Ci spissatus

Relatively thick cirrus spissatus clouds with clear dark shading at sunset approaching the Maltese Islands from Tunisia. These clouds formed at altitudes of between 9.5km and 12km with prevailing temperatures of less than -45°C hence being entirely composed of ice crystals. This composition seems to be confirmed by the first and second thumbnail photos. The first thumbnail shows cirrus with a clear anvil, a cumilform top and very fine drop stirs at the bottom of the cloud similar to falling powdery snow. The second thumbnail shows cirrus in the background terminating in a comma-shape hook similar to the uncinus species or possibly a very small virga indicating very cold temperatures at the cloud level.

Normally, such type of clouds indicate the arrival of bad weather. However, in this case, the photographed cirrus were the remnants of daytime convective isolated thunderstorms forming over Tunisia due to a small low pressure system as indicated on the SLP chart of the fourth thumbnail and confirmed through satellite imagery on the fifth thumbnail. The prevailing very moist air at the upper reaches of the  atmosphere permitted the clouds to survive the whole journey towards the Maltese Islands as they were driven by upper-level NW wind as shown by the weather sounding on the third thumbnail. Hence, since it is confirmed that the cirrus clouds had thunderstorm origins, they are being classifying as cirrus spissatus virga cumulonimbogenitus. Such cirrus had also produced a rare meteorological optical phenomena which is explained here.

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