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wind convergence - divergence

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    Posted: 07 Feb 06 at 11:03am

As the wind weight question seems to have reached a conclusion perhaps you academics out there could help me understand another wind question-

Why is there convergence on the left of the beat with a shore (parallel to the wind direction) on the left and divergence on the right when the shore is on that side .

Spend a while looking for answers yesterday without much sucess (try googleing wind convergence when you have a spare hour or 2.... ) So here's hoping there is a public spirited   out there who can explain....

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Post Options Post Options   Quote Hector Quote  Post ReplyReply Direct Link To This Post Posted: 07 Feb 06 at 1:17pm

Don't know, but just to get things started, could it be something to do with the earth spinning so the land/water is spinning at 1040 miles/hr miles per hour (at the equator) but the air above it isn't (or at least not as fast)?

Hey, if that the case why isn't there always wind?



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Post Options Post Options   Quote KnightMare Quote  Post ReplyReply Direct Link To This Post Posted: 07 Feb 06 at 1:21pm

Ok in researching this a little bit i managed to confuse myself but i think i have got the directions correct. (but the bit about which bank it is may be wrong)

Hopefuly this is correct and helps

The wind flows at slightly differnet angles over open water compared to the land.
Simply this is to do with the amount of drag that the surface creates.
Also at higher altitudes the wind is at yet another angle. This is a similar direction to that of the isobars on water charts.

As the wind closer to the ground gets slowed down by friction it changes angle (consevation of angular momentum) and the direction is controlled by the turn of the earth (corriolis effect) in the northern hemisphere this slower wind turns to the right.

Since the wind flowing along a bank is slowed more then the wind flowing along the water the wind on the bank is turned to the right meaning that this wind colides with the wind flowing along the river causing convergence and a similar thing happens on the right hand bank where the wind is turned to the right which means it flows inland more causeing a divergence in wind.

Not only does convergence mean a strengthening in the wind but it also changes the direction slightly so that you will get headed as you come into a left hand bank but will be lifted away from it (in stronger winds)

Similarly divergence lessens the wind strength but can provide you with a lift as you aproach the bank.

 



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Post Options Post Options   Quote KnightMare Quote  Post ReplyReply Direct Link To This Post Posted: 07 Feb 06 at 2:18pm

ok so i thouroughly confused myself there and being the model employee i decided that pain could be the only thing to sort this out.

As you go up a beat the convergence is on the left hand side.

having just had a debate with my boss, he believes that there is another way of explaining it which is similar to the wave front theory in physics.

But both theories are linked to the wind slowing down as it hits the land (or speeding up as it hits the water - depending on which direction it is in)

You need to know if the convergence is on the right in the southern hemisphere to say which one is true

--- cue response from another member...lol

hope i didnt confuse you as much as i confused myself

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Post Options Post Options   Quote IanP Quote  Post ReplyReply Direct Link To This Post Posted: 07 Feb 06 at 2:24pm

Hum its not easy is it ? I tried the first explanation yesterday but gave it up after a bit as it didn't hang together for me for the following reasons:

 According to Jim S's venue notes(amongst others) convergence is on the left looking upwind. However, if the wind flowing along the left bank is slowed and turns in the direction determined by corriolis effect, then if the wind changes by 180 degrees, won't the deflection still be in the same direction (ie the right bank now has convergent flows) - which doesn't match the facts. Similarly not clear why the faster air on the right bank turns inland when it doesn't on the left (or the slower air on the right bank wants to turn anywhere at all!)

Sorry I'm still confused - how does wave front theory help  

PS you must have an understanding boss

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Post Options Post Options   Quote KnightMare Quote  Post ReplyReply Direct Link To This Post Posted: 07 Feb 06 at 2:55pm

lol my boss is also my RIB owner, my taxi driver and my sisters coach + boat rigger etc.  

no because if the wind changes by 180 degrees the left bank becomes the right bank and vice verser and the wind changes in the same direction compared to the wind not to where you are. ie the wind will always move to ITS left or ITS right

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Post Options Post Options   Quote IanP Quote  Post ReplyReply Direct Link To This Post Posted: 07 Feb 06 at 4:07pm

Hmm, I'm beginning to regret starting this   , but think you are on to something with the wind turning in the same relative direction (and like the diagram too).

According to a 505 web ref the opposite is true in the southern hemishere....

So the story so far, based on the above diagram -  something causes the wind on the left bank to turn right and flow towards the water (perhaps friction / difference in speed/pressure between it and the wind over the water, Coriolis effect) giving convergence, and something (the same effect) causes the faster wind on the right side of the course to deflect to the bank to join the slower wind over the right shore giving divergence. The something is the Coriolis effect which is acting on winds when they change speed  or not

Feeling a bit  and going for a lie down.......



Edited by IanP
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Post Options Post Options   Quote KnightMare Quote  Post ReplyReply Direct Link To This Post Posted: 07 Feb 06 at 4:45pm

LOL yeah do you realy want an explanation including the stuff about relative angular momentum because I could probably work it out (maybe) But simplisitcaly I think it is (off top of head) the slower the wind goes the more it has to turn to keep the same amount of energy. (not technicaly phrased correctly) In the norther hemisphere it turns one way and in the sounthern it turns the opposite (Coriolis effect).

And two winds going into oneanother = stronger winds; two winds going away from oneanother = weaker winds.

This is where some one else comes in and tells me im compleatly wrong.... lol

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Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Posted: 07 Feb 06 at 10:10pm
KnightMare I think your explanation is fantastic.  I do a lot of thinking about sailing and I hadn't worked that one out - but now I understand.  I've really learnt something, thank you.
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Post Options Post Options   Quote KnightMare Quote  Post ReplyReply Direct Link To This Post Posted: 07 Feb 06 at 10:44pm
Wow Cool I managed to do something correct  all that time reading up on it. It must have actualy sunk in.
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