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wind convergence - divergence

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IanP View Drop Down
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Post Options Post Options   Quote IanP Quote  Post ReplyReply Direct Link To This Post Posted: 14 Feb 06 at 8:01pm

I did say rumour, although it was much more fun to watch the plughole effects in NZ and see which way the spiral went I can't remember the result now as its too long ago

Anyway back to the real topic - there's another reference to convergence effects from winds parallell to the shore in the latest issue of Y&Y (First Beat) so are you saying you don't think there's such an effect or its not caused by any mechanism currently under discussion - in which case what's your theory (and lets give KnightMare a break )

Ian Paterson
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Mike B View Drop Down
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Post Options Post Options   Quote Mike B Quote  Post ReplyReply Direct Link To This Post Posted: 14 Feb 06 at 8:05pm

Actually IanP there has already been a lot of research published on this subject. At one extreme a scientist called P Francis published a study of a decade of offshore winds on the East Coast (if my memory serves he looked at the wind direction at Gt Yarmouth and about 20km downwind at a light vessel). At the other extreme there was a major case study in Denmark in 1984 of wind transitions from land to sea. This only looked at a few cases but the area was well instrumented with data buoys, masts, balloons etc. These and others studies confirmed that offshore winds tend to bend left and slow if the sea is relatively cold (=UK in August/September I'm afraid) and veer right and speed up if the sea is relatively warm. The Danish work suggests a gradually bend not a 'S' shape. Of course there is a lot of scatter. Sometimes the wind goes left and speeds up or goes right and slows but no-one ever said meteorology is an exact science.

I think the only gap is in data closer to the coast than a few kilometres. It might be possible to fill this gap with some experiments with party balloons or similar if anyone wanted fame in meteorological circles. Clearly something is going on as winds do vary across courses or from one side of a river to another.

Whether any of this could ultimately help win a race is of course another matter.

 

 

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IanP View Drop Down
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Post Options Post Options   Quote IanP Quote  Post ReplyReply Direct Link To This Post Posted: 16 Feb 06 at 8:06pm

Interesting stuff Mike,  but sure we are talking about the same thing here - just to check - were these studies looking at winds blowing along the shoreline, rather than crossing onshore to offshore ?

Obviously the convergence/divergence effect might help win races otherwise why would the effect be talked about when describing sailing venues, fresh and saltwater,  - can't believe the top types are just conning themselves (or are they conning us )?

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Post Options Post Options   Quote tack'ho Quote  Post ReplyReply Direct Link To This Post Posted: 17 Feb 06 at 8:30am

I think the only gap is in data closer to the coast than a few kilometres. It might be possible to fill this gap with some experiments with party balloons or similar if anyone wanted fame in meteorological circles. [/QUOTE]

that is exactly the area we are interested in!

I might be sailing it, but it's still sh**e!
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Post Options Post Options   Quote Mike B Quote  Post ReplyReply Direct Link To This Post Posted: 20 Feb 06 at 8:33pm

Good point IanP, the work I referred to was about wind crossing the coast. There is obviously a connection though as Knightmares' drawing shows. There is some work on along coast winds but less detailed. It does show that there are no hard and fast rules and, sorry Tack'ho, still applies to ten kilometre plus scales. There is a saving grace in that the relevent distance is the downwind distance so that a coastline 20km long say might produce, at the end, a convergence/divergence zone a kilometre or so wide, possibly.

I'm afraid this is getting confused. I guess all I am saying is that viewed from mainstream meteorology Knightmares' drawing is accurate but on much larger scales than you are interested in and even then is just as likely to work in reverse. Like I said, meteorology is not an exact science.

As regards the top types, I don't doubt for a minute that particular venues have particular characteristics and that good sailors find them quicker and use them better than bad sailors . However I'm a little sceptical about whether these guys have picked up any universal wisdom missed by professional met men. For one thing if you read what the top sailors (and coaches and advisors) write they routinely contradict each other. They can't all be right so it has to follow that someone can succeed at the highest levels without really understanding the weather. When I worked in the Met Office I put a lot of effort into studying sailing wind lore and testing it in case we could learn from it. Sadly I found very little that stood up to scrutiny.

Sorry to be negative but false expectations won't help anyone.

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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 21 Feb 06 at 4:11pm
In accordance to the wind weight question I occurred that your current parallel polarity in which accordance with the portside wind adjustment meter. SO in conclusion you must simply change the wind patterns and crank the motor to full for god sakes you have a yacht! Caviar and crackers.
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Post Options Post Options   Quote IanP Quote  Post ReplyReply Direct Link To This Post Posted: 21 Feb 06 at 8:12pm

Well I'm sure we all learned something from that JR.

Anyway Mike B it looks like there's an interesting piece of science to be done here - can't believe that some of the bright young things at S'hampton or similar haven't put their hands up yet to test the convergence theory on a sailing venue.

Perhaps it will have to wait for my retirement........

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Post Options Post Options   Quote KnightMare Quote  Post ReplyReply Direct Link To This Post Posted: 21 Feb 06 at 9:33pm
Wait until October and I might put my hand up and test it at a sailing venue.
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