March 22, 2009

Ferry Weather

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 ( 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 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 can see it clearly..with northerly winds on the Kingston run and southerly winds on the Bainbridge run.


  1. Cliff, thanks for posting this. I've been meaning to plug your blog over on Puget Sound Maritime, but this post gave me the extra "oomph" I needed to get it done!

  2. Cliff or others,

    Next to the incredibly useful view

    What would be the most useful SINGLE LINK to get an quick overview (e.g. a surface map like the following ones where none is quite perfect: showing too much of the continent where our weather already has been and not enough of the Pacific where it is coming)

    I have spent quite a bit of time searching for the one best link and never quite found it yet.


    (Sorry, I could not make the links clickable).

  3. I agree, the ferry weather website is very useful. I use it all the time before going out on the water. One weakness of the graphic is that it implies real-time conditions as a snapshot (i.e. static in time). I believe it's a collection of data points taken through time as the ferries traverse the Puget Sound. So the data points on the graph may have been collected over a period of hours, right?

  4. Cliff: On your earlier post titled "Weak Front" there was a radar image that I did not recognize. At least it was not one that I had seen on the NWS or UW site? Can you tell me where I can find those radar images.

  5. Some answers...the typical ferry takes roughly a half-hour to get the observations are relatively closely spaced in time.

    The radar image is from the weather loops page of my dept.

  6. Hey Cliff...thanks for posting that site! It`s yet another weather site that I add to my ongoing list of other weather sites, which half them, I hardly use!

    Here`s wind question for Cliff or anyone who can answer.

    On one side of the I-90 floating bridge, the wind will make the water kinda choppy, but on the other side it`s nearly calm. I have always wondered this! Is this because the wind across the bridge is always blowing in one direction?

  7. Isn't the temporal aspect a little more complicated than pooled data from within the last half hour? Especially when one would like to compare wind data between ferry transects. I noticed that the image is updated every three hours and suspect that it should be used to look at general trends over the last three hours.

    I'm not criticizing the value of the service- I think it's highly valuable and hope to see similar maps of other weather parameters in the future. Plus, I love to see cooperation like this between academia and government (or whoever runs WSF).

  8. Andy,
    I'm going to attempt to answer your question regarding why water is choppy on one side of the bridge and calm on the other. Choppiness (also called wave size) is created by wind and there are three variables that influence wave size:
    1. wind duration (larger duration = larger waves)
    2. wind speed (larger speed = larger waves)
    3. the distance that the wind travels which is also called fetch (larger fetch = larger waves)

    Fetch, or lack of it, is the reason why water waves are calm on one side of the bridge and choppy on the other. The floating bridge acts as a barrier to waves and to fetch. So, if a wind is southerly, you should see water waves getting larger as you travel north on Lake Washington. Once the waves meet the bridge, their energy is blocked from traveling further north by the physical barrier of the bridge. On the immediate north side of the bridge, the wind speed and duration are still imparting energy to the sea surface to create waves but the fetch is small and water waves are also small compared to the waves associated with the much larger fetch on the south side.

    Next time you drive across the bridge, notice that the wind direction will always correspond to which side of the bridge is more choppy. It's pretty convenient when you can derive which way the wind is blowing based on looking at a bridge webcam.

  9. Frequent reader...

    What you posted doesn`t look appropriate on Cliff`s blog. Looks kinda like 'spam' to me.

  10. How does the ferry anemometer give accurate readings when the ferry is in motion? I would think the speed of the ferry would give a false reading. I did notice an east west ferry does show wind coming from the north. ???

  11. This comment has been removed by the author.

  12. Cliff,

    I think I found a more informative link to my earlier hypothetical question
    "if you would be stranded on Blake Island with no tourists and just one graphic per day could arrive by Bottle-mail - which would be most useful":

    Close contender

  13. What happens in the middle of the convergence zone?

    When the north-blowing wind and the south-blowing wind collide in the middle, what do they do? Jet upward? Squirt out to the sides?

    That would be something interesting to blog about.

  14. I want to add one other site.
    Here is a link to the Puget Sound area.

    I use it all the time in the summer to check out winds for sailing on Lake Washington.


  15. ah Cliff, if I read that Ferry weather charts at the bottom of your post correctly, it's Southerly winds on the Kingston run and Northerly winds on the Bainbridge/Winslow run.

  16. Hey,

    The ferry weather is sweet, but another good one (shows pressure, cams, forecast,and history) is:
    click on "Washington" and Zoom in a little.

  17. Further to Andy's question and Gator's reply regarding the choppiness of the waves on the windward side of the floating bridges...

    A principle cause of the choppiness in these wave fields is the wave energy reflecting off the windward side of the bridges. this reflected energy travels back into the oncoming waves from (essentially) the opposite direction. The combining of these wave energies near the bridge creates a field of standing waves, double amplitude waves, and cancelled waves and everything in between. That's why it is so chaotic.

  18. Fred... you had mentioned about wind being one of the main factors for the choppy water on one side of the I-90 floating bridge and calm on the other side. Well since wind is a factor, I would think that the blowing wind would just travel up and over the bridge and continue to provide the some what choppy water on the opposing side, though to a lesser extent.

  19. Andy-
    In short, think energy. Virtually all of the wave energy is reflected back from the windward side of the bridge pontoons. No wave energy is able to move past the bridge. Also, "Fetch" starts anew from the leeward side of the bridge (ie. there is initially no fetch). The wind is also much reduced at the waters surface in the lee of the bridge pontoons (there's what, 8-12ft of freeboard blocking the wind?). That's why it's calm for the first 50 yards or so in the lee of the bridge.

  20. How does the ferry anemometer give accurate readings when the ferry is in motion? I would think the speed of the ferry would give a false reading. I did notice an east west ferry does show wind coming from the north. ???

    -- The ferry is moving in a known direction and speed, so that can be factored out of the measurements. Pilots do this all the time -- the difference between the heading and airspeed provided by the aircraft instruments, and the actual track over the ground (plotted the old-fashioned way or, these days, via GPS) provides a very good value for the wind speed and direction. I expect the ferries record GPS data on their position and speed, and transmit that along with the wind readings making the correction trivial.

    When the north-blowing wind and the south-blowing wind collide in the middle, what do they do? Jet upward? Squirt out to the sides?

    That would be something interesting to blog about.

    I think if you do a search you'll find he's already blogged extensively about the convergence zone and how it works. Or you could buy his book.

  21. The ferry weather link has been broken for some time?


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