Probably a once in a lifetime and dramatic form of altocumulus stratiformis opacus mammatus cloud cover taken at sunset. This formation was only observable for several seconds as the sun quickly hid behind further clouds at the horizon. Of course, the cloud type is quite common but the extreme rarity was its over-sized mammatus feature in the middle of the photo and stranger still was their alignment with the SW jet stream wind of the fifth thumbnail. After the sun hid again behind the clouds, this over-sized mammatus feauture became once more invisible to the naked eye hence could not exclude that such a large altocumulus supplementary feauture could go unnoticed. Scientists struggle to explain how mammatus formation occur. However, deeply analyzing the main photo, it seemed that subliming precipitation in the form of snow grains contributed heavily to the extremely over-sized mammatus formation. The resultant precipitation-cooled airmass falling inside a moist upper-level airmass might have been the rare formation of the photographed cloud that occurs in sinking cool air with the evaporation or sublimation of precipitation allowing the air to cool down further and further whilst descending to a much lower level overcoming the heating of compression resulting in rare negative buoyancy as the air descended though there were no thunderstorms in the area to maintain the anti-CAPE. The visible satellite image on the fourth thumbnail shows the cloud situation in the Central Mediterranean area about an hour beforehand which was benign and definitely did not give a clue of what layed ahead at sunset. Again, I emphasize that there were NO upper-level storm clouds but there was suspended desert dust possibly enhancing cloud formation.

The first thumbnail photo is the main photo zoomed in towards the church in the background showing that the cloud base consisted of normal mamma-feautures or virga with clear cloud breaks that allowed for the spectacular photo shot. Both images seemed to indicate that the mammatus clouds had actually formed in lines aligned with the wind potentially acquiring a radiatus feature. The second thumbnail shows a beautiful mosaic of altocumulus and/or cirrus spissatus clouds taken some hours earlier in the afternoon. The third thumbnail is the weather sounding at the end of the day showing that cloud formation was possible at high altitudes between 4500 metres and 11000 metres followed by a deep dry layer underneath it preventing any precipitation from reaching the surface. Notice the extremely strong SW jet and the presence of a potential upper-level cold front which the surface pressure chart did not indicate.

If someone has any feedback or further questions about this unusual and dramatic phenomena, kindly do so by commenting below.