Sunday, March 25, 2018

Virga Fest

Announcement:  The Northwest Weather Workshop is on April 27-28

The NW Weather Workshop is the main annual meeting for those interested in Northwest meteorology.  This year we will have a major session on the meteorology of NW wildfires and others on other aspects of our regional weather.  The gathering takes place at the NOAA facility in Seattle.  To view the agenda and to register, go to the meeting website.  The workshop is open to everyone, but registration is required.
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During the past few days I have received nearly a half-dozen emails with pictures and videos of beautiful tendrils of precipitation falling out of convective clouds, so this is a good time to show you some and talk about this stunning weather feature.

To warm you up, here is an amazing example, one visible from the Seattle Space Needle Panocam at 7 PM Saturday evening.   A lone cumulus tower developed over NE Seattle in the minutes before and if you watch a time lapse you could see the precipitation fall out of the cloud.   It didn't hurt that this was around sunset.

7 PM Saturday March 24

Since I am an academic, let's start with some definitions regarding the phenomenon in question.

Virga is defined as:
"an observable streak or shaft of precipitation falling from a cloud that evaporates or sublimates before reaching the ground."  

So technically, the precipitation (liquid or solid, rain or snow) should evaporate before reaching the surface if it is virga. If it does not evaporate before hitting the ground we use another term:  precipitation shaft.  

But basically, they are the same thing.  So in the pictures above, we have a mixture of the two at the same time.

Now it turns out we had a photographer just to the west of the same virga/precipitation shaft.   Her is a wonderful shot by Kathy Scott at 7 PM from her vantage point in Maple Leaf in North Seattle.  Clearly the precipitation shaft is to the east of her (you can see the Cascades in the background).


The radar image at the same time shows the cumulus cell's precipitation, but is not impressive:
NWS radar image at 6:59 PM 24 March
Virga is usually associated with evaporative cooling, as precipitation falls into air that is unsaturated (less than 100% relative humidity) below.   If you are below a cloud that starts to precipitate...and thus producing virga... you might well feel a blast of cooler air because the precipitation had evaporated before it reached the surface where you are.

And because cool air is denser and thus heavier than warmer air, the evaporating air tends to accelerate downward.  And when the downward moving air slams into the surface it splays out, producing enhanced horizontal winds.

Did that happen last night?  You bet.  Let's pick a location west of Maple Leaf (see map below) from the stations available from the wonderful weatherunderground.com site.


Here are the observations at the location (click on the image to expand).  If you look closely in the first panel you will see a sudden drop in temperature and a rise in dew point (a measure of the amount of moisture in the air) around 7 PM.

In the second panel there is wind speed....a notable acceleration was observed.  The fourth panel should an abrupt change in wind direction from the cool, downdraft air.  But there was no accumulation of precipitation (third panel)...it has evaporated.


Interestingly, I live close to the above station and I experienced some very, very light showers at that exact time.  I know that because I trying to repair my old wheelbarrow at that time. And a chill developed in the air.

Finally, if you really want to experience virga, one has to view an animation.   So here is a wonderful example provided by Greg Johnson of SkunkBay Weather for a convective line that moved through his north Kitsap location of Wednesday.  Enjoy.

Squall Moving Through... From The Air and Ground.... 3/25/18 from SkunkBayWeather on Vimeo.


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