Wall Cloud

What a Wall Cloud Is

A wall cloud is:

• A section of a supercell's cloud base that hangs noticeably lower than the surrounding cloud
• Located under the rain-free base of the storm (usually southwest of the visible rain)
• Typically rotating - visible rotation is a critical distinguishing feature
• 1-4 miles wide typically
• The site where tornadoes usually descend from

How to Identify a Wall Cloud

1. Look at the Cloud Base

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.

2. Check for Rotation

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.

3. Location Relative to Rain

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).

4. Persistent, Not Fleeting

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.

Wall Cloud vs. Shelf Cloud

These two are often confused:

• Wall cloud: Located under the rain-FREE base, rotating, associated with rotating supercells, precedes tornadoes
• Shelf cloud: Located AT the leading edge of a rain shaft, wedge-shaped and horizontal, moves outward with the storm's cold outflow, associated with straight-line winds (not usually tornadoes)

Wall clouds are pre-tornado. Shelf clouds are pre-derecho.

Timeline: Wall Cloud to Tornado

When a wall cloud develops, the typical progression is:

1. 0 minutes: Wall cloud forms
2. 5-15 minutes: Wall cloud tightens and rotation becomes visible
3. 15-25 minutes: Funnel cloud may appear, extending downward from the wall cloud
4. 20-30 minutes: Funnel touches ground - tornado has formed

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.

What to Do If You See a Wall Cloud

• Note the direction from you - the wall cloud is usually southwest of the storm's motion
• Report to local NWS (via their online storm report form) or dial 911 for immediate reports
• Move to shelter - do not stay outside to watch
• Continue monitoring weather radio for warnings

Wall Cloud Photography

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.