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 (and even then… those pop-up storms can happen at what seems like any time!). 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, and we even get images from people concerned about some cloud formations… The biggest culprits? Shelf Clouds and Wall Clouds. These are different things, but both are archetypal SLCs, or “Scary Looking Clouds”. So what’s the difference?
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“, and is actually generally unrelated to Wall clouds. The elongated cloud structure is usually 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, but as mentioned earlier, they can even be seen when storms are WEAKENING – so aren’t always a sign of bad things to come! (…but you should probably go indoors if you see one barreling towards you).
Wall Clouds, on the other hand, are what often what many people THINK they are seeing and, as such, are concerned about – and understandably so. 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. They also often have more distinct features – you can usually see the rotation occurring, and have intense upward motions, and at times you can even see “barber pole” like striation features up and along the surface of a wall cloud.
Have you ever seen a shelf or wall cloud before? They’re intimidating forces of nature, but truly exemplify the beauty of these beasts.