Wall clouds are some of the most eye capturing parts of storm chasing, and are often the visual appearance that a thunderstorm (or likely Supercell if a wall cloud is present) is taking the next step to becoming even more dangerous. The wall cloud typically indicates where the area of strongest updraft is occurring and rarely producing any precipitation itself, but rotating wall clouds can be a clear cut indication of a rotating mesocyclone within the thunderstorm which can produce tornadoes, some of which may become strong and deadly.
Wall clouds are formed through a process called entrainment. This is when warm, moist inflow air, rises and converges. It overpowers the rain-cooled air which is normally associated with the downdraft of a thunderstorm. As the warm air entrains the cooler air, the temperature and dew points increase. As the air continues to rise, it becomes statured which then leads to a process of further cloud development occurring and sometimes this process leads to the wall cloud developing. The wall cloud will also develop at the base of a thunderstorm and may also develop through the formation of rising scud. Wall clouds never produce precipitation as they are in an area called a ‘rain-free base’. The wall cloud is a clear cut indicator that a thunderstorm has entered supercell mode as its the clear cut line between the updraft and downdraft. The separation of these 2 drafts is vital for supercell production as this separation causes the mesocyclone to develop and work in perfect harmony. As long as this process takes place, a thunderstorm will remain a supercell.. as soon as the downdraft and updraft become mixed, then the storm begins to weaken.
Their structure is unique as they can be anything from minuscule in size (in the big picture) to absolutely mammoth (between 1km and 8km+). As stated above, they form in the warmer inflow region of the storm which is coinciding with the direction of the steering winds. In the Northern hemisphere, wall clouds typically form on the Southern / South-West regions of a supercell, however they normally form over the Northern or North-East regions of a supercell in the Southern hemisphere.
Attached to many wall clouds can be what is known as a tail cloud, which is a ragged band of cloud extending from the wall cloud to the precipitation core. Another accessory is the flumen cloud which is commonly known as a beavers tail. This is formed by warm, humid inflow in a strong thunderstorm but can often be mistaken for a tornado. Despite the presence of a beavers tail normally being associated with tornadic risk, the cloud itself does not rotate.