Wall clouds, and what radar shows beneath them

A wall cloud is a localized, persistent, often abrupt lowering of cloud beneath a thunderstorm’s rain-free base, marking where the storm’s strongest updraft draws air in from below. It is one of the most important things a storm spotter can see — and it is something a weather radar, for practical purposes, cannot see at all.

That gap is the subject of this page. The supercell guide makes the point in passing; here is the longer version: what a wall cloud is, how spotters tell it apart from its look-alikes, and what the radar does show in the part of the storm where one forms.

What a wall cloud is

The National Weather Service’s SKYWARN spotter glossary defines it as “a localized, persistent, often abrupt lowering from a rain-free base,” ranging “from a fraction of a mile up to nearly five miles in diameter,” and normally found on the south or southwest — inflow — side of the storm. The rain-free base itself is, in the same glossary’s words, a dark horizontal cloud base with no visible precipitation beneath it that “typically marks the location of the thunderstorm updraft.”

Tetsuya Fujita coined “wall cloud,” along with “tail cloud” and “collar cloud,” from his photogrammetric study of the Fargo, North Dakota, tornado of June 20, 1957 (Fujita 1960, as reviewed by Schultz et al. 2011 in the National Weather Digest). The Fargo wall cloud was first photographed about 10 miles west of the city, roughly 25 minutes before the storm reached it.

How it forms

A mature storm’s updraft pulls in low-level air from miles around. Most of it is warm, moist inflow from ahead of the storm. Some of it is air drawn in from the rain area — the outflow of the forward-flank downdraft. NSSL’s Severe Weather 101 explains the consequence: that rain-cooled air is very humid, so its moisture “quickly condenses below the rain-free base to form the wall cloud.”

Put simply, air that is already close to saturation needs less lifting to form cloud, so it condenses at a lower height than the air forming the surrounding base. The NWS spotter guide describes the same thing as warm, moist inflow meeting cool, moist outflow from the forward flank, with the combined air ingested by the updraft. Moist inflow along the forward flank can also condense into a tail cloud that feeds into the wall cloud.

Rotating and non-rotating

Not every wall cloud rotates, and the NWS glossary says so explicitly. A non-rotating wall cloud still marks an updraft, but it is not by itself evidence of a mesocyclone.

A rotating wall cloud is. The glossary notes that rotating wall clouds usually develop before strong or violent tornadoes, “by anywhere from a few minutes up to nearly an hour,” and that wall clouds should be watched for “persistent, sustained rotation and/or rapid vertical motion.” NSSL adds the other visual cues: strong surface winds flowing into it, and small cloud elements rising quickly into the rain-free base. The operative words are persistent and sustained. Brief turbulent churning is common; organized rotation that holds is the signal.

The converse also matters: many rotating wall clouds never produce a tornado, just as most mesocyclones do not.

Wall cloud, shelf cloud or scud

Spotter training spends real time on this, because the three are routinely confused in reports. The distinctions below come from NWS Louisville’s shelf-cloud-versus-wall-cloud material and the SKYWARN glossary.

NWS Louisville puts the shelf-cloud test plainly: with a shelf cloud, “the wind will come first with rain following behind it.” A wall cloud, by contrast, is “much smaller and more compact” and “usually under a rain free cloud base.” Scud, the glossary says, consists of small ragged fragments unattached to a larger base, generally associated with cool moist air such as thunderstorm outflow.

Why radar cannot see it

Three reasons, any one of which would be enough.

• It is cloud, not precipitation. The WSR-88D transmits at a wavelength of about 10 cm, and the return from a volume of targets depends steeply on their size. Cloud droplets return a tiny fraction of what raindrops and hail do, and radar sensitivity falls off with range. Close to the radar a WSR-88D can register very weak non-precipitating echo, but a wall cloud has no distinct radar signature that separates it from the clear-looking inflow air around it.
• It is under the beam. The lowest routine tilt at most sites is 0.5°. Under standard refraction, the center of that beam is about 1.5 km (roughly 4,800 ft) above the radar’s height at 100 km range, and with a beamwidth just under 1° the beam is well over a kilometre deep there. A lowering from the storm’s base, a short distance off the ground, is below or smeared across that sample at most ranges.
• It sits in the updraft, where radar echo is weakest. The whole reason a rain-free base exists is that precipitation is not falling there. The wall cloud forms in exactly the part of the storm the radar shows as low reflectivity.

What radar does show in that region

The radar cannot show you the cloud, but it can show you the structure the cloud hangs beneath. In a classic supercell these features line up around the same small patch of ground.

• The inflow notch. An indentation in low-level reflectivity on the inflow side of the storm — usually in the right quadrant of a classic supercell, per the NWS glossary. It is the radar view of the rain-free base: air going up, nothing coming down.
• The weak echo region and BWER. Stepping up through tilts over the notch, a strong updraft shows as low reflectivity at low levels capped by an overhang of strong echo aloft. When the weak echo is enclosed on all sides, it is a bounded weak echo region. The glossary notes it is “almost always found in the inflow region” and “cannot be seen visually” — the mirror image of the wall cloud, which can be seen and cannot be radar-detected.
• The hook and the RFD. The rear-flank downdraft wraps around the mesocyclone and is often visible from the ground as a clear slot curling around the wall cloud. Precipitation along that boundary is what can appear on radar as a hook or pendant. The NWS spotter guide places the wall cloud on the northern side of the RFD, which on radar puts it near where the hook wraps in.
• The mesocyclone in velocity. Rotation is where radar and spotter overlap. Inbound beside outbound, persistent across scans and deep across tilts, in the same place as the notch and hook, is the radar evidence for what a rotating wall cloud shows visually. Storm-relative velocity makes it easier to see by removing the storm’s own motion.

None of these proves a wall cloud is present. Together they tell you where one would be, and that is why the warning process pairs radar with ground truth from spotters.

Next: tornadoes on radar, and how much each signature beneath a wall cloud should raise your confidence.

WeatherStory Radar reads full-resolution Level II from NOAA’s public archive, so every tilt over the inflow notch is there to step through.