just why, exactly, is wind over tide rough?

I think the non-linerarity of the effect of wind, that you mentioned, is a lead here. Also I suspect that a knot of water movement may be "worth more" than a knot of air movement for some reason, perhaps becuase water is more massive and hence carries more kinetic energy. Don't know how it all ties in though. There may be a PhD in all this...
 
First it helps to think of the components of water movement separately. Current is the horizontal motion and waves are vertical motion. Wave height is a factor of wind strength, time and fetch. Assuming these factors remain unchanged in your three situations, the wave height will be the same - but the steepness of the wave will be greatly changed. If you consider a wave passing over stationary water (no current) - you can picture it as successive columns of water, which when they fall force the column of water next to them to rise up. If these columns of water are falling onto a surface that is moving along in the same direction as their fall, then the fall is drawn out and made more gentle. If they are falling onto a surface that is moving against, then they tend to trip over themselves, making the waves steep and turbulent. Think about moving walkways (like those in airports that travel about twice walking speed) - when you step onto one and accelerate from walking speed to running speed, the transition is smooth, but when you step off and return to walking speed, it's more abrupt. Either that or it's just PFM.

Kevin
 
There are somethings in life that you have to analyse and somethings you have to accept - and this is a case in point.

Don't analyse it, accept it - wind over tide is bad - wind with tide is better.

An old fisherman friend explained it to me in these terms - think of the surface of the water as a piece of carpet - if it's is moving in a direction and the wind is blowing in the same direction nothing much happens - if however the wind starts to blow in the opposite direction it's like pushing the carpet against itself and it rucks up, the more the wind blows the more and steeper rucks you get.

But always remember that once the wind gets above F6 whilst the same still applies it will still be "rough" and local conditions will apply IE headlands, shallow water etc will determine what the actual wave/sea conditions are.

I accepted his knowledge without question and never tried to analyse it - just lived with it!

Peter.
 
Thanks cruiser2b. I now have a descriptive mental picture that squares with all I knew before, and explains the problem.

Wow, that took some doing. Thanks to all who contributed. I feel more comforable knowing rather than just accepting
 
Fluid mechanics. The problem is conservation of energy. Both wind and wave energy act on the water. As the energy cannot be smoothly dissipated the result is 'hydraulic leap' or 'hydraulic jump'. Which results in a variety of hydraulic phenomna which we are familiar with /forums/images/graemlins/frown.gif

http://en.wikipedia.org/wiki/Hydraulic_jump

may help
Hydraulic_jump_in_sink.jpg



but you can probably set up this illustration of hydraulic leap yourself. /forums/images/graemlins/smile.gif

More examples here http://www.fluidmech.net/gallery/waves/hang.htm
 
You must log in or register to reply here.

Share this page

Top