Base of cumulonimbus calvus clouds (the tower indicated a pretty smooth swelling though the sub-species type cannot be defintely concluded) due to a line of bad weather (indicated as a solid black line on the surface pressure chart) which was passing directly over the Malta representing an upper level trough line but still a front in the real sense of the word because it involved a clash between two air masses (the cooler air advancing over the warmer air). The first thumbnail photo (courtesy of Samuel Cutajar) shows a resemblance of what looked like a wall of sand as the storm was progressing. The weather sounding indicated lots of instability at mid-levels and also lots of wind shear. This storm was surface-based due to strong convective inhibition but was preceded by a particular altostratus cloud as photographed and explained here.