Individual bundles of cotton-like clouds which appeared together with castellanus. Such clouds indicate instability due to mixture of warm air from below combined with an approaching cold airmass and these along with altocumulus castellanus clouds are harbingers of a thunderstorm yet to come. In fact, 2 hours later, during the night, a thunderstorm did occur effecting mainly the Gozo area. The first thumbnail of the weather sounding depicts the explained scenario whereby a cold front just below the cloud level is inducing the airmass above it to rise upwards forming these clouds and heralding the approach of a thunderstorm. The second thumbnail shows that the thunderstorms were still over Tunisia (encircled in red), at the time these harbinger clouds of bad weather appeared and the third thumbnail shows the heavy patch of rain over Gozo due to the thunderstorm.