Storm Station 247

The Field Book/Vol. III, The Sky/Cloud classification/FB-SKY-019

Cumulonimbus

Cumulonimbus is the thunderstorm cloud: a heavy, dense tower from a low dark base through all three étages, its top turning to fibrous ice and often spreading into an anvil. Any cloud that makes lightning or thunder is a cumulonimbus.

Plate FB-SKY-019The generaRevision 1, 2026-09-24Status draftAlso called Cb, thunderstorm cloud, thunderhead, anvil, incus, cumulonimbus capillatus, cumulonimbus calvus

A painted section of a cumulonimbus over a prairie, heights to scale from the ground to 49,000 feet (15 kilometres): a tall billowing tower from a dark base near 4,000 feet (1.2 kilometres), an updraft arrow rising through it, the freezing level marked across it, its top spread into a flat anvil along the tropopause at 39,000 feet (12 kilometres) and carried downwind to the right, a small dome of overshooting top above the anvil, rounded mamma hanging under the downwind anvil, a grey rain shaft with a downdraft arrow beneath the tower, and a low dark shelf cloud along the gust front.
Fig. A Cumulonimbus in section. The storm cloud, from its base through all three étages to the anvil.MaximizeThe drawing, to scale

What it is

Cumulonimbus is the thunderstorm cloud: a heavy, dense cloud of great vertical extent, a mountain or huge tower, with a dark base in the low étage and a top that has turned smooth, fibrous or streaked with ice, and often spread out flat into an anvil. It is the one genus that spans all three étages, Cloud classification FB-SKY-001. Any cloud that makes lightning or thunder is a cumulonimbus, by the Atlas's own rule.

The Atlas lists two species, the two ends of its top's change to ice: calvus, the top softening and losing its cauliflower outline, and capillatus, the top plainly fibrous, often an anvil. It has no varieties. It carries more supplementary features than any other genus, eight: praecipitatio, virga, incus the anvil, mamma, arcus the shelf cloud, tuba the funnel, murus the wall cloud and cauda the tail cloud. Four accessory clouds can come with it. Fig. B draws each.

What it is not

It is not cumulus congestus, Cumulus FB-SKY-018, however tall: a congestus has a sharp cauliflower top and no ice, lightning or thunder. It is not nimbostratus, Nimbostratus FB-SKY-015, which rains steadily from a layer; a cumulonimbus rains in showers that start and stop. A thunderstorm, Thunderstorm FB-EVT-001, is the event; cumulonimbus is the cloud that makes it.

Lookalikes

Cumulus congestus
A cauliflower top with sharp outlines, no ice fibres and no thunder.
Nimbostratus
A layer with steady rain, not showers, and no thunder.
Altocumulus castellanus
Turrets from a middle level base, the forerunner of storms, not a storm.

The machine

A cumulonimbus is a cumulus that kept rising. Moist air near the ground, lifted by a front, an outflow or a mountain, is warmer than the air around it all the way up through a deep unstable layer, and rises in an updraft that can exceed 60 mph (27 m/s) in a strong storm. Fig. A draws it as the hollow arrow up the tower. Above the freezing level the droplets are supercooled; ice, snow and hail grow among them, and the collisions between them separate electric charge, which is where lightning comes from.

The updraft rises until it meets the tropopause, near 39,000 ft (12 km) in the middle latitudes in summer, where the air stops cooling with height and the cloud can rise no farther. The top spreads sideways along it as the anvil, carried downwind. A strong updraft carries air on past the tropopause for a while, in a dome above the anvil: the overshooting top, Overshooting top and above anvil plume FB-SKY-042. Under the anvil, sinking air makes pouches, mamma.

Rain and hail falling through the cloud drag air down with them and cool it by evaporation; the downdraft reaches the ground and spreads out as a cold outflow. Its leading edge is the gust front, and where it lifts the warm air ahead a shelf cloud forms along it, arcus. The architecture of the cloud, and how it changes in a supercell, has a plate of its own, Cumulonimbus architecture FB-SKY-041.

Small paintings of cumulonimbus, one for every name the International Cloud Atlas lists for it, each labelled with its full name and its rank: the species calvus and capillatus; the supplementary features praecipitatio, virga, incus, mamma, arcus, tuba, murus and cauda; and the accessory clouds pannus, pileus, velum and flumen; with the count of each rank beneath, a hyphen where the Atlas gives none.
Fig. B The cumulonimbus inventory. Every name the Atlas lists for cumulonimbus, 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 a cumulonimbus from the ground to 49,000 feet (15 kilometres): a tower from a base near 4,000 feet (1.2 kilometres) through the low, middle and high étages, water droplets below the freezing level and ice above it; an updraft rising through the tower; the top spread into an anvil along the tropopause near 39,000 feet (12 kilometres) and carried downwind, with an overshooting dome above it near 44,000 feet (13.4 kilometres); mamma hanging under the anvil; rain and a downdraft beneath the tower; and a shelf cloud along the gust front where the cold outflow spreads.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-019-ATHE TOWER FROM A LOW BASE TO THE TROPOPAUSE, THE ANVIL AND OVERSHOOTING TOP, RAIN, DOWNDRAFT AND GUST FRONT; HEIGHTS TYPICALFIG. A CUMULONIMBUS IN SECTION01020304002468101214kft(km)LOWMIDDLEHIGHTROPOPAUSEFREEZING LEVEL, 32 °F (0 °C)KEYIce crystalsWater dropletsRain or hail reaching the groundUpdraftDowndraft1234567TITLECumulonimbus, a section to height scaleVOL. III THE SKY · THE GENERATYPESECTIONSCALEHEIGHT TO SCALE, WIDTH NTSREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-24IDFB-SKY-019-ADRAWN AS linework on paperSOURCES WMO, AMS, NWS
Fig. A, the drawing A section to height scale. The tower from a low base to the tropopause, the anvil and overshooting top, rain, downdraft and gust front; heights typical HEIGHT TO SCALE, WIDTH NTSMaximizeThe sheet, SVG, 11 by 17The painting
  1. The updraft
  2. The overshooting top
  3. The anvil, incus, spread along the tropopause
  4. Mamma under the anvil
  5. Rain and the downdraft
  6. The gust front and its shelf cloud, arcus
  7. The freezing level, 32 °F (0 °C)
Inventory of cumulonimbus as the International Cloud Atlas lists it, each name with a small drafted sketch and one line on what it is: the species calvus, capillatus; the varieties a hyphen; the supplementary features praecipitatio, virga, incus, mamma, arcus, tuba, murus, cauda; the accessory clouds pannus, pileus, velum, flumen.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-019-BEVERY SPECIES, VARIETY, FEATURE AND ACCESSORY CLOUD THE INTERNATIONAL CLOUD ATLAS LISTS FOR CUMULONIMBUS; SKETCHES NOT TO SCALEFIG. B THE CUMULONIMBUS INVENTORY1SPECIES2, AT MOST ONEcumulonimbus calvusThe top softening and smooth; the caulifloweris going, and no fibres yetcumulonimbus capillatusThe top fibrous and streaked with ice, oftenspread as an anvil2VARIETIES-3SUPPLEMENTARY FEATURES8, ANY THAT APPLYcumulonimbus praecipitatioRain, snow or hail reaching the groundcumulonimbus virgaFall streaks that evaporate before they reachthe groundcumulonimbus incusThe anvil: the top spread out flat, smooth orfibrouscumulonimbus mammaPouches hanging from the undersidecumulonimbus arcusA dense horizontal roll on the lower frontedge: a shelf or roll cloudcumulonimbus tubaA funnel of cloud from the base: a funnelcloud or tornadocumulonimbus murusA lowering of the base beneath the updraft:the wall cloudcumulonimbus caudaA tail of cloud running horizontally into thewall cloud4ACCESSORY CLOUDS4, ANY THAT APPLYcumulonimbus pannusRagged shreds beneath the cloud: scudcumulonimbus pileusA small smooth cap just above the topcumulonimbus velumA wide thin veil the tops rise intocumulonimbus flumenA band of low cloud feeding into a supercell:the beaver's tailTITLECumulonimbus, its inventory in the AtlasVOL. III THE SKY · THE GENERATYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-24IDFB-SKY-019-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 cumulonimbus; 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
  • An unstable atmosphere, in which rising air stays warmer than its surroundings through a deep layer
  • Something to start the lift: a front, an outflow, a sea breeze or a mountain

Scales

time
30 minutes to an hour for one cell; hours for a supercell
horizontal
3 to 15 miles (5 to 25 km) for one tower; an anvil can spread over 60 miles (100 km)
vertical
from a base near 1,000 to 6,500 ft to tops of 39,000 to 60,000 ft (12 to 18 km)
orlanski
meso gamma to meso beta

Signatures

surface
CB in the sky report; thunder heard; a sharp gust, a pressure jump and a temperature drop as the outflow arrives
satellite
the coldest tops on the map; an anvil spreading downwind; overshooting tops as bumps above it
radar
strong echo from rain and hail; a hook or a rotation signature in a supercell

The numbers

QuantityValue, and the kind of number it is
AbbreviationCbStandard, International Cloud Atlas
ÉtageBase in the low étage; top usually in the highStandard, International Cloud Atlas
Made ofWater droplets below, supercooled droplets, ice crystals, snow and hail aboveStandard, International Cloud Atlas
ThunderA cloud that makes lightning or thunder is a cumulonimbusStandard, International Cloud Atlas
Anvil heightNear 39,000 ft (12 km) in the middle latitudes in summer, at the tropopause; higher in the tropicsTypical, Wallace 2006
Species2: calvus, capillatusStandard, International Cloud Atlas
Varieties-Standard, International Cloud Atlas
Supplementary features8: praecipitatio, virga, incus, mamma, arcus, tuba, murus, caudaStandard, International Cloud Atlas
Accessory clouds4: pannus, pileus, velum, flumenStandard, International Cloud Atlas
ReportedCB added to the sky report when a cumulonimbus is seenStandard, Federal Meteorological Handbook No. 1
Severe thunderstormHail of 1 inch (2.5 cm) or more, a gust of 58 mph (50 knots, 26 m/s) or more, or a tornadoStandard, Directive 10-511
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 reports cumulonimbus by what it does: thunder heard, a sudden gust, a jump in pressure and a fall in temperature as the outflow arrives, heavy rain or hail. The observer adds CB to the sky report. Radar sees its rain and hail, and satellites see its top as the coldest cloud on the map, the anvil spreading and the overshooting tops as bumps above it.

How it is warned

Every severe thunderstorm warning and nearly every tornado warning is a warning about a cumulonimbus. A storm is severe with hail of 1 inch (2.5 cm) or more, a gust of 58 mph (50 knots, 26 m/s) or more, or a tornado, Thunderstorm FB-EVT-001. The wall cloud under its updraft, Wall cloud FB-SKY-040, and a funnel from its base are what spotters report to the Weather Service, Tornado FB-EVT-009.

See also

Sources

  1. World Meteorological Organization. International Cloud Atlas.
  2. American Meteorological Society. Glossary of Meteorology.
  3. National Weather Service. Directive 10-511, WFO Severe Weather Products Specification.
  4. Office of the Federal Coordinator for Meteorology. Federal Meteorological Handbook No. 1, Surface Weather Observations and Reports.
  5. Wallace, J. M. and P. V. Hobbs. Atmospheric Science, An Introductory Survey, 2nd ed. (2006).
  6. National Weather Service. Automated Surface Observing System (ASOS) User's Guide.

Definition after the International Cloud Atlas. Plate FB-SKY-019, revision 1, 2026-09-24. The number is permanent; cite it.