Arcus clouds


A shelf cloud is a common type of arcus clouds. It is a low, wedge-shaped, horizontal arcus cloud, attached to the base of the thunderstorm.

A shelf cloud forms along the gust front (cyan). As the warm, moist air ahead of the storm, the inflow (orange) is forced upwards by the advancing gust front, cooler air from the outflow (blue) is entrained (picked up with the rising warm air), causing the moisture to condense and to form the shelf cloud.

When looking at a shelf cloud you are still in the inflow; the air is warm and moist.
When looking at a whale’s mouth you are already in the outflow; the air is cool(er).

Shelf clouds can be of different heights/depths: some only reach a small distance below the base of the thunderstorm, while others can reach all the way to the ground.

Strong, very sharp gust fronts produce a low, ragged shelf cloud. Rising fractus clouds are frequent, often wispy in case of rapid rising motion. Note the lowermost part of the shelf cloud in the photo below displaying distinct wispy structure.

– shelf clouds tend to be best developed over flat terrain, either flat plains or over sea or lake surfaces; any topography (

In some cases, multi-level shelf clouds develop, displaying a striking striated appearance. These are often associated with particularly strong thunderstorms and are often a sign of an approaching severe windstorm.