Widespread formation of altocumulus castellanus clouds identified by the cumuliform towers in the cloud formation giving it a castle-like appearance. This appearance is evident by a zoomed in cloud cover photo (first thumbnail) looking akin to small cumulus clouds because of their higher base at 3700 metres as indicated by the weather sounding on the second thumbnail. As typical for these species, the clouds are taller than they are wide. They are also seperated hence of the variety floccus because the photographed clouds are lacking a common base. On the centre-left of the photo, some clouds seem to have merged together changing the variety to perlucidus because of the remaining small gaps in the cloud cover. Such clouds have the capability to transition into cumulus congestus and cumulonimbus.

In fact, these produce the mid-level Spring thunderstorms over the Maltese Islands with some big drops of rain (but generally only with an accumulation of a 1 or 2mm). However, it is not always the case that such clouds precede a thunderstorm, in fact no storms occured on the day (though the rest of the week was unstable with altocumulus clouds being observed daily). The trigger that led to such widespread formation of the clouds in photo was a cold front that crossed the Maltese Islands as indicated by the surface pressure chart on the third thumbnail and proved by the freezing levels packed lines shown in the fourth thumbnail chart indicating the contrast between the warm air over the Maltese Islands and an advancing cold airmass, a weather situation which is the most ideal to observe turrents of altocumulus castellanus along with mid-level moisture at altitudes of just between 3.7km and 4.4km preventing them from turning into something bigger.