Virga or Fallstreaks.   Virga week on OpticsPOD continues with this shoal of aerial jellyfish imaged by David Lynch of Color & Light in Nature (more images). Taken at Topanga, California on June 2, 2009. ©David Lynch, shown with permission.

Virga or fallstreaks (more 1,2) are streams of raindrops, snowflakes or halo forming ice crystals precipitating from clouds but evaporating before reaching the ground.

David Lynch describes these particular clouds "as 'glaciating altocumulus'. The supercooled altocumulus droplets freeze. Because the vapor pressure of  water vapor over ice is so much lower than over water, the ice crystals grow quickly to large sized (0.1 - 1 mm) and as a result fall much faster than the droplets which are much smaller (0.01 mm). Thus the ice appears to "fall" from the altocumulus, producing the feathery cirrus 'beard'."

Fallstreaks are characteristically hook shaped. Differences in wind speed with height (wind shear) can be responsible. Another factor is that as the water droplets or ice crystals evaporate and shrink they fall ever more slowly and finally drift horizontally with the wind.


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