Roll cloud draws eyes to sky

The roll cloud, which is one of two types of arcus clouds, is identified by being low, horizontal and cylindrical in shape and isolated from other clouds. It is rare and will only form if very specific and uncommon conditions are met. When warm air is pushed above cool, the wind direction and speed is altered, creating the cloud, which appears to roll.
Thunderstorms often follow these clouds and this was definitely the case in South Durban, with reports of a heavy storm following the sighting. Bluff residents said the storm, which included hail stones and flash flooding in some areas, was intense for a few minutes, but abated quickly.
– erinh@dbn.caxton.co.za

See photos of the cloud here.