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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: 08 Feb 06 at 5:10pm

 Good work Knightmare, I'm now convinced (although the green in your diagram hurts my eyes  ), so award yourself    status. Presumably the effect on the lake wind is  local and shallow, relative to the prevailing wind, so the shore winds on either side of the lake will tend not to be affected allowing the pressure differences to occur?

Anyway, thanks, this problem has kept me entertained over the past few days, but its back to the grindstone tomorrow

Ian Paterson
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KnightMare View Drop Down
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Post Options Post Options   Quote KnightMare Quote  Post ReplyReply Direct Link To This Post Posted: 08 Feb 06 at 5:18pm

lol soz about the green it was either that or somethign which was too dark for the arrows to show properly.

woohoo  status ... should i be happy or not?

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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: 08 Feb 06 at 5:26pm

... I think that depends on how you feel about s

Alternatively you could award yourself a ....

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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: 12 Feb 06 at 4:00pm

Hi

I'm intrigued by the varied discussion on convergence/divergence. I'm a meteorologist by profession and spent some time looking at such matters a decade or so ago. Knightmares's diagram could have come out of some text books except for two big problems: -

Big Problem 1 Temperature. The drawing assumes the wind experiences more drag over land than over water. All things equal that might be the case but temperature differences dominate the atmosphere. Given cooler water than land the opposite will usually be the case and your diagram will then work in reverse. In fact round chilly UK waters the common rule that offshore winds always veer right away from the coast is pretty likely to be wrong.

Big Problem 2 Scale. How big is the lake in your drawing? The effects w'ere talking about take at least kilometres and ideally tens of kilometres to work. You're unlikely to see these effects on Grafham Water, you might in the Caspian.

Sorry for such an academic tome but I would be interested to hear if anyone thinks they can consistently predict and use coastal convergence (or coastal jets as the 'trade' now likes to call them). 

 

 

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KnightMare View Drop Down
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Post Options Post Options   Quote KnightMare Quote  Post ReplyReply Direct Link To This Post Posted: 12 Feb 06 at 4:51pm
Oh fun fun fun. Give me a day and il come up with some kind of answer to that.
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Jon Emmett View Drop Down
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Post Options Post Options   Quote Jon Emmett Quote  Post ReplyReply Direct Link To This Post Posted: 12 Feb 06 at 8:47pm
Unfortunately there is very little in sailing you can consistently predict!!!
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Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Posted: 12 Feb 06 at 10:37pm
I'm sure Mike B is right - but there has to be some explanation of the local effects we expereince every weekend.  I'm continually looking for some edge over the opposition and I commonly come upon variations over a distance of a few 100m which I cannot explain.  There's very little research made at the resolution we race in except for our own observations.  Invariably there is an advantage on one side of the river compared with the other - it could be a million different things but what Knightmare mentioned could be a factor.  So I think we should still voice our theories.
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Post Options Post Options   Quote tack'ho Quote  Post ReplyReply Direct Link To This Post Posted: 13 Feb 06 at 9:55am

So are we saying that where we have a cold sea, say early in the seaon, we will find that the wind doesn't change speed and therfore direction.  But doesn't that assume that the temp difference is able to take immeadiate affect rather than the air coming off the land being cooled as it passed over the sea.  So in effect we would almost see a 'S'  as the wind turns right as it accelerates as as the drag decreases as it crosses the coast then swinging left again as it is cooled by the sea temp???

 

The second question about scale.  Its one of those weird things which doesn't quite obey accepted physics.  It shouldn't work at small lake and river size butI 'm sure i've seen it (or something) happen  even on lakes the size of grafham and over narrow rivers.  Now this can't be anything to do with the coriolis force at this scale but somethings going on!!  

I might be sailing it, but it's still sh**e!
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Post Options Post Options   Quote IanP Quote  Post ReplyReply Direct Link To This Post Posted: 13 Feb 06 at 8:02pm

Rumour has it that Coriolus forces can work at the micro scale (see your washbasin plughole!).

However not that I doubt the wisdom of Jim S's venue guides etc, or KnightMare's good work above, but I think what's needed now is some observational evidence involving data collection with thermometers, anemometers, wind vanes, compasses etc,etc and probably computer models too. Sounds like a projects for an eager student - any takers?



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tack'ho View Drop Down
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Post Options Post Options   Quote tack'ho Quote  Post ReplyReply Direct Link To This Post Posted: 14 Feb 06 at 9:25am
Not sure about coriolis forces working at that scale.  They are generated by the differences in velocity of the rotation of the earth over latitude.  Not convinced that the difference in velocity over the 5cm of the plughole would be anywhere near enough in comparison to all the other forces acting on the water. Want proof, next time you have a bath use your finger to generate a whirlpool in any direction you like when you take the plug out then stop it and generate one in the other direction and see what happens.
I might be sailing it, but it's still sh**e!
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