Arcus clouds are low, horizontally spread out cloud formations. Arcus clouds contain two different types of cloud - Roll clouds and Shelf clouds.

Roll clouds (pictured above and below) are long horizontal pipe shaped clouds which lie at a low altitude. Their appearance can be very dominating although they are somewhat rare.

Roll clouds indicate a microburst, or strong downdraft of wind, may be happening in the atmosphere. Contrary to popular myth, roll clouds are not capable of causing tornadoes.