What are mammatus clouds?

Mammatus clouds were captured over Scotland this week as Storm Dudley travelled overhead. These strange, but harmless, clouds can look otherworldly but have a simple cause.

Derived from the Latin word mamma meaning udder or breast, these clouds form as warm saturated air sinks within a cumulonimbus thunder cloud.

The subsiding air eventually appears below the cloud bases as rounded pouch-like structures called mammatus. These clouds are usually seen after the worst of a thunderstorm has passed and the imposing formations can extend for hundreds of miles.

They can look particularly spectacular early and late in the day as the Sun's rays hit each pouch from a low position.