A gray-bluish featureless cloud as a relatively stable warm conveyor belt engulfed the Central Mediterranean. This was triggered by an upper level trough placed unusually far south over NorthWest Africa directing cool air over these areas and warm air over the Maltese Islands. The main difference from altostratus opacus is that such cloud allows the sun to shine dimly through (as proved on the first thumbnail) while opacus totally covers the sun. When such cloud does produce significant precipitation, then it is classified as nimbostratus. In fact, the weather sounding indicated a very moist upper level but with little instability and clouds produced from ice crystals. The cloud top temperature indicated that the upper cloud level was at an altitude of at least 10km which corresponds to that indicated by the weather sounding.