Monday, January 2, 2012

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.

10 comments:

LatigoLiz said...

First heard that term from my brother-in-law...he made this movie:
http://www.imdb.com/title/tt0979253/
and
http://dryrain-themovie.blogspot.com/

Elf said...

LOL. I love ya Cliff and your book. But if you're talking about virga on Jan. 2 instead in July, you know its a lame winter.

Record of 2011 now into 2012. The year of no summer AND now winter!!! I'm serious, you need to write about this in your sequal book. Absolutely unbelievable....

Unknown said...

Loving this wind! I live in the hills 3 miles NNE of Monroe in a somewhat sheltered place... only see wind a few times a year. It's been whooping up the last couple days, complete with power failures. Fun stuff!

Super said...

I was somewhat surprised to see a post in virga rather than the wind. If this wasn't the strongest wind of the season, it was close, at least for south Snohomish County. Lots of branches and debris down this morning. It sure felt more severe than the Christmas Day Storm.

nathaniel said...

In a somewhat unrelated note, I found these sky pictures on a fishing board and thought the readers here might like them:
http://www.piscatorialpursuits.com/forum/ubbthreads.php/ubb/showflat/Number/729592/gonew/1/2012_Hopin_things_are_lookin_u.html#UNREAD

izzit said...

There were "chinese lanterns" that lifted into the sky shortly before midnight...they seemed to stay lit high into the sky.

NOT a lame winter this week - you try finding yourself in 50+ winds after NOAA has TWICE failed to predict severity on Lake WA! What gives? I'd hoped that new weather ball wold reveal all.. ??

Mark said...

This is pretty wild Midwestern style clouds up at Arlington Airport yesterday!

http://www.pacificnorthwestflying.com/index.php?topic=8481.0

ILoveWinter! said...

50 mph winds and 60 degrees this morning in Eatonville. Really wierd going outside this morning without a jacket! It felt warmer outside than it did in the house!

Unknown said...

Look at the winds @ Camp Muir today! 1/4/12
http://www.nwac.us/weatherdata/campmuir/10day/

Justin said...

Ok Cliff, what's the deal today (Jan 4)? I'm watching the forecast and the new coastal radar, thinking the rain will break so I can go on a mtn bike ride after work.

The radar loops show little rain off the coast, but rain forming on land, and real rain dumping once the clouds hit the sound. This looks a different than normal. What's going on here, it keeps tricking me into thinking its about to dry up.