Wall Cloud vs. Shelf Cloud: What's the Difference?
In the warm seasons here in the South, we see almost weekly some form of stormy or unsettled weather, if we aren't in some sort of drought. With those storms often come heavy rain, lightning and more often than not, scary skies sweeping across the Valley. We all recognize some of the more iconic forms of significant weather - tornadoes, funnels, anvil clouds, and more - but the line sometimes gets hazy (literally, at times) between certain other formations, often causing some confusion and worry. The biggest culprits? Shelf Clouds and Wall Clouds.
We'll start with the more common phenomena of the two - the Shelf Cloud. It looks ominous, no doubt, but can frequently be associated with non-severe storms, and even showers fading out. While it may appear like a "wall" of cloud, the technical name is "Arcus cloud". The elongated cloud structure is caused by cold air in the thunderstorms downdraft spreading out, condensing into this formation and often precludes higher winds and precipitation. Unlike a wall cloud, which is usually at the rear of a storm, this is often in the front of a storm. They can still indicate dangerous weather, though - derechos, or severe lines of 60+mph winds - often are lead by a significant shelf cloud.
Wall Clouds, on the other hand, are what often are behind a majority of the concern surrounding such formations. Wall clouds are in many ways opposites to the shelf cloud - as mentioned previously, they most commonly reside in the rear sector of the storm, and instead of cold, downbursting air, their formation is due to warm and humid inflowing air from the surrounding atmosphere. Wall clouds often - but not always - preclude tornadoes, and are typically a sure sign of a supercell as opposed to a more coherent line of thunderstorms.
Have you ever seen a shelf or wall cloud before? They're intimidating forces of nature, but truly exemplify the beauty of these beasts.