Breast-Formation Clouds Appear in Hertfordshire,UK

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.