Ac virga
/ 0 Comments / in Altocumulus, Medium LevelsFalling snow flurry from altocumulus sublimating before reaching the ground. Of course, it would certainly have melted to rain should its precipitation made it all the way to the surface as the temperature on the day was well above freezing (ie: around 18C). The first thumbnail refers to the day`s weather sounding on which I will base my explanation of this particular phenomena. Well, the weather sounding showed two cloud layers, one at low-levels and another at mid-levels between 500mb and 450mb (5.7km – 6.6km). The former cloud layer can be excluded as the photographed cloud is clearly at a relatively high altitude and lower clouds were observed in the area (not photographed). Hence the latter matches the photographed cloud perfectly. If one takes this scenario, the surrounding temperature where this cloud formed was around -16C with high relative humidity. Because of this cold and moist surrounding, the cloud would have certainly produced snow as precipitation, the dry powdery snow type in fact. Upon falling into lower altitudes, this isolated snow flurry would have immediately encountered a much drier airmass but still not warm enough to melt it down to rain as the weather sounding indicated a temperature of around -1C and with a relative humidity of just 5% at only 2km below the cloud base at an altitude of 3.7km. Besides, with such conditions, strong evaporative cooling would have kicked in causing certain sublimation rather tham melt and evaporation. As the snowflake produced was probably light, the prevailing Force 7 wind at that altitude has blowed the falling snow in such a way as to produce this very beautiful photo which is quite rare in Malta. This should not be mistaken with mammatus which in this photo, the snow virga seems to be very similar to mamma-like feautures. The second thumbnail shows a close-up of the same cloud base.
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