For the last few days, social media is awash with photographs of an extraordinary roll of cloud that has acquired the moniker of “cloud tsunami” hitting the coast of New South Wales, Australia. The ominous cloud measuring several kilometers long that swept across Sydney on Friday last week is actually a shelf cloud.

A shelf cloud is technically an arcus cloud — a kind of low, horizontal, wedge-shaped cloud — that forms at the base of another cloud, usually a thunderstorm cloud, like in this case. When rain from the thunderstorm comes vertically down it drags the air with it, which spreads across the land surface creating a leading edge called a gust front. This outflow cuts under warm air being drawn into the storm's updraft. As the lower cooler air lifts the warm moist air, its water condenses, creating a cloud which often rolls with the different winds above and below. A shelf cloud has a rising motion on the leading edge, while the underside often appears turbulent and wind-torn.

Photo credit: guysebastian/instagram