Formation of cumulonimbus reaching a stable layer in the atmosphere hence forming an anvil as pictured. This formation was the direct result of an upper cold pool of air brought down by a cut-off low at 500mb over the Central Mediterranean. This also led to the formation of a shallow low surface pressure around the Maltese Islands. The first thumbnail also shows a cumulus congestus cloud forming over Valletta harbour due to the contrast of a relatively warm airmass underneath a cold pool of air in the upper levels.