What are they?: Mammatus clouds spotted in Reading

Mammatus clouds were spotted over Reading this weekend, the large, fluffy clouds can look otherworldly but what's actually going on?

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

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.