February 2023

Arcus – Nature’s Front Bumper

If ever there were a cloud forewarning of chaos, it would be the arcus. Known more colloquially as a shelf cloud, it is a feature that can appear attached to the front of an approaching storm system. This arcus cloud was spotted by Jan Webster (Member 58,250) as it rolled in over Toolleen, Victoria, Australia about 30 minutes before the deluge arrived. 

The formation of this cloud feature begins with the heavy precipitation falling in the shower cloud advancing behind it – which is usually a Cumulonimbus, but occasionally also a Cumulus congestus. As the precipitation falls in a deluge within the cloud, it pulls cool air down with it, creating often strong downdrafts. Any evaporation of rain or melting of ice within the shower cools the air, adding energy to its descent. The precipitation lands on the ground and the air has nowhere to go but sideways. The cool air therefore flows out from the shower or thunderstorm, and it burrows beneath the warmer, less dense air in the vicinity down near the surface. This outflow from the storm is most pronounced ahead of the storm’s direction of travel. There, it lifts the less dense surface air, encouraging its moisture to condense into droplets that appear as a ridge of cloud, like the front bumper of some huge atmospheric truck.

Arcus formations can vary considerably in appearance. Some look scruffy and ragged, like a fender that has been shredded into strips. Others appear smooth, perhaps sculpted into horizontal ridges stacked one on top of another. February’s Cloud of the Month is an example of the latter. 

As an arcus cloud approaches, you begin to feel cool air flowing towards you from the storm beyond. Winds can pick up considerably when it is overhead – to a damaging extent if the arcus is associated with a large multicellular thunderstorm. Once the cloud feature has passed overhead, the heavy precipitation usually begins to fall in a matter of minutes. In this brief window of time, one can admire the turbulent, roughened appearance of the back of the arcus. This inside view, between front lip and showers, of the advancing beast of the storm is sometimes likened to the inside of a whale’s mouth. The deluge that inevitably follows means any cloudspotter should be reaching for their waterproofs when they see an arcus cloud approaching.

Arcus spotted over Toolleen, Victoria, Australia by Jan Webster (Member 58,250).

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