shapes with sharp outlines,over and downwind of hillsaltocumulus castellanusTurrets rising from a common base, likebattlementsaltocumulus floccusSmall tufts with ragged bases, often trailingfall streaksaltocumulus volutusA long horizontal tube that seems to rollabout its axis2VARIETIES7, ANY THAT APPLYaltocumulus translucidusThin enough to show where the sun or moon isSKY BETWEENaltocumulus perlucidusGaps between the elements show sky or a highercloudaltocumulus opacusThick enough to hide the sun or moonaltocumulus duplicatusTwo or more layers at slightly differentheightsaltocumulus undulatusWaves in the sheet or its elementsaltocumulus radiatusParallel bands that seem to meet at thehorizonaltocumulus lacunosusA net of round holes, often with fringed edges3SUPPLEMENTARY FEATURES5, ANY THAT APPLYaltocumulus virgaFall streaks that evaporate before they reachthe groundaltocumulus mammaPouches hanging from the undersidealtocumulus fluctusShort lived curls or breaking waves on the topaltocumulus asperitasA rough, chaotic wave pattern on the undersidealtocumulus cavumA round hole with fall streaks hanging in itsmiddle4ACCESSORY CLOUDS-TITLEAltocumulus, its inventory in the AtlasVOL. III THE SKY · THE GENERATYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-24IDFB-SKY-013-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 altocumulus; 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 at middle levels, lifted or cooled to saturation
  • A layer stirred from within, by cooling at its top or by the wind changing with height, which breaks it into cells
  • Waves in the air flowing over mountains, for the lens shaped kind

Scales

time
minutes for one cloudlet; hours for a sheet
horizontal
a patch of a few miles to a sheet of hundreds of miles
vertical
a few hundred feet to about 3,300 ft (1 km) deep, in the middle étage
orlanski
meso

Signatures

surface
a layer between 6,600 and 12,000 ft in the sky report when it is low enough for the ceilometer
satellite
a patchy layer, often in rows or waves
radar
no echo, except for heavy virga

The numbers

QuantityValue, and the kind of number it is
AbbreviationAcStandard, International Cloud Atlas
ÉtageMiddle: 6,600 to 23,000 ft (2 to 7 km) in the middle latitudesStandard, International Cloud Atlas
Made ofWater droplets, often supercooled; ice crystals when very coldStandard, International Cloud Atlas
Element sizeMost elements 1° to 5° wide, seen more than 30° above the horizonStandard, International Cloud Atlas
Species5: stratiformis, lenticularis, castellanus, floccus, volutusStandard, International Cloud Atlas
Varieties7: translucidus, perlucidus, opacus, duplicatus, undulatus, radiatus, lacunosusStandard, International Cloud Atlas
Supplementary features5: virga, mamma, fluctus, asperitas, 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), and reports each layer's cover and base height in eighths of the sky. Altocumulus with its base below that height appears in the report as a layer; the station does not name the genus. Above 12,000 ft it is added from sight at staffed stations. Satellites show it as a patchy layer, often in rows or waves.

How it is warned

Altocumulus is not warned. Its lens shapes mark strong mountain waves, which forecasters watch for aircraft turbulence, and its castellanus is one of the older signs of an unstable afternoon. Thin altocumulus in front of the sun or moon can make a corona or iridescence, Corona, iridescence and glory FB-SKY-052. Neither is a warning; both are the sky showing how the air at its middle levels is moving.

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-013, revision 1, 2026-09-24. The number is permanent; cite it.