Gerry Ellis / Cloudy skies over crashing waves, Bathsheba Beach, windward coast of Barbados, Caribbean

Jim Brandenburg / Cloudy sky over tallgrass prairie, Wind Cave National Park, South Dakota

Niall Benvie / Mammatus cloud formation, Brechin, Scotland, February, 2012.

Niall Benvie / Mammatus cloud formation, Brechin, Scotland, February, 2012.

Niall Benvie / Mammatus cloud formation, Brechin, Scotland, February, 2012.

Niall Benvie / Mammatus cloud formation, Brechin, Scotland February, 2012.

Niall Benvie / Mammatus cloud formation, Brechin, Scotland February, 2012.

Niall Benvie / Mammatus cloud formation, Brechin, Scotland, February, 2012.

Alex Hyde / Alto-cumulus clouds. UK, December.

Alex Hyde / Alto-cumulus clouds. UK, December.

Mike Read / Rainbow over Scots Pine (Pinus sylvestris) and autumn coloured Bracken (Pteridium aquilinum) on heathland at Steephill Bottom, New Forest National Park, Hampshire, England, UK, with rain bearing cumul...

Tim Fitzharris / Avery Peak reflected in beaver pond, San Juan Mountains, Colorado