Storm Station 247

The Field Book/Vol. III, The Sky/Cloud classification/FB-SKY-018

Cumulus

Cumulus is a detached, heaped cloud with a flat base and a sharply outlined top that grows in rising mounds, brilliant white where the sun strikes it. Each one sits on a column of rising air, and its base marks the height at which that air cools to its dew point.

Plate FB-SKY-018The generaRevision 1, 2026-09-24Status draftAlso called Cu, fair weather cumulus, cumulus humilis, cumulus mediocris, cumulus congestus, towering cumulus, TCU, thermal, low cloud

A painted section of cumulus over a prairie, heights to scale from the ground to 23,000 feet (7 kilometres): three white cumulus with sunlit tops and dark flat bases on one condensation level at 4,000 feet (1.2 kilometres), each above a dashed arrow of a rising thermal; a small flat cumulus humilis, a cumulus mediocris to about 9,500 feet (2.9 kilometres), and a towering cumulus congestus to about 18,000 feet (5.5 kilometres) with a thin pale cap of pileus above its top.
Fig. A Cumulus in section. Three cumulus on one flat base at the condensation level, each over its thermal.MaximizeThe drawing, to scale

What it is

Cumulus is a detached cloud, dense and sharply outlined, growing upward in mounds, domes or towers whose tops look like a cauliflower. Its sunlit parts are brilliant white; its base is darker and nearly flat. It forms with its base in the low étage, Cloud classification FB-SKY-001, and the tallest grow up into the middle étage and beyond.

The Atlas lists four species, which are also the stages of its growth: humilis, small and flat, wider than it is tall; mediocris, about as tall as it is wide; congestus, tall and growing fast, the towering cumulus; and fractus, ragged shreds. It has one variety, radiatus, for cumulus in long rows. Five supplementary features and four accessory clouds can come with it, among them pileus, a smooth cap over a rising top, and tuba, a funnel from the base. Fig. B draws them all.

What it is not

It is not cumulonimbus, Cumulonimbus FB-SKY-019. A cumulus that makes lightning or thunder, or whose top has turned to fibrous ice, is cumulonimbus by definition; until then it is cumulus, however tall. It is not stratocumulus, Stratocumulus FB-SKY-016, which spreads sideways as a layer under a lid. It is not altocumulus castellanus, whose turrets rise from a base in the middle étage.

Lookalikes

Stratocumulus
A layer of joined rolls, spreading sideways under an inversion.
Cumulonimbus
Lightning, thunder, or a top turned to fibrous ice.
Altocumulus castellanus
Turrets rising from a middle level base, not from a low one.

The machine

Each cumulus stands on a thermal. The sun warms the ground, the ground warms the air above it, and a parcel of that air, warmer and lighter than the air around it, rises. As it rises it expands and cools. At the height where it has cooled to its dew point its vapour condenses, and the cloud begins: every thermal in the same air reaches its dew point at the same height, so a field of cumulus shares one flat base. Fig. A draws three on that base.

The height of the base can be estimated from the ground: about 230 ft for each °F between the temperature and the dew point (125 m for each °C). A spread of 20 °F puts the base near 4,600 ft (1.4 km).

What happens above the base depends on the air aloft. Where the air above is stable, the cloud stops as a small humilis. Where it is unstable, the rising air stays warmer than its surroundings, keeps rising, and the cloud grows to mediocris and then congestus. A rising top that pushes a moist layer above it into saturation wears a smooth cap, pileus. A congestus that keeps growing, until its top turns to ice, has become a cumulonimbus.

Small paintings of cumulus, one for every name the International Cloud Atlas lists for it, each labelled with its full name and its rank: the species humilis, mediocris, congestus and fractus; the variety radiatus; the supplementary features virga, praecipitatio, arcus, fluctus and tuba; and the accessory clouds pileus, velum, pannus and flumen; with the count of each rank beneath, a hyphen where the Atlas gives none.
Fig. B The cumulus inventory. Every name the Atlas lists for cumulus, 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 cumulus from the ground to 23,000 feet (7 kilometres): three clouds of water droplets on one flat base at the condensation level, 4,000 feet (1.2 kilometres), each above a thermal of rising air; a small flat cumulus humilis to about 5,400 feet (1.6 kilometres), a cumulus mediocris to about 9,500 feet (2.9 kilometres), and a towering cumulus congestus to about 18,000 feet (5.5 kilometres) wearing a thin cap of pileus.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-018-ATHREE CUMULUS ON ONE FLAT BASE AT THE CONDENSATION LEVEL, EACH ON ITS THERMAL; HEIGHTS TYPICAL OF A SUMMER AFTERNOONFIG. A CUMULUS IN SECTION0510152001234567kft(km)LOWMIDDLEHIGHCONDENSATIONLEVELKEYWater dropletsThermal, rising air1234567TITLECumulus, a section to height scaleVOL. III THE SKY · THE GENERATYPESECTIONSCALEHEIGHT TO SCALE, WIDTH NTSREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-24IDFB-SKY-018-ADRAWN AS linework on paperSOURCES WMO, AMS, Wallace
Fig. A, the drawing A section to height scale. Three cumulus on one flat base at the condensation level, each on its thermal; heights typical of a summer afternoon HEIGHT TO SCALE, WIDTH NTSMaximizeThe sheet, SVG, 11 by 17The painting
  1. A thermal: rising air
  2. The flat base at the condensation level
  3. Cumulus humilis
  4. Cumulus mediocris
  5. Cumulus congestus
  6. Pileus: a cap of cloud over the top
  7. The low étage
Inventory of cumulus as the International Cloud Atlas lists it, each name with a small drafted sketch and one line on what it is: the species humilis, mediocris, congestus, fractus; the varieties radiatus; the supplementary features virga, praecipitatio, arcus, fluctus, tuba; the accessory clouds pileus, velum, pannus, flumen.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-018-BEVERY SPECIES, VARIETY, FEATURE AND ACCESSORY CLOUD THE INTERNATIONAL CLOUD ATLAS LISTS FOR CUMULUS; SKETCHES NOT TO SCALEFIG. B THE CUMULUS INVENTORY1SPECIES4, AT MOST ONEcumulus humilisSmall and flat, wider than it is tallcumulus mediocrisAs tall as it is wide, the top with smallbulgescumulus congestusTall and growing, its top like a cauliflowercumulus fractusRagged shreds, their outlines changing fast2VARIETIES1, ANY THAT APPLYcumulus radiatusParallel bands that seem to meet at thehorizon3SUPPLEMENTARY FEATURES5, ANY THAT APPLYcumulus virgaFall streaks that evaporate before they reachthe groundcumulus praecipitatioRain, snow or hail reaching the groundcumulus arcusA dense horizontal roll on the lower frontedge: a shelf or roll cloudcumulus fluctusShort lived curls or breaking waves on the topcumulus tubaA funnel of cloud from the base: a funnelcloud or tornado4ACCESSORY CLOUDS4, ANY THAT APPLYcumulus pileusA small smooth cap just above the topcumulus velumA wide thin veil the tops rise intocumulus pannusRagged shreds beneath the cloud: scudcumulus flumenA band of low cloud feeding into a supercell:the beaver's tailTITLECumulus, its inventory in the AtlasVOL. III THE SKY · THE GENERATYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-24IDFB-SKY-018-BDRAWN AS linework on paperSOURCES WMO, AMS, Wallace
Fig. B, the drawing Its inventory in the Atlas. Every species, variety, feature and accessory cloud the International Cloud Atlas lists for cumulus; 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

  • Ground warmed by the sun, which warms the air above it
  • Parcels of that air rising on their own because they are warmer than the air around them: thermals
  • Enough moisture for the rising air to reach its dew point and condense

Scales

time
10 to 30 minutes for one small cumulus; longer for a towering one
horizontal
a few hundred feet to a few miles across
vertical
a few hundred feet for humilis to 20,000 ft (6 km) or more for congestus
orlanski
meso gamma

Signatures

surface
scattered or broken cloud at one base height in the sky report; TCU added when towering cumulus is seen
satellite
bright separate cells in the visible, often in rows or streets
radar
no echo until the tallest begin to rain

The numbers

QuantityValue, and the kind of number it is
AbbreviationCuStandard, International Cloud Atlas
ÉtageBase in the low étage; tops can reach the middle and the highStandard, International Cloud Atlas
Made ofWater droplets; ice in the tops of the tallestStandard, International Cloud Atlas
The baseFlat, at the condensation level of the rising airTextbook, Wallace 2006
Base height, estimatedAbout 230 ft for each °F between temperature and dew point at the ground (125 m for each °C)Textbook, Wallace 2006
Species4: humilis, mediocris, congestus, fractusStandard, International Cloud Atlas
Varieties1: radiatusStandard, International Cloud Atlas
Supplementary features5: virga, praecipitatio, arcus, fluctus, tubaStandard, International Cloud Atlas
Accessory clouds4: pileus, velum, pannus, flumenStandard, International Cloud Atlas
Towering cumulusReported as TCU when seenStandard, Federal Meteorological Handbook No. 1
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 cumulus bases with the laser ceilometer, which reports scattered or broken cloud at one height: the flat base. It does not name the genus, but the observer adds TCU to the report when towering cumulus is in sight. On radar a cumulus has no echo until its drops grow large enough to fall.

How it is warned

Cumulus is not warned. It is watched, because a congestus is a thunderstorm forming, and the warnings that follow are for the storm, Thunderstorm FB-EVT-001. A funnel from the base of a growing cumulus, tuba, is reported as a funnel cloud, and on rare occasions it reaches the ground as a tornado, Tornado FB-EVT-009.

See also

Sources

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

Definition after the International Cloud Atlas. Plate FB-SKY-018, revision 1, 2026-09-24. The number is permanent; cite it.