Storm Station 247

The Field Book/Vol. III, The Sky/Cloud classification/FB-SKY-016

Stratocumulus

Stratocumulus is a low layer or patches of grey or whitish cloud in rolls, lumps or rounded masses, joined or with gaps, each element more than five degrees wide as seen from the ground. It usually lies under an inversion that stops it growing upward.

Plate FB-SKY-016The generaRevision 1, 2026-09-24Status draftAlso called Sc, stratocumulus stratiformis, stratocumulus castellanus, roll cloud, low cloud

A painted section of stratocumulus over a prairie, heights to scale from the ground to 8,000 feet (2.4 kilometres): six large grey and white rolls from about 2,600 to 3,500 feet (0.8 to 1.1 kilometres), with gaps of blue sky between them; a warm tinted band of inversion above them from 3,600 to 4,300 feet (1.1 to 1.3 kilometres); wavy lines of the air beneath stirred from the ground; and an observer for whom each roll spans more than five degrees.
Fig. A Stratocumulus in section. A low layer of large rolls with gaps between them, capped by an inversion.MaximizeThe drawing, to scale

What it is

Stratocumulus is a low layer or set of patches of grey or whitish cloud, almost always with dark parts, made of rolls, lumps or rounded masses that may be joined or have gaps between them. It stands in the low étage, from the ground to 6,600 ft (2 km), Cloud classification FB-SKY-001. Seen more than 30 degrees above the horizon, most of its elements are more than five degrees wide: wider than three fingers held at arm's length.

The Atlas lists the same five species for it as for altocumulus: stratiformis, lenticularis, castellanus, floccus and volutus. Seven varieties and six supplementary features can apply, from perlucidus, with sky showing between the rolls, to asperitas, a rough wave pattern on its underside. The Atlas gives it no accessory clouds. Fig. B draws every name.

What it is not

It is not altocumulus, Altocumulus FB-SKY-013, whose elements are smaller, one to five degrees wide, and stand in the middle étage. It is not stratus, Stratus FB-SKY-017, which is an even grey layer with no lumps or rolls. It is not cumulus, Cumulus FB-SKY-018: cumulus stand as separate heaps with flat bases and grow upward, where stratocumulus spreads sideways in a layer. Cumulus that spread out under a lid become stratocumulus, and the Atlas has a name for that too.

Lookalikes

Altocumulus
Smaller elements, 1° to 5° wide, higher.
Stratus
Even and featureless, with no lumps or rolls.
Cumulus
Separate heaps with flat bases, growing upward.

The machine

Stratocumulus lives under an inversion: a layer where the air warms with height, which stops rising air. Fig. A draws the inversion as the dense ruling above the cloud. Moist air below it is stirred upward, by the wind over rough ground or by warming from the ground or the sea below, and cools to its dew point near the top of the stirred layer. It cannot rise farther, so the cloud spreads out along the lid.

The cloud then keeps itself going. Its top radiates heat to space and cools, the cooled air sinks through the layer, and the sinking and the rising between them break the layer into cells and rolls. Where the air sinks the cloud thins into gaps; where it rises the rolls thicken. Over cool oceans off western coasts this can hold a sheet of stratocumulus in place for weeks.

Most stratocumulus gives nothing more than a grey day. A thick layer can give light drizzle, rain or snow, and its supplementary features record that: praecipitatio when it reaches the ground, virga when it does not.

Small paintings of stratocumulus, 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, praecipitatio, mamma, fluctus, asperitas and cavum; with the count of each rank beneath, a hyphen where the Atlas gives none.
Fig. B The stratocumulus inventory. Every name the Atlas lists for stratocumulus, 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 stratocumulus from the ground to 8,000 feet (2.4 kilometres): a layer of rolls of water droplets from about 2,600 to 3,500 feet (0.8 to 1.1 kilometres), in the low étage, with gaps between the rolls; an inversion above it, from 3,600 to 4,300 feet (1.1 to 1.3 kilometres); the air beneath stirred from the ground; and an observer for whom each roll spans more than five degrees.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-016-AROLLS OF WATER UNDER AN INVERSION IN THE LOW ÉTAGE, THE AIR BENEATH STIRRED FROM THE GROUND; HEIGHTS TYPICALFIG. A STRATOCUMULUS IN SECTION01234567800.511.52kft(km)LOWMIDDLEINVERSION: AIR WARMER ALOFTEACH ELEMENT OVER 5° WIDE,SEEN FROM THE GROUNDKEYWater dropletsInversion123456TITLEStratocumulus, a section to height scaleVOL. III THE SKY · THE GENERATYPESECTIONSCALEHEIGHT TO SCALE, WIDTH NTSREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-24IDFB-SKY-016-ADRAWN AS linework on paperSOURCES WMO, AMS, Office of the Federal Coordinator for Meteorology
Fig. A, the drawing A section to height scale. Rolls of water under an inversion in the low étage, the air beneath stirred from the ground; heights typical HEIGHT TO SCALE, WIDTH NTSMaximizeThe sheet, SVG, 11 by 17The painting
  1. Rolls of water droplets
  2. A gap between the rolls
  3. The inversion that caps the layer
  4. The air beneath, stirred from the ground
  5. The observer: each element over 5° wide
  6. The low étage
Inventory of stratocumulus 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, praecipitatio, mamma, fluctus, asperitas, cavum; the accessory clouds a hyphen.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-016-BEVERY SPECIES, VARIETY, FEATURE AND ACCESSORY CLOUD THE INTERNATIONAL CLOUD ATLAS LISTS FOR STRATOCUMULUS; SKETCHES NOT TO SCALEFIG. B THE STRATOCUMULUS INVENTORY1SPECIES5, AT MOST ONEstratocumulus stratiformisSpread in a wide sheet or layerstratocumulus lenticularisLens or almond shapes with sharp outlines,over and downwind of hillsstratocumulus castellanusTurrets rising from a common base, likebattlementsstratocumulus floccusSmall tufts with ragged bases, often trailingfall streaksstratocumulus volutusA long horizontal tube that seems to rollabout its axis2VARIETIES7, ANY THAT APPLYstratocumulus translucidusThin enough to show where the sun or moon isSKY BETWEENstratocumulus perlucidusGaps between the elements show sky or a highercloudstratocumulus opacusThick enough to hide the sun or moonstratocumulus duplicatusTwo or more layers at slightly differentheightsstratocumulus undulatusWaves in the sheet or its elementsstratocumulus radiatusParallel bands that seem to meet at thehorizonstratocumulus lacunosusA net of round holes, often with fringed edges3SUPPLEMENTARY FEATURES6, ANY THAT APPLYstratocumulus virgaFall streaks that evaporate before they reachthe groundstratocumulus praecipitatioRain, snow or hail reaching the groundstratocumulus mammaPouches hanging from the undersidestratocumulus fluctusShort lived curls or breaking waves on the topstratocumulus asperitasA rough, chaotic wave pattern on the undersidestratocumulus cavumA round hole with fall streaks hanging in itsmiddle4ACCESSORY CLOUDS-TITLEStratocumulus, its inventory in the AtlasVOL. III THE SKY · THE GENERATYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-24IDFB-SKY-016-BDRAWN AS linework on paperSOURCES WMO, AMS, Office of the Federal Coordinator for Meteorology
Fig. B, the drawing Its inventory in the Atlas. Every species, variety, feature and accessory cloud the International Cloud Atlas lists for stratocumulus; 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 near the ground, stirred upward by the wind or by warming from below
  • An inversion above, a layer where the air warms with height, which caps the cloud
  • Cooling at the cloud top, which stirs the layer from above and breaks it into cells

Scales

time
hours to days; over cool oceans it can last for weeks
horizontal
a few miles to sheets over hundreds of miles of ocean
vertical
about 650 to 3,300 ft (200 m to 1 km) deep, in the low étage
orlanski
meso and synoptic

Signatures

surface
a broken or overcast layer low in the sky report
satellite
bright in the visible, warm in the infrared; cells or rolls over the ocean
radar
no echo, or light drizzle

The numbers

QuantityValue, and the kind of number it is
AbbreviationScStandard, International Cloud Atlas
ÉtageLow: from the ground to 6,600 ft (2 km)Standard, International Cloud Atlas
Made ofWater droplets; snow or ice crystals when very coldStandard, International Cloud Atlas
Element sizeMost elements more than 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 features6: virga, praecipitatio, 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
Sky cover reportedIn eighths of the sky: FEW 1 to 2, SCT 3 to 4, BKN 5 to 7, OVC 8Standard, Federal Meteorological Handbook No. 1

How the station sees it

An airport station measures stratocumulus well: it is low enough for the laser ceilometer, which reports each layer's base height and cover in eighths of the sky, FEW, SCT, BKN or OVC. A broken or overcast layer of it sets the ceiling. The station does not name the genus. Satellites show it bright in the visible images and nearly as warm as the ground in the infrared, often in rows or cells over the sea.

How it is warned

Stratocumulus is not warned. It matters most to pilots, for the ceilings it sets, and to forecasters of the day's temperature: a layer that does not break keeps an afternoon several degrees cooler than a clear one, and one that breaks late surprises a forecast that expected sun.

See also

Sources

  1. World Meteorological Organization. International Cloud Atlas.
  2. American Meteorological Society. Glossary of Meteorology.
  3. Office of the Federal Coordinator for Meteorology. Federal Meteorological Handbook No. 1, Surface Weather Observations and Reports.
  4. National Weather Service. Automated Surface Observing System (ASOS) User's Guide.

Definition after the International Cloud Atlas. Plate FB-SKY-016, revision 1, 2026-09-24. The number is permanent; cite it.