Pascal Tordeux / Lone Maple (Acer) in a field against cumulo-nimbus and mammatus clouds. Picardy, France, May.

Jim Brandenburg / Prairie under threatening thunder clouds, South Dakota

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.

Luiz Claudio Marigo / Clouds of the type Mammatus above Rio de Janeiro city, these clouds usually indicate the imminent arrival of a cold front cold, Brazil, June 2011

Bryan and Cherry Alexander / Mammatus cloud formation at sunset over Gardemoen Airport, Oslo, Norway, June 2008.

Floris van Breugel / A clearing storm and mammatus clouds at sunset, over sand dunes, Death Valley National Park. California, USA. February 2009.