Saturday, July 20, 2013

Stratus Secrets

Day after day we have experienced the same pattern of stratus developing overnight west of the Cascade crest, with clearing during the morning.   But this morning we had "superstratus":  thicker, slower to burn off, and accompanied by fog and drizzle in some locations (like north Seattle!).

The enhanced stratus this morning was associated with stronger onshore flow at low levels, with a deep marine layer capped by a strong inversion (see plot at Quillayute Washington, line on right is temperature, on left dew point;  when the lines are on top of each other the air is saturated). The vertical coordinate is height (in terms of pressure); 850 is roughly 5000 ft. Air is very dry above 925 hPa (about 3000 ft).

It is fascinating to watch the stratus burn off using high-resolution weather satellite imagery.  One thing we learned from the satellite pictures is how stratus and fog burns off:  from the edges.   Let's watch it happen today (Saturday). 

Let's start Friday afternoon at 4 PM, when the stratus of the previous day had burned off.  Plenty of low clouds offshore!
By 8 PM, the low clouds had begun to move inland (look carefully around Hoquiam and the western Strait of Juan de Fuca).

The images you are looking at are called "visible" imagery--it is what you would see from space.  The trouble is that visible light is not available during the night and infrared imagery (that is good 24-h the day by looking at the temperature of the emitting surface) is not very useful at night for low clouds (since they have a similar temperature to the surface).   No worries!  By combining a number of wavelengths, NOAA has developed what is known at "fog imagery" that can show the fog even when it is dark out.  Here is an example for 4:00 AM.  If you look carefully, you can see the low clouds has spread over the western lowlands.

Extensive low clouds are  confirmed by the visible imagery at 7:30 AM---coverage is pretty extensive.
 Now let's watch it burn off. By 9:30 AM, the low clouds were pulling back from the Cascade valleys.

 Pulling back more at 11:30 AM
 Shrinking further at 12:30 PM
And then a rapid pull back by 1:30 PM.  This is hours later than yesterday, the product of the deeper marine stratus/fog layer.

So stratus/fog burns off from the sides.  In addition, there is a tendency for the base of the fog and stratus to lift, as some solar radiation gets through to warm the surface.  Meteorologists have a fancy device called the laser-ceilometer that can measure the base of clouds by reflecting a laser beam off of them.  The UW got some surplus ceilometers from the National Weather Service and have one running in real time (and viewable on the web).  Here the latest can see the base of the low clouds rise and weaken in time.
Another perfect weather day in meteorological paradise.


cayuga said...

The technical tools available with the information coming from them are more than fascinating magic, and your explanations are quite good (they get my sleepy mind after I wake up into a sharper state as I work through the details from your blog). Looking out toward the Sound this morning from WS, with the subtle comings and goings of the grey fog bank (with a touch of a breeze on the tree leaves) is wondrous - combining your recounting of the science of the weather and the aesthetics of the wrapping of the fog is , to coin a phrase, just super.


Joel David said...

We recently moved to Bellingham and I have noticed that during the past month or so on many days when the marine layer is covering much of western Washington many areas of Whatcom county are still sunny. Any ideas why?? Could the Fraser River Valley have any effect on the marine layer even thought the flow in onshore?