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wind convergence - divergence

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Post Options Post Options   Quote IanP Quote  Post ReplyReply Direct Link To This Post Posted: 08 Feb 06 at 11:16am

Good try Knightmare, but not sure we've really nailed the mechanism because: 

The wind approaching the lake (lets stay away from the sea to keep it simple)  and the wind on the landward side of either shore must be travelling at the same speed. The only thing changing as far as I can see is the wind over water which accelerates as it moves onto the water due to the lower friction. So isn't the Coriolus effect working on the faster wind on the lake  and if so why does the left shore wind turn and cross the lake edge to give the convergence? 

Hang in there.....

Ian Paterson
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Post Options Post Options   Quote tack'ho Quote  Post ReplyReply Direct Link To This Post Posted: 08 Feb 06 at 12:12pm

Right, if you assume that the wind is flowing over the land at say 5 m/s and it crosses the bank at 90 degrees to the shoreline as it crosses the bank the friction decreases so it accelerates to say 7m/s.  As is accelerates in order to conserve angular momentum it is deflected right (in the northern hemisphere) by say 10 degrees.

Eventually this packet of air will meet the right hand bank (right hand from the airs perspective). Flowing on the landward side of the bank the air is still flowing in its orginal direction and speed. So there will appear to be more pressure (slight simplification but see all the other post vis a vi speed vs pressure) on this bank. The opposite is true as the deflected air will be flowing away from the left hand bank.

Whats worth remembering is this isn't an 'on off' affect but occurs over a distance.hence the apparent lift on the left bank as you look at it going up wind as the wind converges with the slower air over the land and adjusts it heading to suit.

 

I think

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Post Options Post Options   Quote tack'ho Quote  Post ReplyReply Direct Link To This Post Posted: 08 Feb 06 at 12:44pm

Obviously i meant a lift on port as you sail away from the bank. this lift will dissappear as you sail back into the air flowing over the lake. 

To my mind this means, and i may be out in a limb here, but as you sail into the left hand bank you will be knocked until you tack where you will be on a lift that dissapears as you sail away from the bank, and you will have better pressure.  On the right hand bank you'll be lifted as you sail into the bank but the pressure will decrease. when you have to eventually tack you will slowly be lifted as you sail back into the lake.

I stand ready to be corrected!

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Post Options Post Options   Quote tack'ho Quote  Post ReplyReply Direct Link To This Post Posted: 08 Feb 06 at 12:48pm

Post 3, i'm having a mare the wind is deflected left as it accelerates, all this trying to reverse left and right for beating has confused me!!  So reverse everything I have just said

Doh!!

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Post Options Post Options   Quote KnightMare Quote  Post ReplyReply Direct Link To This Post Posted: 08 Feb 06 at 1:00pm

Originally posted by IanP

The wind approaching the lake (lets stay away from the sea to keep it simple)  and the wind on the landward side of either shore must be travelling at the same speed. The only thing changing as far as I can see is the wind over water which accelerates as it moves onto the water due to the lower friction.

Not quite the wind over the water is actualy closer to the speed and direction of the high level winds so it is the wind flowing over the land that is changed more.

But basicaly the wind as it changes speed has to change its direction to keep the same amount of angular momentum. It is not a sudden change as with all things wind orientated the canges happen over a distance - there will be a gradual change in wind speed away/towards the bank and this will mean a gradual change in direction but the over all effeect is a convergence on one shore and a divergence on another.

 

I think it is quite confusing to think about the wind coming directly off of one bank as this does mean there isnt any convergence because all of the wind is shifted in the same direction. The effect is most noticable on a river with the wind flowing almost down the river so you have convergence one side and divergence the other.

The divergence is caused by the angle of the wind on the shore and the angle of the wind on the lake being different and heading towards one another.



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Post Options Post Options   Quote tack'ho Quote  Post ReplyReply Direct Link To This Post Posted: 08 Feb 06 at 2:18pm

Actually thinking about it i was right all along a wind that is slowed turns left in the northern hemisphere, which is a contradiction of what knightmare says I know but...errrr... it does.  Which agrees with Jim S assertion on convergence on the left (going upwind).

 

I'm only doing this to try and get my next star

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Post Options Post Options   Quote KnightMare Quote  Post ReplyReply Direct Link To This Post Posted: 08 Feb 06 at 2:58pm
Iv given up on trying to tell you which direction it turns to because it is the opposite direction depending on whether you are behind the wind or infront or it.  But my (very rough) diagram does have convergence ont he left as you go up wind
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Post Options Post Options   Quote KnightMare Quote  Post ReplyReply Direct Link To This Post Posted: 08 Feb 06 at 3:30pm
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Post Options Post Options   Quote tack'ho Quote  Post ReplyReply Direct Link To This Post Posted: 08 Feb 06 at 3:40pm
That looks exactly like the scrawl that is covering my desk where real work should be.  It could be we are all agreeing for the first time in forum history
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Post Options Post Options   Quote KnightMare Quote  Post ReplyReply Direct Link To This Post Posted: 08 Feb 06 at 3:51pm

lol yeh my dest is the same I have countless pieces of paper with said diagram drawn from many points of view. I keep on having to take notes into my boss with convergence and divergence written all over them

Now lets just see if IanP agrees or do we have to dig out the phys text book and give that explanation and it looks like a get the award for being the "public spirited ..." ?

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