The feature spotters are most often looking for, its definition in full, and the three signals that decide whether it is worth a radio call.

In this lesson you will

  • State the NWS definition of a wall cloud.
  • List the signals that make a wall cloud reportable: rotation, rapid vertical motion, persistence.
  • Distinguish a rotating from a non-rotating wall cloud.
  • Locate and identify a tail cloud.

The definition, in full

Wall cloud, SR-145, verbatim
A localized, persistent, often abrupt lowering from a rain-free base. Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm. When seen from within several miles, many wall clouds exhibit rapid upward motion and cyclonic rotation. However, not all wall clouds rotate. Rotating wall clouds usually develop before strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour.

Four things in that paragraph do real work. Localized and persistent rule out the ragged lowering that appears for thirty seconds and dissolves. Inflow side places it: south or south-west, on the rear flank, under the updraft base, not out front on the gust front. Not all wall clouds rotate is the sentence most summaries drop. And a few minutes up to nearly an hour is the lead time you may be working with.

A dark, rounded lowering hangs beneath a storm base over open farmland, with a narrow cone-shaped tornado extending from it to the ground and a lighter slot cutting into the cloud base beside it
Wall cloud near Alfalfa, Oklahoma, 22 May 1981. Compact, localized, hanging beneath the updraft base rather than out along a gust front. The bright slot cutting in beside it is lesson 4. Photo courtesy of NSSL, public domain, via SPC

What makes it reportable

A wall cloud is a feature. A rotating wall cloud is a report. Three signals, and you want more than one:

  1. Sustained rotation. Not one apparent turn. Persistent, in one direction, over a watching period. Lesson 7 is the technique for establishing this honestly.
  2. Rapid vertical motion. Cloud material visibly ripping upward into the base, often fastest where the tail cloud joins.
  3. Persistence. It is still there, and still doing both of the above, minutes later.
Teaching point
A wall cloud with none of the three is a lowering, and reporting it as rotation is a false report with your name on it. The Field Guide's instruction is the whole of the discipline: report only what you see, not what you think you see.

How often does a wall cloud become a tornado?

Nobody knows, and this course is not going to tell you. There is no defensible peer-reviewed conversion rate, and a precise-sounding percentage here is one of the things this material is specifically audited to keep out. What can be said is qualitative and comes from NSSL: a wall cloud can exist for ten to twenty minutes before a tornado appears, but not always.

If you meet a figure for this in a class or a forum, ask what it cites. The honest answer is that most wall clouds do not produce tornadoes and the fraction that do has never been pinned down in a way that survives review.

The tail cloud

Tail cloud, SR-145, verbatim
A horizontal, tail-shaped cloud (not a funnel cloud) at low levels extending from the precipitation cascade region of a supercell toward the wall cloud. Cloud motion in the tail cloud is away from the precipitation and toward the wall cloud, with rapid upward motion often observed near the junction of the tail and wall clouds.

The motion is the identification. Cloud material moving out of the rain and into the lowering, accelerating upward where they meet, is rain-cooled air being ingested by the updraft. That is also, in one sentence, how a wall cloud forms.

Check yourself

You watch a localized lowering under the updraft base for three minutes. It persists, but shows no rotation and no obvious vertical motion. What do you report?

  1. A wall cloud, with no rotation observed, if your office wants wall clouds at all
  2. A rotating wall cloud, since wall clouds rotate by definition
  3. Nothing at all, since only tornadoes are reportable
  4. A funnel cloud, since a persistent lowering is a developing funnel
Hint

The definition says something explicit about whether wall clouds rotate.

Answer

A wall cloud, with no rotation observed, if your office wants wall clouds at all

SR-145 says plainly that "not all wall clouds rotate", so a wall cloud is not a rotation report on its own. Describe what you saw and what you did not: a persistent wall cloud, no rotation observed. That is a useful thing for an office to hear, and it is honest. Keep watching, because this can change in minutes.

Sources

  1. Branick, A Comprehensive Glossary of Weather Terms for Storm Spotters, NOAA Technical Memorandum NWS SR-145
  2. NWS Weather Spotter's Field Guide (YPA-201154, June 2011)
  3. NSSL Severe Weather 101, tornado basics and tornado types
  4. SPC public domain tornado images, scanned from the NSSL storm intercept archive