Shelf cloud versus wall cloud
- View
Two lowerings. One is the storm breathing out and one is the storm breathing in. Everything about how you report them is different, and this is the call spotters get wrong more than any other.
In this lesson you will
- Given a photograph and the storm's motion and precipitation location, classify a lowering as a shelf cloud or a wall cloud.
- State the location rule: wall clouds sit on the rear inflow side under the updraft base, shelf clouds on the leading edge attached to outflow.
- State that a shelf cloud is an outflow feature and is not reportable as rotation.
- State that a wall cloud is reportable only with rapid rotation, upward motion and warm inflow.
- Apply the cues about sloping toward or away from the rain, and the axis of rotation.
Why this lesson is twice the size of the others
Because forecast offices keep writing pages about it. NWS Louisville states plainly that shelf clouds "many times... are reported as wall clouds, funnel clouds, or rotation", and that scud clouds "are often mistakenly called wall clouds or funnel clouds".
Note what that is and is not. It is the National Weather Service saying, qualitatively, that this is a frequent error. No study quantifies it, and you should be suspicious of anyone who gives you a percentage for it. What it is enough to justify is spending thirty minutes here instead of fifteen.
The definition, in full
The Field Guide glossary on wall clouds, quoted at length because every clause is load-bearing: "A localized, persistent, often abrupt lowering from a rain-free base... 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."
Two things fall out of that. A wall cloud is not by itself a tornado warning, because not all of them rotate. And a rotating one can precede a tornado by up to nearly an hour, which is a very long time in this business and the reason a report of one is valuable rather than merely interesting.
Where each one lives
The checklist
| Ask | Shelf cloud | Wall cloud |
|---|---|---|
| Where in the storm? | Leading edge | Rear, inflow side, usually SW |
| Wind or rain first? | Wind first, from the storm | Warm wind toward the storm |
| Attached to what? | The outflow, along the gust front | A smooth, flat updraft base |
| Axis of rotation | Horizontal roll, if any | Vertical, sometimes strongly |
| Slopes and points | Down and away from the rain | Down and toward the rain |
| Motion relative to rain | Can run far ahead of it | Holds its position with it |
| Size and shape | Wide wedge, can span the horizon | Smaller, compact, localized |
| Reportable? | As wind, never as rotation | Only with rotation, rapid rise, warm inflow |
Assembled from NWS Louisville, the NWS Fort Worth spotter handout and the Field Guide glossary. The two amber rows are the ones that decide a report.
The axis row is the one worth memorising, because it survives bad light and bad angles better than the others. NWS Louisville puts it directly: a shelf "may appear to rotate on a horizontal axis", whereas "Wall clouds will rotate on a vertical axis, sometimes strongly". A shelf cloud tumbling toward you is not rotation in the sense anybody wants reported.
The motion cues come from the NWS Fort Worth handout, and they are the ones to use when you can see the rain: shelf clouds "slope down and away from rain" and "can move far ahead of the rain", while wall clouds "slope down and towards rain", "maintain their position with respect to the rain", and "form under a smooth, flat updraft base".
What each one looks like
Set those two photographs side by side and the difference is obvious. In the field it is not, because you will see one of them at a time, in worse light, from a worse angle, without the other to compare against. That is exactly why the checklist above is built out of where you are and what the wind is doing, rather than out of what the cloud looks like.
Three errors to have already made
- The swap itself. Calling a shelf cloud a wall cloud. Almost always a spotter who is looking at the leading edge of a storm and has not asked which side of it they are on.
- "Any lowering is a wall cloud." Scud lowers. Beaver tails lower. Ragged cloud under a gust front lowers. A wall cloud is a persistent, localized lowering from a smooth updraft base, and the other three are not.
- "Every wall cloud gets reported." No. Not all wall clouds rotate, and one that is not rotating, not rising rapidly and not fed by warm inflow is a cloud. Reporting all of them is how an office learns to discount your net.
Check yourself
You are ahead of an approaching storm. A long, low, wedge-shaped lowering spans the horizon, tilting downward away from the rain behind it, and it is running out ahead of the precipitation. The wind at your back has just picked up sharply, blowing from the storm toward you. Report?
- Shelf cloud. Report the wind threat, and do not report rotation
- Wall cloud with possible rotation, because it is a persistent lowering and it is large
- Funnel cloud, because the leading edge is descending toward the ground
- Nothing yet. Wait to see whether it develops rotation
Hint
Four cues are given: your position, the slope direction, the motion relative to the rain, and the wind direction. All four agree.
Answer
Shelf cloud. Report the wind threat, and do not report rotation
Every cue points the same way. You are on the leading edge, the feature slopes away from the rain, it is outrunning the precipitation, and the wind is blowing from the storm toward you. That is outflow, so it is a shelf cloud, and a shelf cloud is never reported as rotation. It is still worth a report: damaging straight-line wind is directly behind it.
Sources
- NWS Weather Spotter's Field Guide (YPA-201154, June 2011), glossary W (wall cloud)
- NWS Louisville, "Shelf Cloud versus a Wall Cloud"
- NWS Fort Worth, Spotter Handout 2022, and the Storm Spotter's Dictionary
- SPC public domain tornado images, scanned from the NSSL storm intercept archive