It is formed by the warm, humid inflow of a strong thunderstorm, and is often mistaken for tornadoes. Some wall clouds also have a band of cloud fragments encircling the top of the wall cloud where it meets the ambient cloud base this feature is a collar cloud.Īnother accessory cloud is the flumen, commonly known as the beaver's tail. Most movement is horizontal, but some rising motion is often apparent as well. Cloud elements may be seen to be moving into the wall cloud, as it is also an inflow feature. It can be thought of as an extension of the wall cloud in that not only is the tail cloud connected to the wall cloud but also that condensation forms for a similar reason. Some wall clouds have a feature similar to an "eye", as in a mesoscale convective vortex.Īttached to many wall clouds, especially in moist environments, is a cauda ( tail cloud), a ragged band of cloud and cloud tags (fractus) extending from the wall cloud toward the precipitation core. Rotating wall clouds are visual evidence of a mesocyclone. This is in the rear of the supercell near the main updraft and most supercells move in a direction with northeasterly components, thus for supercells forming in northwest flow situations and moving southeastward, the wall cloud may be found on the northwest or back side of such storms. In the Northern Hemisphere wall clouds typically form at the south or southwest end of a supercell. Wall clouds form in the inflow region, on the side of the storm coinciding with the direction of the steering winds (deep layer winds through the height of the storm). Wall clouds can be anywhere from a fraction of 1.6 km (1 mi) wide to over 8 km (5 mi) across. Wall clouds may form as a descending of the cloud base or may form as rising scud comes together and connects to the storm's cloud base. As this air continues to rise, it becomes more saturated with moisture, which results in additional cloud condensation, sometimes in the form of a wall cloud. As the warm air continues to entrain the cooler air, the air temperature drops and the dew point increases (thus the dew point depression decreases). Wall clouds are formed by a process known as entrainment, when an inflow of warm, moist air rises and converges, overpowering wet, rain-cooled air from the normally downwind downdraft.
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