Formation of altocumulus stratiformis opacus mammatus following a squall line that had just missed the Maltese Islands as indicated by the satellite image on the sixth thumbnail. The weather sounding on the third thumbnail indicated that the photographed cloud could have been up to 5.6km thick with a base at altitude of 2.4km. In fact, thunderstorms were observed preceding this rare beautiful mammatus cloud formation which was definitely caused by a sinking airmass. In fact, my opinion is that the cold front had actually overrun the clouds that it produced. With the mid-level clouds still present and the air following the cold front becoming more stable, it began to sink forming these beautiful clouds from the extensive sheet of mid-level clouds hence the species stratiformis and being thick enough to hide the sun hence of the variety opacus. The first thumbnail shows a feautureless extensive cloud cover with saharan dust approaching the Maltese Islands whilst the second thumbnail is the extensive high level clouds observed on the same evening due to persistent upper-level moisture. The first thumbnail clouds, probably mid-level cumulonimbus, was actually the tail-edge of a squall line approaching the Islands because of the cold front from a low pressure system that was over the Balearics as shown in the surface pressure chart on the fourth thumbnail. This corresponded perfectly with the cloud position as shown in the visible satellite image of the fifth thumbnail and the satellite image on the sixth thumbnail showing the thunderstorm line before approach and after it passed.