June 06, 2012

Coastal Clouds

Yesterday I told people who wanted to view the transit of Venus along the coast to go as close to the water as possible...why?

There is often a major difference between the amount of clouds offshore or right on the coast and just a short distance inland, with clouds increasing inland--so being right on the coast really increased the chances of seeing the transit of Venus yesterday.

Here is an example of the effect.  First, a visible satellite picture from the GOES satellite at 3 PM  yesterday (about when the transit started!).  There was some clouds and convective showers offshore, but look what happens as the air crosses the coast...you can see lines of small cumulus developing, aligned in cloud "streets" parallel to the winds.  You can also view the convergence zone over Puget Sound and the enhanced clear zones to the north and south...many of you wisely headed there.

We can get a better view of the development of coastal clouds from another satellite: the polar-orbiting MODIS system.  Here are two pictures from it, with horizontal resolutions of 250 meters.  The first image is along and east of the central WA coat and the second southern Vancouver Is.  You can really see the effect in this images...relatively cloud free offshore and on the immediate coas,t with clouds developing inland.  You notice that some of the clouds even get bigger inland.

High-resolution weather forecasting models, like the UW WRF model run at 1.33 km grid spacing, can produce this observed cloud distribution.  Here is a sample from yesterday
for a forecast valid at 3 PM.  Not bad...and it also got the convergence zone clouds and the clear zones to the north and south.  

So why do clouds form just inland along the coast, particularly during the day (this phenomenon is far less prevalent at night).

This time of the year the sun is quite strong and is able to heat up the land substantially when a solid overcast is not overhead.  The water offshore is cool (roughly 50F).  When cool air moves over the warm land, we have a large change of temperature with height.   Such large vertical temperature changes can cause the air to convect, producing upward thermals and cumulus clouds.  This is quite similar to what happens in your saucepan when you turn on the burner...a large change in temperature with height causes your cereal to convect.

This convection takes a time to develop, so there it is a mile or so inland before clouds start forming in a significant way.

The development of coastal clouds is also helped along by what is called coastal frictional convergence.  This is the kind of terminology that will impress your friends and fascinate folks at cocktail parties.  The ocean surface is aerodynamically relatively smooth (little friction) and thus winds over water are strong.   Over land the surface is far rougher and that slows down the winds.

So if air is moving inland (eastward)....fast moving air from the ocean meets air that is moving more slowly over land.   Air get piled up and some of the air is forced to rise...this is the coastal convergence.   Rising air produces clouds.

Finally, in many parts of the Pacific  coast there is terrain just inland, terrain that causes air to rise...and yes....forming clouds.

So for a number of reasons, expect more clouds as one moves inland from the shore.  Another reason that hotels on the coast can charge more than their brethren  a few miles inland.


  1. Oh yeah, my friends would love it if I used that kind of terminology. I suspect at most cocktail parties mention of "frictional convergence" would lead to social difluence and/or conversational bombogensis.

  2. Thanks Cliff! You said likely clear in Olympia and you were absolutely spot on! The sight through a #10 welding filter was wonderful. For the really super view, check out this NASA composite just posted.

  3. Thank you Cliff! I went to work miserable on Tuesday that I was not going to see the transit. I saw your 11am report about coastal clearing and left work and headed to Aberdeen. Although there were some clouds there, my view of the sun was almost uninterrupted. Wind was a minor issue, but the sight of Venus' silhouette through a solar-filtered telescope was tremendous. The sunspots were astounding too. I'm convinced one group looked just like the Hawaiian Islands in the transposed view I had through my telescope.

    After a few hours I went to Ocean Shores just for kicks and set up on the beach where there was even clearer skies. Driving home I was hopeful I could show my kids the transit before sunset, but the clouds were thick and rainy by the time I made it back to the Eastside.

    Thanks again.

  4. We got to see a few minutes up in Bellingham. It was great.

    Have you seen this site http://hint.fm/wind/ A really neat graphic although not your level I thought you might find it interesting.


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