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| © Eugene Brennan |
Some ominous mammatus-like clouds (often associated with thunderstorms) from October 2004. They were actually this colour. The billowing on the underside is caused by hail in suspension, being continually circulated by strong updraughts. Hail can move upwards and downwards, ice accumulating like the layers of the skin of an onion on each pass so that the stones can potentially grow to great size.