Storm Station 247

The Field Book/Vol. III, The Sky/Cloud classification/FB-SKY-033

Accessory clouds

An accessory cloud is a smaller cloud seen with a main one, joined to it or close by: a cap over a rising top, a veil the tops push into, ragged scud under a raining base, or a band of low cloud feeding a storm. The International Cloud Atlas lists four.

Plate FB-SKY-033The ranksRevision 1, 2026-09-25Status draftAlso called accessory cloud, pileus, cap cloud, velum, pannus, scud, flumen, beaver tail, inflow band

Small paintings of the 4 accessory clouds of the International Cloud Atlas, each painted in one genus and labelled with its full name, with every genus the Atlas gives it to beneath: cumulus pileus, in cumulus and cumulonimbus; cumulus velum, in cumulus and cumulonimbus; nimbostratus pannus, in altostratus, nimbostratus, cumulus and cumulonimbus; cumulonimbus flumen, in cumulus and cumulonimbus.
Fig. A The four accessory clouds. Each accessory cloud painted, with the genera it is seen with.MaximizeThe drawing, to scale

What it is

An accessory cloud is a smaller cloud seen with a main one, joined to it or close by. The International Cloud Atlas lists four, drawn in Fig. A with the genera each is seen with, and a cloud may show any that apply. They come last in a name: cumulus congestus pileus is a towering cumulus wearing a cap.

Pileus is a small smooth cap just above a rising top. Velum is a wide thin veil that the tops rise into and often pierce. Pannus is ragged scud beneath a cloud, most often under rain. Flumen is a band of low cloud feeding a storm, known to spotters as the beaver's tail.

What it is not

It is not a supplementary feature, which is a part of the main cloud itself, such as an anvil or a wall cloud, Supplementary features atlas FB-SKY-032. A wall cloud is attached to the base and may rotate; pannus is loose and does not. A lens cloud standing in a mountain wave is altocumulus lenticularis, Orographic clouds FB-SKY-035; a pileus sits on a rising top and moves with it.

Lookalikes

Supplementary features
Parts of the main cloud itself, such as an anvil or a wall cloud; see FB-SKY-032
Wall cloud
A lowering of the base under the updraft, attached and often rotating; pannus is loose and does not rotate
Lens cloud
A lenticularis stands in a mountain wave on its own; a pileus sits on a rising top and is pierced by it

The machine

Pileus and velum form the same way. A strong updraft pushes a moist layer upward ahead of it, the layer cools as it rises, and it condenses into a thin smooth cap. If the layer is wide, the cap is a veil across the tops of many towers; if the updraft keeps rising, it bursts through. Pannus forms under a raining base, where the falling rain moistens the air until it condenses into shreds that drift with the wind. Flumen forms in the moist air flowing into a supercell's updraft from the side of the rain, Cumulonimbus architecture FB-SKY-041.

Fig. B draws all four on one sky.

A painted sky, not to scale: a towering cumulus wearing a thin pale cap, pileus; two growing towers rising into a wide thin veil, velum; a raining storm with dark ragged scud, pannus, beneath its base; and a thin band of low cloud, flumen, running into the storm from the right above an arrow of the inflow.
Fig. B The four on one sky. Each accessory cloud with the cloud it comes with.MaximizeThe drawing, to scale

The drawings

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

The 4 accessory clouds of the International Cloud Atlas, each with a small drafted sketch, one line on what it is, and the genera the Atlas gives it to: pileus in cumulus and cumulonimbus; velum in cumulus and cumulonimbus; pannus in altostratus, nimbostratus, cumulus and cumulonimbus; flumen in cumulus and cumulonimbus.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-033-ATHE FOUR ACCESSORY CLOUDS THE INTERNATIONAL CLOUD ATLAS LISTS, AND THE GENERA EACH IS SEEN WITH; SKETCHES NOT TO SCALEFIG. A THE FOUR ACCESSORY CLOUDSpileusA small smooth cap just above the topIn Cu, CbvelumA wide thin veil the tops rise intoIn Cu, CbpannusRagged shreds beneath the cloud: scudIn As, Ns, Cu, CbflumenA band of low cloud feeding into a supercell: the beaver'stailIn Cu, CbEACH SKETCH SMALL AND NOT TO SCALE; UNDER EACH NAME, THE GENERA THE ATLAS GIVES IT TO, BY THEIR ABBREVIATIONS.TITLEAccessory clouds, the four accessory cloudsVOL. III THE SKY · THE RANKSTYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-25IDFB-SKY-033-ADRAWN AS linework on paperSOURCES WMO, AMS
Fig. A, the drawing The four accessory clouds. The four accessory clouds the International Cloud Atlas lists, and the genera each is seen with; sketches not to scale NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17The painting
  1. pileus
  2. velum
  3. pannus
  4. flumen
Elevation of one sky, not to scale: a towering cumulus wearing a thin smooth cap of pileus; a wide thin veil, velum, that two growing towers rise into; a raining storm with ragged scud, pannus, beneath its base; and a band of low cloud, flumen, running into the storm from the right with the inflow beneath it.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-033-BTHE FOUR WHERE EACH FORMS; NOT TO SCALEFIG. B THE FOUR ON ONE SKY1234NOT TO SCALE. AN ACCESSORY CLOUD IS SEEN WITH ANOTHER CLOUD, JOINED TO IT OR CLOSE BY.TITLEAccessory clouds, the four on one skyVOL. III THE SKY · THE RANKSTYPEELEVATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-25IDFB-SKY-033-BDRAWN AS linework on paperSOURCES WMO, AMS
Fig. B, the drawing The four on one sky. The four where each forms; not to scale NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17The painting
  1. Pileus
  2. Velum
  3. Pannus
  4. Flumen

Ingredients

  • A rising top that lifts a moist layer above it to saturation: pileus, and velum when the layer is wide
  • Rain falling into moist air beneath a base: pannus
  • Warm moist air flowing into a supercell's updraft: flumen

Scales

time
minutes for a pileus; an hour or more for a velum or flumen
horizontal
a cap as wide as the top it sits on; a velum or flumen tens of miles long
vertical
a few hundred feet deep
orlanski
micro to meso

Signatures

surface
low pannus under rain can set a station's lowest layer, often broken or overcast
satellite
too small to see, except a velum spread wide over the tops
radar
no echo of their own

The numbers

QuantityValue, and the kind of number it is
Accessory clouds4: pileus, velum, pannus and flumenStandard, International Cloud Atlas
How many to a cloudAny that applyStandard, International Cloud Atlas
PannusWith altostratus, nimbostratus, cumulus and cumulonimbusStandard, International Cloud Atlas
Pileus and velumWith cumulus and cumulonimbusStandard, International Cloud Atlas
FlumenWith cumulus and cumulonimbus; on a supercell, the beaver's tailStandard, International Cloud Atlas
Where the name goesLast: cumulus congestus pileusStandard, 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

Pannus under rain often sets a station's lowest layer, reported as broken or overcast a few hundred to a few thousand feet up by its ceilometer, which reaches 12,000 ft (3.7 km). The others are named from sight.

How it is warned

Accessory clouds are not warned. A pileus over a cumulus says its updraft is strong and rising fast. A flumen feeding a storm is one of the signs spotters look for on a supercell.

See also

Sources

  1. World Meteorological Organization. International Cloud Atlas.
  2. American Meteorological Society. Glossary of Meteorology.

Definition after the International Cloud Atlas. Plate FB-SKY-033, revision 1, 2026-09-25. The number is permanent; cite it.