Formation of altocumulus after the passage of a cold front over the Maltese Islands. Some wind shear was present in the middle levels of the atmosphere, enough for some undulations to form within the cloud layer. The first photo of the thumbnail shows a cumulus cloud piercing through the mid-level cloud cover. Usually, when they combine it would be likely that isolated thunderstorms form due to instability (remember that Ac clouds form in unstable airmasses) and the fact that moisture would be already available. However, only a few cumulus had developed on the day due to weak forcing as the upper low (check weather charts thumbnail) remained just to the north of the Maltese Islands. Weather charts of the day are available in this web page as thumbnails.