Cumulonimbus cloud with anvil (including possible mammatus) as seen from the Western side of Malta due to the movement of cold air in the upper atmosphere over a still warm sea. In fact, the 500mb chart had indicated a large cut-off low with a huge cold pool area also covering the Maltese Islands. Most of the weather activity had occured offshore. It is noted however, that the smaller a cold pool is, the better chances are of forming cumulonimbus clouds (of course it has to be over your area) as a large cold pool spreads its energy over a wide area hence no focus for thunderstorms.