Mid-level clouds (Cumulus at higher altitudes) passing over the Maltese Islands from Africa due to a surface low pressure over that area leading to its formation and being carried by southwesterly winds aloft. Some thunder and few drops of rain were associated with this cloud. This formation occurs when a pocket of cooler air slides across a deck of altocumulus. This creates instability causing local air bubbles to rise from within the mid-level cloud. The end result is this cloud photo. The first thumbnail shows another torrent of cloud very similar to cumulus congestus but with a high base (mid-level altitude).