From Snowshaft to Rainshaft

Virga, the word for precipitation extending down from a cloud and not reaching the surface, is not due to evaporation of the cloud droplets as they fall. Rather, it is an optical backscatter problem.

A. B. Fraser of Penn State Explains. “Consider a snowflake falling into warmer air (>0 C). It melts and there is a transition in the precipitation shaft. It switches from a snow shaft to a rain shaft. Optical cross section can be reduced by a factor of about 2. And because drops fall faster than crystals the optical cross section is reduced even more as they speed up on passing from snow shaft to rain shaft. Put the two together and the reduction in optical density is 10X. Dendrite or star flakes have higher optical densities than drops made from the said crystal fall very slowly and so dendritic snow shafts are very dark and when they fall and melt the optical density declines a whole lot, like 20X. You can see right through the rain shaft below the snow haft. If there were a grauple shaft extending down from a cloud and the grauple melted there would be almost no observed virga because the drop size and fall rate of grauple and the resulting rain drop are nearly the same.”

The take-home (thinking about it all message is that virga is real, however the traditional explanation of evaporation at the bottom of the shaft is not. Think radar. Radio-wave out, measure the optical back-scatter of the return. Weather radar “sees the hydrometeors” (size and fall rate).

Fraser is a professor emeritus at Penn State. He and his website are major assets.

Malene Thyssen,, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license


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