Few will have seen a sky like it. Yet this extraordinary-looking cloud formation wasn't photographed in exotic climes, but in St Albans, Hertfordshire, on a recent August evening. What is it and how did it form? MAMMATUS FORMATION The bobbly clouds that make this image so startling are called mammatus clouds - a name derived from the Latin word for breast. They hang under the main body of other clouds. "There is one prerequisite for a mammatus formation - a big thunderstorm - because they form on the back edge of retreating storms," says BBC broadcast meteorologist Peter Gibbs. In storms, air moves in rapid "up draughts" and "down draughts" and mammatus clouds are essentially pockets of air and water droplets which have descended in downward draughts, he says. While the whole cloud might measure 0.5 of a mile (0.8km) across, each mammatus cloud or "udder" is several metres wide. They are also referred to as "supplementary features" because they are arguably not clouds in their own right, says Gavin Pretor-Pinney, who runs the Cloud Appreciation Society. Mammatus formations can be seen on any type of cloud but look most dramatic on the cumulonimbus thunderclouds seen here, he says. They will usually last no more than five to 10 minutes. Return CUMULONIMBUS THUNDERCLOUDS Grey, brooding cumulonimbus clouds cause thunderstorms. They are menacingly dark because they are about four to five miles (6.4 - 8km) thick or tall, according to Mr Gibbs. They contain ice crystals which frequently produce hail stones and torrential rain. Return LOW SUN According to experts, the low lying sun at 7.30pm, when this picture was taken is what makes this formation so spectacular, because it highlights the edge of the clouds and brings out their formation. "The lighting is just right in this photo - but mammatus formations are not just a British phenomenon, they can be seen all over the world," adds Mr Gibbs. Return OTHER OBSERVATIONS Mammatus formations are not that uncommon, they occur after most thunderstorms, says Mr Gibbs.

But dramatic scenes like this are only captured about once a year, says Mr Pretor-Pinney.

The clouds are about 15 miles (24km) away in this photo, he estimates.

Mammatus clouds might be behind some UFO sightings.

"These visually arresting clouds draw attention to the beauty and drama of what is happening in the sky - and people spend too long looking at their feet," says Mr Pretor-Pinney.

The picture was taken by Ian Pattison, 31, on 12 August.



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