Sunday, March 22, 2009
If you ever do any boating...or just interested in weather over the water...few web sites are more educational than the Ferry Weather web site (http://i90.atmos.washington.edu/ferry/Ferryjs/mainframe1.htm). Many of the Washington State ferries have weather sensors on them and send the information back to WSDOT and the University of Washington in real-time (see screenshot). At the UW we plot the winds and temperatures on this web site...and add nearby land observations to complete the picture. As you see by the second image...one thing is clear...winds are generally stronger over the water than over the land...often by 50-200%!! Why is that? Water is much smoother aerodynamically than the land..with its trees, buildings, and hills, and this rougher surface slows the winds.
I am sure you have noticed this effect when you drive across one of the local floating bridges (which have weather instrumements as well). As you can see from the image, when land is upwind of a ferry, the winds really drop. I have seen winds blowing at 30 mph in the central Sound...and only 5-8 mph in harbor. When air goes from land to water the wind speed does eventually speed up...takes a few miles to do so, as higher momentum air from aloft is mixed down. The ferry weather web site is heavily used by recreational boaters, the WA State Ferries themselves, and commercial shipping. It is also invaluable for meteorologists to learn about the details of our local winds.
Today is a nice example...a Puget Sound convergence zone has developed (see radar image wind SW-NE band across Puget Sound). Look at the Ferry Weather...you can see it clearly..with northerly winds on the Kingston run and southerly winds on the Bainbridge run.
Posted by Cliff Mass at 1:59 PM