Both the main picture and the first thumbnail show the formation of altocumulus castellanus clouds given away by their turrent-like castles protuding at the top. In the Maltese Islands, altocumulus castellanus clouds usually occur in Spring and to a lesser extent in Summer (the reason is the dominance of anticyclonic regimes in the upper levels). These clouds are observed during warm air advection when an upper-level trough is about to cross the Mediterranean or the extreme NW of Africa but when Malta is still situated in the warm side as evident in the 850mb temp on the third thumbnail. These mid-level clouds were relatively low-based forming at a height of between 3km and 5km as indicated by the lifting level of 3.3km on the weather sounding on the second thumbnail. The uplift was provided by a solid black line in the surface pressure chart on the fourth thumbnail clearly corresponding with thunderstorms shown in the satellite image of the fifth thumbnail and that the airmass over the Islands was unstable at mid-levels. Despite mid-level instability, no thunderstorms occured.