While lightning illuminated Dorset skies, it was another meteorological phenomenon which caught people’s attention.

As storms moved in, they brought with them an ominous looking cloud formation which saw people whipping out their smartphones and recording the strange celestial sight.

Being witnessed was what is known as a ‘shelf cloud’ – a type of ‘arcus cloud’ – but what are they and why do they happen?

Shelf cloud by Andy Lyons

READ MORE: Pictures of thunderstorm and shelf cloud in Dorset

READ MORE: Shelf cloud seen ahead of storm in Dorset

What is a shelf cloud?

According to the Met Office, arcus clouds are “spectacular low-level, long and thin clouds associated with powerful thunderstorms”.

There are two types of arcus clouds – shelf clouds and roll clouds.

Usually appearing at the leading edge of a storm, shelf clouds are not rain clouds themselves, but heavy showers generally follow, thanks to those following cumulonimbus clouds.

They are formed when the cold air coming down from cumulonimbus clouds spreads quickly along the ground and pushes warm, moist air up, which then condenses into these long cloud ‘shelves’.

Are shelf clouds dangerous?

As shelf clouds are usually attached to the base of a thunderstorm cumulonimbus (although they can be formed from any type of convective clouds), they are generally associated with strong, gusty winds, heavy rain or hail, along with thunder and lightning.

In certain conditions, a very low shelf cloud can indicate potentially violent and sudden but lengthy wind gusts, know as wind squalls, are approaching.