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Old 12-09-2010, 11:29   #1
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Winds for Various Sailing Locations - A Brief Study

Hi All,

So last weekend I committed some rather poor seamanship and weather preparedness. (as an e.g. of what were many errors that no doubt belong in the sailors confessional: The exhaust pipe on the transom is no longer hooked up to a muffler or engine. I've been meaning to get around to plugging it, but it hadn't been a problem in several weekends of previous sails so I didn't bother. Last weekend, when there was some real wind (forecast said 12 kn, wind gauges said 16 with 20 gusts and a small craft advisory). Apparently when the boat heels over far enough the exhaust pipe goes below the waterline. I discovered this when the bilge pump went off and I saw a nice stream of water flowing into the bilge......)

As penance for my poor seamanship and failure to look at the weather, I decided to look into how often conditions are as squirrely as last Saturday.

I also thought it would be interesting to compare the sailing in the Chessie vs. Miami and SF Bay. So below are some nice histograms showing how often the wind is at certain speeds in different months over the past five years (2005-2009 inclusive). I reproduce nicely the fact that Chessie winds are more variable and that sailing is best done in the fall and spring. Furthermore, in general, the Chesapeake has higher winds than both Miami and SF, which surprised me. It also surprised me how high of a windspeed one should be prepared to deal with in the fall months on the Chesapeake (15-20 not uncommon), which has me rethinking my priorities in terms of sail selection.

Without further adieu, enjoy the data presented below. If you want a chart for another US based port, just let me know and I can probably put it up in pretty short order.

Chesapeake Bay: Entrance


Miami, FL



San Francisco Bay

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Old 12-09-2010, 11:37   #2
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I had a site I used that had all sorts of data for cities all over.

I can't find it anymore, in my favorites nor with searches.

I wonder if you are using that site for your data and if so would you share it.
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Old 12-09-2010, 11:42   #3
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The site that I used is the NOAA National Data Buoy Center:

NDBC - Historical Data Download

Weather underground also has lots of historical data, but it's hard to see the big picture, which is why I did my own analysis.

Example:
Weather Station History : Weather Underground
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Old 12-09-2010, 12:37   #4
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Originally Posted by ad_astra View Post
Furthermore, in general, the Chesapeake has higher winds than both Miami and SF, which surprised me.
Part of that may be that you've chosen to measure the wind 18nm offshore, which is not the same as the wind in SF Bay. Try getting measurements for Point Blunt, and you'll get the real story of SF Bay winds. (Understand that our summer wind is caused by heating/convection in the valley sucking the wind through the Golden Gate "slot." It's not uncommon to have huge winds in the slot and then get a few miles offshore only to be becalmed.)
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Old 12-09-2010, 14:51   #5
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Thanks ad a.
I sure wish I could find that old site though.
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Old 12-09-2010, 17:22   #6
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Not the same data, but this site has very useful wind statistics: Windfinder - Wind & weather statistic Eagle Point
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Old 12-09-2010, 18:04   #7
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Quote:
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Part of that may be that you've chosen to measure the wind 18nm offshore, which is not the same as the wind in SF Bay. Try getting measurements for Point Blunt, and you'll get the real story of SF Bay winds. (Understand that our summer wind is caused by heating/convection in the valley sucking the wind through the Golden Gate "slot." It's not uncommon to have huge winds in the slot and then get a few miles offshore only to be becalmed.)
Very true! Many times returning from Drakes Bay to the north or Half Moon Bay from the south, I'd be motoring with the sails up to maintain 5 knots, and put a least one reef in the main before I entered the Bay because I knew I'd need it. We'd never linger in the slot because there would be at least two races in progress.
Check the current winds in SF Bay at this site
Real-Time San Francisco Bay Wind Patterns
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Old 12-09-2010, 21:38   #8
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Another thing to consider: I suspect that those numbers are averages for 24 hours as well as being out to sea 18 miles. The winds in SFB are driven by the pressure differential between the sea and the the thermal low that is generated each day in the central valley (as mentioned above). That low pressure fills in at night, and the winds usually drop way off, thus skewing the 24 hr average well downward. A more useful number for sailors might be the daily maximum 10 minute average recorded, and I'm sure that velocity in SFB would be a bit more than in the Chessie!

Cheers,

Jim and Ann s/v Insatiable II lying Cairns, Qld, Oz

PS: It's been 24 years since we sailed away from SFB... maybe it has changed (ho,ho)!
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Old 13-09-2010, 15:52   #9
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Actually, what I plotted are in fact the 10 minute averages. However, they are over 24 hours and are offshore (so they include night and are diminished). It seems that the windy part of SF Bay doesn't have a NOAA bouy there, but it's pretty clearly marked in the forecast that John A links to. My motivation was not winds during the day, but winds at any time of day i.e. for passage planning purposes.
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