Altocumulus castellanus indicating that thunderstorms are on their way. In fact, thundery activity occured during that night and on the following day. These form when wind at the suface is relatively light hence warm, moist air bubbles can rise gradually upwards. As indicated in the 500mb chart, cool air flowing from the north clashed with the warm, humid conditions present around the Maltese Islands. A squall line developed on the east side of Malta the following day in the afternoon producing heavy showers with thunderstorms that fortunately moved away quickly. Due to low clouds, the proper cumulonimbus clouds making up the squall line could not be photographed.