Virga!

Tonight the wind is blowing outside as another trough passes by...in fact, wind gusted above 50 mph at some exposed locations near water.   But in this blog, lets talk about something much more benign...

On New Year's Eve lots of folks were at the Space Needle to watch the fireworks display and it was precipitating....but not at the ground.  At ground level it was completely dry at midnight, but precipitation was falling from the clouds above and evaporating before it reached the ground--that's virga.

We had a few ways to tell that precipitation was falling...the Camano Island radar--whose lowest beam is about 2000 ft above the city--showed the precipitation  at midnight (see image):

The vertically pointing Seattle profiler also showed precipitation falling from above and evaporating.   But at the surface there was nothing until 1 AM.  Here is the observations at Seattle-Tacoma Airport:

L- at 0853 UTC (12:53 AM PST) was the first precipitation report--even though the radar had showed precipitation for a few hours.  Earlier in the evening there was a large difference between the temperature (lower 40s) and the dewpoint (mid 20s) at this location, indicated low relative humidities.  In fact, here is the plot at the UW...the air got drier and drier during the period before midnight (the RH is found in the fouth panel down).

During the day you can see virga...here is an example:

Virga is often noted around here during the beginning stages of precipitation events.  And having dry air at low levels helps insure precipitation evaporates before reaching the surface.   The deeper the dry air the better.  West of the Cascades a situation with easterly flow at low levels with moist, southwesterly air aloft is a good virga producer, and is one that often occurs as storms approach us from the west.

From below, the clouds lose their hard edges and there is a "softness" to the underside of the clouds during virga periods.   Sometimes you can fell a cool chill at the ground as the air, cooled by evaporation, reaches the ground.

Anyway, virga might be a .5 on the meteorological Richter scale, but still is worthy of note.