Philippe Clement / Altocumulus undulatus clouds in blue sky, Brittany, France, September 2010

Gerry Ellis / Altocumulus thunder head dumping rain on pan, Amboseli National Park, Kenya

Anup Shah / Sunrise over the Masai Mara National Reserve, Kenya.

Franco Banfi / Ice floes, on the north of Spitsbergen, with mackerel sky, Svalbard, Norwegian archipelago, Norway, Arctic Ocean

Philippe Clement / Nimbostratus cloud covering cirrocumulus / altocumulus and blue sky

Kirkendall-Spring / Lamar River in snow, Lamar Valley of Yellowstone National Park, Wyoming, USA. February 2015.

Juergen Freund / Devils Marbles silhouetted at sunset, with mackerel sky, Devils Marbles Conservation Reserve, Northern Territory, Australia.

Georgette Douwma / Altocumulus undulatus cloud formations and Oak (Quercus) trees. Epping Forest, London, UK, January.

Georgette Douwma / Altocumulus undulatus cloud formations and Oak (Quercus) trees. Epping Forest, London, UK, January.

Georgette Douwma / Altocumulus undulatus cloud formations and Oak (Quercus) trees at dawn. Epping Forest, London, UK, January.

Vincent Grafhorst / Desert and altocumulus clouds, Namib Desert, Namibia

Peter Cairns / Sunset over Loch Insh, Cairngorms National Park, Scotland, October 2013.