Formation of shallow or marginal cumulonimbus cloud identified with the species ‘calvus’ due to its very fuzzy appearance at its top perhaps rendered as such by falling graupel precipitation. The upper part of the photographed cumulonimbus looks indistinct and flattened having a whitish mass appearance without sharp outlines. A prominent feature is the arcus feature at the base of the cloud which was very hard to discern through human eyes (in fact contrast was needed) and this indicated downdrafts. In fact, the wind had picked up suddenly to 57kmh (Force 7) and temperature dropped from 9C to 6C. The arcus feauture was explained in detailhere. The whole cloud is therefore classified as Cumulonimbus capillatus arcus praecipitatio. The first thumbnail is another cloud photo taken during that late afternoon. This one looks more like a cumulus congestus cloud. However, it base has blobs making it a possible murus cloud which is defined as an abrupt lowering of the cloud base making it a type of a wall cloud. However, it is not clear whether the cloud elements were attached to the main cloud or just seperate features which joined later with other clouds.

According to the weather sounding and Luqa METAR reading as shown on the second and third thumbnails respectively, the clouds formed at heights of between 1km and 4.8km due to huge thermal contrast between the very cold air and warm surface temperatures within a highly undulating jet stream as shown on the fourth thumbnail leading to vertical instability with high ‘Thermal’ values. Indeed as depicted by the fifth thumbnail 850mb chart, parts of the Arctic Circle were actually warmer than the Maltese Islands.