From a search on the Internet, a cloud photo very similar to this one was found in clouds-online website. The subspecies `arcus` means `With gust collar at the bottom side of the cloud`. However, no wind gusts (at least on ground level) were observed by me (the observer) while taking this shot. The photographed cumulonimbus could have also been `pannus` meaning `with frazzles at the bottom side of the cloud.` The weather situation at the time was that a frontal system had just passed over the Maltese Islands. The weather sounding provides a good measure on the column of the atmosphere at that time.