Formation of cirrostratus nebulosus and fibratus clouds taken before sunset over the Maltese Islands. The main difference between both species is that the former is fog like and which produced the halo whilst the latter produced the stratiations in the background. Cirrostratus dominated almost the whole day as evident by the fifth thumbnail photo taken early afternoon showing a panoramic view of the Upper Barracks. From the weather sounding on the sixth thumbnail, the cloud base was estimated at an altitude of at least 7.3km with an upper estimate of 12.5km. As shown in the visible satellite image and the 500mb chart on the seventh and eighth thumbnails respectively, it is virtually certain that these high clouds were the result of an upper-level trough crossing the heart of North Africa bringing with it the cloud edges. Furthermore, I am also virtually certain that the photographed clouds were present at between 7.4km and 8.4km above sea-level as the same sounding showed a katabatic cold front at that altitude which led to the further cooling of very moist upper-level airmass leading to the formation of ice fog at temperatures of lower than -25°C. No bad weather occurred afterwards and in the following day. The air was clear and unusually dry with just 38% humidity in the afternoon allowing the phenomena to be better viewed from the ground.

A faint solar halo and very bright sun dogs on both sides of the sun were observed and better illustrated on the first two thumbnail photos showing the left and right parhelia respectively. As explained by the figure on the third thumbnail, sundogs, scientifically known as “parhelia”, form as sunlight is refracted by hexagonal plate-like ice crystals with diameters larger than 30 micrometers and their flat faces horizontally oriented. Sundogs are only visible just before the sun sets or some time after sunrise. Hence, such a phenomenon is more likely in the Winter months when the sun is low in the sky for longer periods of time and with colder upper-level temperatures. To observe this phenomenon, the observer must be on the same horizontal plane of both the sun and the ice crystals. As sunlight passes through the ice crystals, the ray of the sun is bent by 22 degrees before reaching the observer. Hence, the photographed parhelia was the result of light bending by hexagonal ice crystals that were oriented with their flat faces horizontal. If these ice crystals were randomly oriented, only the faint solar halo would have been visible.

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