In the warm seasons here in the South, especially this time of year, we see almost weekly some form of stormy or unsettled weather, if we aren’t in some sort of major drought. With those storms often come heavy rain, lightning and more often than not, scary skies sweeping across the Valley – we’ve all seen the dark brooding skies at the horizon before, while the opposite direction is as sunny as can be. Moreover, I’m sure we all recognize some of the more iconic forms of significant weather – tornadoes, funnels, and more – but the line sometimes gets hazy (literally, at times) between certain other formations, often causing some confusion and concern. The biggest culprits? Shelf Clouds and Wall Clouds. Let’s discuss their differences.
We’ll start with the more common phenomena of the two, and one of the more commonly mistaken phenomena when it comes to “could this produce a tornado?” – 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, but many times in the Southeast, these form when storms are gusting out, too.
Wall Clouds, on the other hand, are what often behind a majority of the concern surrounding such formations – and they are what many more meteorologists would be concerned to see. 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. These don’t develop often in the Summer… the conditions are just not as favorable.
Have you ever seen a shelf or wall cloud before? Let us know in the comments… in the South, they’re certainly more common sights than many of us may prefer, but that’s hey… that’s why we’re here.