Glaciating Virga Over Newcastle, England
November 18, 2006
The above photo showing a fine example of glaciating virga beneath these altocumulus clouds (the light gray veils) was taken over Newcastle, United Kingdom on July 21, 2006 (at 21.00 UT or 9:00 p.m. local time). Virga is precipitation that evaporates before reaching the ground surface -- rain or snow that falls into a much drier environment. It seems that in this case, it's not rain drops but rather tiny ice crystals falling from the clouds. Glaciating altocumulus clouds are composed of supercooled water droplets, and when they freeze, the resulting ice crystals grow very rapidly because the vapor pressure of water vapor over ice is small compared to the vapor pressure of water vapor over water. This allows the crystals to become quite large, and they fall rapidly compared to the tiny water droplets in the cloud. See also the Earth Science Picture of the Day for November 9, 2005.