Whipped into fantastical shapes, these clouds hang over the darkening landscape like the harbingers of a mighty storm. But despite their stunning and frequent appearances, the formations have yet to be officially recognised with a name.
They have been seen all over Britain in different forms - from Snowdonia to the Scottish Highlands - and in other parts of the world such as New Zealand, but usually break up without producing a storm
And some experts believe the stormy weather phenomenon deserves its very own classification. Experts at the Royal Meteorological Society are now attempting to make it official by naming it 'Asperatus' after the Latin word for 'rough'. If they are successful, it would be the first variety of cloud formation to be given a new label in over half a century.
'It is a bit like looking at the surface of a choppy sea from below,' said Gavin Pretor-Pinney, founder of the Cloud Appreciation Society, who identified the cloud from photographs sent in by members. (Daily Mail)
