October 16, 2013

Why Do Low Clouds Burn Off on Some Days and Not Others?

There were some disappointed folks today (Wednesday) as low clouds (stratus and stratocumulus) held in over portions of western Washington through the afternoon, even though the official forecasts were for afternoon sun.  A visible satellite image at 5 PM illustrates the situation depressing situation for some Puget Sound residents. 

What controls where the low clouds develop and when they will burn off?  Why are the predictions often very poor for low cloud development and burn off?

Let me begin by saying that predicting low clouds is very hard and our computer models are very poor at dealing with them.  To show how bad it can get, here is the forecast from the UW high-resolution model of low clouds at 8 AM this morning.  Nearly no clouds over Puget Sound.   Embarrassing.  And this weaknesses in shared by most other weather prediction systems.

Forecasting low clouds plays to all my profession's weaknesses.  Our computer models often don't have enough resolution in the vertical to properly simulate shallow clouds.  Our boundary-layer schemes, the software used to simulate the lowest layers of the atmosphere, often fail when the atmosphere is stable (warm air above cold air is an example of a stable situation), producing anomalous mixing in the vertical that destroys low clouds.   We often don't have enough information from observations to properly describe the lower atmosphere, a description that forecast models need to make skillful forecasts.

The good side of this?  Plenty of work to keep folks like me and graduate students busy for years!  And fortunately, my experienced colleagues at the National Weather Service know about these model weaknesses and try to compensate for them...but there is only so much one can do subjectively.

Although predicting low clouds is hard, it is very important, particularly for aviation.  Low clouds can greatly alter who can fly, aircraft spacing, and which runways are used.  And low clouds, and particularly fog, can also impact marine traffic, and if bad enough highway safety.  Every year there are large, multi-car accidents in dense fog.

So what controls the development and dissipation of fog and low clouds?  I could write a book on this, but here are a few elements.

(1) Strength and period of sun.   Strong sun over long days works against low clouds, since solar heating warms the surface and tends to cause the lower atmosphere to mix, destroying fog and stratus.  That is why fog and low clouds are less frequent in July and increase by September and October.   Our day is roughly 16h in late June and 10h 45 minutes today--the day is approximately 5 hr shorter. And the intensity of the sun is far weaker during the fall compared to mid-summer. 

(2)  Depth of the cool, moist layer near the surface. The deeper the cool, cloud layer near the surface, the harder to burn off.   One reason why the clouds stuck around today was that the lower layer (around here often called the marine layer) was thickened by the passage of an upper level disturbance last night.

(3)  Strength of offshore easterly flow.   When we have strong easterly flow, the air sinking over the western slopes of the Cascades is warmed as it is compressed by the higher pressure at lower levels.   That warming tends to destroy low clouds, if the the descending air can approach the surface.

(4) Strength of the inversion capping the cool, marine air.  During the fall and winter, the cool, moist air near the surface is often capped by an inversion, in which temperature warms with height.  The stronger the inversion, the harder to get rid of the low clouds.

(5)  Low cloud/fog situations are often associated with upper level high pressure over the eastern Pacific.   If the high is a bit offshore, there is onshore flow at low levels, which promotes low clouds.  If the high is nearly over us, we typically get offshore flow, which works against low clouds.
The upper level ridge was fairly offshore today, which promotes onshore flow and low clouds.

Another element is location, as illustrated by the visible satellite photo at 2 PM today.  You notice that the low clouds are in an arc behind the Olympics?  Many low cloud
 situations are associated with high pressure offshore and northwesterly flow approaching our coast.   There is often a wake region of dead air behind the big obstacle--the Olympics.  An analog would be the weaker flow behind a big rock in a stream.

 Low clouds are protected in that dead air, and clouds can be enhanced on the lower windward slopes of the Cascades.

(6)  Low clouds, if thick enough, can maintain themselves and be resistant to burn-out, particularly during the weaker-sun portion of the year.  Clouds reflect a lot of the sun's radiation falling on them...radiation that then is not available to warm the surface and thus burn off the clouds.   I am afraid we are in a bit of this situation now.

You see how complex forecasting low clouds is?  And I haven't told you the half of it.  So you can understand how sometimes we get it wrong...like today.

Dawg Dash Forecast

A beloved tradition at the University of Washington is the Dawg Dash fun run/walk that will take place this Sunday, October 20th (information here). This event helps provide scholarship funding, which is acutely needed.  The weather looks fine..completely dry, with of course a good chance of morning low clouds. 


  1. Fair enough. But what if you just predicted low clouds every day? Wouldn't you actually end up being more accurate?

  2. I was bummed out a bit that you guys got it wrong today as I took the family out to the valley to the pumpkin patch... but this time of year, I always take a "fog burning off by noon" forecast as a 50-50 thing.
    Please write the book on he development and dissipation of fog and low clouds. I have my finger poised over the invisible button on Amazon for that. :-)


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