Arcus clouds are low-level, wide ranging clouds typically associated with powerful storm clouds and thunderstorms.
Height of base: up to 6500 ft
Arcus comes from the latin word for 'arch'. There are two types of arcus clouds: shelf clouds or roll clouds. They can normally be seen beneath powerful storm clouds or cumulonimbus clouds. Shelf clouds are wedge shaped and attached to the storm cloud whereas roll clouds are a horizontal column separated from the storm cloud that can appear to be rolling. They form within warm air from near the ground which has been pushed up by the cold air exiting out of the storm cloud in the downdraft.
The images below have been sent in by Met Office Twitter followers.