Clouds turn the sky into a big art gallery, complete with icy jellyfish and clouds that look like breaking waves. Two new books explore the varieties, and our gallery illustrates some of the most intriguing types

© The cloud collector's handbook by Gavin Pretor-Pinney

Fallstreak holes

These are crisp gaps in mid or high-level cloud layers, below which dangle trails of ice-crystals. To form a fallstreak hole, the cloud layer must consist of supercooled droplets - that is, water is in liquid form - even though temperatures at cloud level are well below 0° celsius. A fallstreak hole forms when one region of the cloud finally starts to freeze, starting a chain reaction.

All the moisture from the supercooled droplets in the area rushes to join the ice crystals, which quickly grow big enough to fall below. A form of "virga", the trail of ice crystals doesn't tend to reach the ground, but evaporates away before doing so.

© Extraordinary clouds by Richard Hamblyn

Mammatus clouds (from the Latin for "udders") are associated with unstable, often stormy weather - though they can also be seen in relatively calm condition, long after bad weather has passed. Their appearance is the result of pockets of cold, saturated air sinking rapidly from the top of a storm cloud, forming downward bulges or ripples at the base.

Their shapes can vary considerably, from long, undulated ripples covering many square kilometres, to smaller patches of near-spherical pouches. This rather menacing display of globular mammatus, contorting itself over a college sports stadium in Hastings, Nebraska in June 2004, is the latter type.

© Extraordinary clouds by Richard Hamblyn
Pink UFOs

A stack of altocumulus lenticularis clouds hovers over the Alpujarra Mountains in southern Spain, stained red by the rays of the setting sun.

Lenticular clouds (Latin for "lens-shaped") are a common sight in mountainous regions. They have often been mistaken for UFOs.

See Gallery for more extrordinary pcitures.