This series of Mammatus clouds where taken on December 27 2004 just as a severe summer thunderstorm had formed over my house in Perth, Western Australia.
From what I read, the Mammatus clouds are caused by ice in the clouds becoming heavy and causing the cloud to sag.
A very rare site!
EOS 20D And 17-40 f/4 L.
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Extreme Weather, Lightning and Storms, Put Out! Or Get Out!!!, South West Art And Photography.