A wall cloud is a distinctive lowered, rotating segment of a supercell thunderstorm's base. If you see one, it's the strongest single indicator that a tornado may form in the next 10 to 30 minutes. Trained storm spotters are trained specifically to recognize wall clouds because they're the pre-tornado warning sign.
A wall cloud is:
Storms have a "cloud base" - the height at which cloud material begins. A wall cloud is a section of that base hanging 500-2,000 feet lower than the rest.
Watch a wall cloud for 30 seconds to 1 minute. If the cloud is rotating (usually counter-clockwise in the Northern Hemisphere), it's a rotating wall cloud - the immediate tornado precursor. If it's not rotating, it may still be significant but the tornado risk is much lower.
A wall cloud is under the rain-free base of a supercell - the part of the cloud that is NOT dropping heavy precipitation. If a lowered cloud is in the rain shaft itself, it's more likely just a rain-fed shelf cloud (not tornadic).
Real wall clouds persist for at least 5-10 minutes. Transient lowered clouds that appear and disappear quickly are usually scud - fragmented low clouds pushed around by turbulence. Wall clouds are stable structures.
These two are often confused:
Wall clouds are pre-tornado. Shelf clouds are pre-derecho.
When a wall cloud develops, the typical progression is:
Not every wall cloud produces a tornado - roughly 20-40% do. But every wall cloud should trigger sheltering behavior. The NWS often issues Tornado Warnings based on wall cloud reports from spotters.
Storm chasers photograph wall clouds constantly - they're one of the most photogenic severe weather features. If you're chasing legally at safe distance (over 2 miles from the storm), a wall cloud makes excellent time-lapse footage as it develops.
Amateur storm photography: only shoot from behind safe cover. Never stand exposed near a rotating wall cloud.
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