Formation of a wedge-shaped cloud after the passage of mundane altocumulus undulatus. In this photo, it is very clear that this cloud has lower base than the clouds on the photo`s left hand side. The leading edge of this thunderstorm cloud was very layered while the bottom portion clearly appeared wind-torn with cumulus cloud formation due to clear evidence of wind shear. This is is very different from a roll cloud because the photographed cloud is clearly attached with the parent cloud while roll clouds are not. In the surface pressure chart, it was very clear that the formation of this cloud was initiated by the passage of a cold front over the Maltese Islands during the afternoon. The mechanism behind the formation of a shelf cloud is that cool, sinking air from a storm cloud`s downdraft spreads out across the land surface, with the leading edge called a gust front. This outflow cuts under warm air being drawn into the storm`s updraft. As the lower cooler air lifts the warm moist air, its water condenses, creating such cloud. This cloud formation is quite rare over the Maltese Islands. Wind had increased suddenly while the air cooled significantly from the MAX of 16C reached on this day. The first thumbnail shows a combined two photos of this cloud.