Storm Station 247

The Field Book/Vol. III, The Sky/Cloud classification/FB-SKY-040

Wall cloud

A wall cloud is a lowering of a thunderstorm's base under its strongest updraft, often at the rear of the rain in a supercell. It forms where rain cooled, moist air is drawn into the updraft and condenses lower down. A wall cloud that rotates is where most strong tornadoes form.

Plate FB-SKY-040Storm cloudsRevision 1, 2026-09-24Status draftAlso called murus, wall cloud, tail cloud, cauda, rotating wall cloud, lowering, rain free base

A painted section of the lower part of a supercell over a prairie, heights to scale from the ground to 13,000 feet (4 kilometres), the storm moving left to right: a dark billowing storm cut off at 12,500 feet (3.8 kilometres) above a flat rain free base near 4,500 feet (1.4 kilometres); a dark wall cloud lowered beneath the updraft to about 2,300 feet (0.7 kilometres), banded as it turns, with a slender funnel beneath it; a tail cloud running from the wall toward the rain; two arrows of inflow from the rain side; the rain and hail core on the right; and on the left a clear slot of blue sky carved by the rear flank downdraft.
Fig. A The wall cloud under its storm. The lower part of a supercell from the side, moving left to right.MaximizeThe drawing, to scale

What it is

A wall cloud is a lowering of a thunderstorm's base, hanging beneath its strongest updraft. Its sides are steep and its base can be well below the rest of the storm's. In a classic supercell, Supercell FB-EVT-007, moving toward the northeast, it hangs at the rear of the rain, often on the storm's southwest side. It is typically 1 to 4 mi (1.6 to 6.4 km) across. The International Cloud Atlas names it murus, one of the supplementary features of cumulonimbus, Cumulonimbus FB-SKY-019.

Fig. A draws it under its storm. A tail cloud, cauda, often runs from the wall back toward the rain, with the moist air flowing along it into the wall. Behind the wall, the rear flank downdraft can carve a clear slot of sky.

What it is not

It is not a shelf cloud. A shelf cloud is a wedge on the storm's leading edge, ahead of the rain, with the cold outflow running out beneath it; it sits where air is sinking and spreading, and a wall cloud sits where air is rising. It is not scud, the loose ragged shreds that drift under a storm and are not attached to its base. Fig. B draws the three side by side, as spotters are taught to tell them apart.

It is not a tornado. Most wall clouds never produce one.

Lookalikes

Shelf cloud
A wedge on the storm's leading edge, ahead of the rain, with the cold outflow beneath it; it does not rotate about a vertical axis
Scud
Ragged shreds, loose and detached, drifting with the wind
Funnel cloud
A funnel descends from the wall cloud; the wall cloud is the broader lowering it comes from

The machine

The wall cloud is the updraft made visible. A supercell's updraft draws in air from near the ground, and some of that air has passed through the rain. The rain has cooled and moistened it, so as it rises it reaches its dew point sooner, at a lower height than the air elsewhere under the storm: the base lowers there, into a wall. The tail cloud is the same moist inflow condensing on its way in.

When the storm's updraft rotates, as in a supercell's mesocyclone, the wall cloud can be seen to rotate with it, and cloud can be seen rising rapidly into it. The rear flank downdraft wraps around the wall from behind. It is in that meeting of rotating updraft and wrapping downdraft that most strong tornadoes form, often within minutes to an hour of a wall cloud beginning to rotate, Tornado FB-EVT-009. A funnel that descends from the wall cloud is the first sign.

Three small paintings under a dark storm base. A wall cloud: a steep sided block lowered from the base, banded as it turns, with an arrow of air rising into it. A shelf cloud: a wedge sloping up and back into the storm ahead of a grey rain curtain, with an arrow of outflow running out beneath it. Scud: ragged loose shreds below the base with an arrow of the wind.
Fig. B Wall cloud, shelf cloud and scud. Three low clouds under a storm, and the test that tells them apart.MaximizeThe drawing, to scale

The drawings

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

Section of the lower part of a supercell from the ground to 13,000 feet (4 kilometres), the storm moving left to right: a flat rain free base near 4,500 feet (1.4 kilometres); a wall cloud lowered beneath the updraft to about 2,300 feet (0.7 kilometres) with a funnel beneath it; a tail cloud running from the wall toward the rain, with the inflow flowing along it into the wall; the rain and hail core ahead to the right; and the rear flank downdraft behind, carving a clear slot.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-040-ATHE LOWER PART OF A SUPERCELL SEEN SIDE ON, MOVING LEFT TO RIGHT: THE BASE, THE WALL CLOUD, THE TAIL CLOUD, THE RAIN AND THE DOWNDRAFTFIG. A THE WALL CLOUD UNDER ITS STORM051001234kft(km)LOWMIDDLETHE STORM ABOVE, CUT AT 12,500 ftINFLOW, FROM THE RAIN SIDECLEAR SLOTKEYWater dropletsRain and hail reaching the groundUpdraftDowndraft12345678TITLEWall cloud, the wall cloud under its stormVOL. III THE SKY · STORM CLOUDSTYPESECTIONSCALEHEIGHT TO SCALE, WIDTH NTSREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-24IDFB-SKY-040-ADRAWN AS linework on paperSOURCES WMO, Markowski, Lemon
Fig. A, the drawing The wall cloud under its storm. The lower part of a supercell seen side on, moving left to right: the base, the wall cloud, the tail cloud, the rain and the downdraft HEIGHT TO SCALE, WIDTH NTSMaximizeThe sheet, SVG, 11 by 17The painting
  1. The rain free base
  2. The wall cloud, murus
  3. The tail cloud, cauda
  4. The inflow, from the rain side
  5. The rain and hail core
  6. The rear flank downdraft and its clear slot
  7. A funnel
  8. The updraft
Three small elevations with their names and a line each: a wall cloud with steep sides hanging under the updraft, rotating, with cloud rising into it; a shelf cloud as a wedge on the storm's leading edge ahead of the rain with the outflow running out beneath it; and scud as ragged loose shreds drifting with the wind, not attached to the base.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-040-BTHREE LOW CLOUDS UNDER A STORM, TOLD APART BY WHERE THEY HANG AND HOW THEY MOVEFIG. B WALL CLOUD, SHELF CLOUD AND SCUD1WALL CLOUD, MURUSHangs under the storm's updraft, beside or behindthe rain; its sides are steep and it may rotate,with cloud rising into it. The one to report.2SHELF CLOUD, ARCUSA wedge on the storm's leading edge, sloping upand back into the storm, ahead of the rain; thecold outflow runs out beneath it.3SCUD, PANNUS AND FRACTUSRagged shreds, low and loose, drifting with thewind or blown out ahead of the storm. They do notrotate and they are not attached to the base.THE TEST SPOTTERS USE: A WALL CLOUD LOWERS FROM THE BASE UNDER THE UPDRAFT AND PERSISTS; A SHELF CLOUD LEADS THE STORMWITH THE OUTFLOW BENEATH IT; SCUD IS LOOSE AND DETACHED. A WALL CLOUD THAT ROTATES, AND KEEPS ROTATING, IS REPORTED AT ONCE.EACH DRAWN SMALL AND NOT TO SCALE, THE STORM MOVING FROM LEFT TO RIGHT.TITLEWall cloud, wall cloud, shelf cloud and scudVOL. III THE SKY · STORM CLOUDSTYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-24IDFB-SKY-040-BDRAWN AS linework on paperSOURCES WMO, Markowski, Lemon
Fig. B, the drawing Wall cloud, shelf cloud and scud. Three low clouds under a storm, told apart by where they hang and how they move NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17The painting
  1. Wall cloud, murus
  2. Shelf cloud, arcus
  3. Scud, pannus and fractus

Ingredients

  • A strong, lasting updraft, as in a supercell
  • Moist air near the ground drawn into it, cooled by the rain so it condenses at a lower height
  • For rotation, a mesocyclone: the updraft turning, from wind that changes with height

Scales

time
minutes to an hour
horizontal
typically 1 to 4 mi (1.6 to 6.4 km) across
vertical
a few hundred to a few thousand feet below the storm's base
orlanski
meso gamma

Signatures

surface
seen, not measured: spotters report its rotation, its motion and any funnel
radar
a hook of echo wrapping round the updraft, and a rotation couplet in the velocity data above the wall cloud
satellite
an overshooting top above the updraft

The numbers

QuantityValue, and the kind of number it is
Atlas nameMurus, a supplementary feature of cumulonimbus; the tail cloud is caudaStandard, International Cloud Atlas
SizeTypically 1 to 4 mi (1.6 to 6.4 km) acrossTypical, Markowski 2010
Where it hangsUnder the updraft, at the rear of the rain in a classic supercell moving northeast, so often on the storm's southwest sideTextbook, Lemon 1979
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

No instrument sees a wall cloud; spotters do. A station under the storm records what comes with it: the gust front, the pressure jump and the rain. Radar sees the updraft region as an echo free vault with a hook of rain wrapping around it, and the rotation above the wall cloud in its velocity data. What the radar cannot see, near the ground under the wall, is what a spotter reports.

How it is warned

A rotating wall cloud is one of the reports that leads the Weather Service to issue or extend a Tornado Warning. Spotters report its location, its direction of motion, whether it is rotating, and any funnel; the report goes into the warning's text as the source, How to read a warning FB-STN-001. For anyone who sees one, the warning's advice applies at once: move to shelter before the funnel reaches the ground.

See also

Sources

  1. World Meteorological Organization. International Cloud Atlas.
  2. Markowski, P. and Y. Richardson. Mesoscale Meteorology in Midlatitudes (2010).
  3. Lemon, L. R. and C. A. Doswell III. Severe Thunderstorm Evolution and Mesocyclone Structure as Related to Tornadogenesis, Monthly Weather Review 107 (1979).
  4. National Weather Service. Directive 10-511, WFO Severe Weather Products Specification.
  5. National Weather Service. Automated Surface Observing System (ASOS) User's Guide.

Definition after the International Cloud Atlas. Plate FB-SKY-040, revision 1, 2026-09-24. The number is permanent; cite it.