Mammatus clouds are strange, pouchlike protrusions that can form on the underside of a thunderstorm's anvil- shaped cloud-top. They are caused by cold air sinking downwards and are often a sign of strong storms.  This sinister-looking example was captured near Aberdyfi in the Autumn of 2004. Product number 2000-2009013. Email me to enquire about this image: click HERE.