Shelf cloud

A shelf cloud is a low, horizontal, wedge-shaped arcus cloud. A shelf cloud is attached to the base of the parent cloud, which is usually a storm, but can form in any type of convective clouds. The upward movement of the cloud can usually be seen on the top (outer) of the shelf cloud, while the underside usually appears turbulent and torn by the wind. Fresh air, sinking from a storm cloud downdraft, spreads across the earth's surface, with the attacking edge called the facade. This exit is cut under hot air and drawn into the rising current of the storm. As the colder air makes the air hot and humid, the water condenses, creating a cloud that usually rolls with different winds above and below (wind shear).

People who see a cloud of shelves may believe they have seen a wall cloud. This is probably a mistake since an approaching shelf cloud appears to form a wall made of cloud. A shelf cloud usually appears at the edge of a storm, and a wall cloud will usually be at the rear of the storm.

A strong gust front will cause the lower part of the front edge of a shelf cloud to be jagged and lined with rising fractus clouds. In more severe cases, there will be vortices along the edge, with torrential slag masses that may reach the ground or be accompanied by increased dust. A very low shelf cloud, accompanied by these signals, is the best indicator that a potentially violent windstorm is approaching. An extreme example of this phenomenon looks almost like a tornado and is known as a gustnado.