just why, exactly, is wind over tide rough?

shmoo

shmoo

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Consider three situations:
1) water is flowing at 3kt and the wind is blowing the same way as 10kt. Surface of water experiences 7kt apparent wind. Not rough

2) water is still flowing at same speed but wind has got upto 16kt. Surface of water experiences 13kt apparent wind. Not rough

3) tide turns. Water is flowing at 3kt the other way, withwind back down to 10kt. Surface of water experiences 13kt apparent wind. Rough

Why is 3 rougher than 2 when the surface (the only interface between water and wind) is experiencing same apperent wind?
 
I believe it has something to do not only with the "apparent" wind that the surface feels, but also the fact that the motion of the water compresses (or lengthens) the wavelength. Kind of in the way that a siren on an emergency vehicle travelling towards you sounds like its period is faster than after it has gone past you and is moving away.

Short answer - increase in apparent wind makes the waves bigger, motion of the water makes the wavelength shorter therefore waves are steeper therefore rougher.

Someone else will tell me I am full of sh*t, no doubt.
 
[ QUOTE ]
..its period is faster than after it has gone past you and is moving away.
..

[/ QUOTE ]

I always thought that was caused by the feeling of relief causing your heartbeat to slow down.
 
My understanding of this is pretty much the same as yours: if the direction of the waves is against the current then the wavelength becomes shorter so the wave face is steeper and therefore more dangerous and the crests are more likely to break.

However I think this must only be true in shallow waters! If the water is deep how do the waves 'know' that there's any current at all? This makes me think that in deep water the only effect that the current will have is just that due to the change in apparent wind speed. ...wadya fink?
 
Ok, I get the analogy with Doppler but I think it is wide of the mark. The change in pitch is only apparent. The pitch of the waves actually in the air remains constant. Someone observing from a privileged frame of reference (way off to one side, say) would see this.

The analogy with Doppler does work for a boat sailing into the waves (sees an apparent shorting of wavelength) or sailing away (sees an apparent lengthing of wavelength). The the wavelength really remains constant, however.

The difference between Dopper and wind over tide is that in the latter the waves actually do get rougher: its not a matter of frames of reference.
 
Sorry to "do a Gludy", but...

"The pitch of the waves actually in the air remains constant. Someone observing from a privileged frame of reference (way off to one side, say) would see this."

Are you sure about this?
 
Of course mystique is quite right. Pitch does change. Wash my mouth out...

However.... I still think Doppler is a red herring. As bbg put it
"how do the waves 'know' that there's any current at all?", when the only interface between water and wind is the surface?
 
I think you are right that the wind holds up the face of the waves. In deep water it has the same effect; crossing the Gulf Stream between Florida and the Bahamas is life threatening in a north wind over 20 knots, but quite do-able without the northerly component.
 
[ QUOTE ]
...crossing the Gulf Stream between Florida and the Bahamas is life threatening in a north wind over 20 knots...

[/ QUOTE ]Yes - been there - Miami to Bimini on a couple of occasions. The Nly wind was less than that but it was still pretty rough!
 
If you sailed in tidal waters you would know why! /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif
 
You're quite right, of course. You just need to know that wind over tide is a condition that can lead to rough water, not why this is so. You don't need to know how a carburettor works to drive a car. Or even how to spell carburettor to make this point intelligently. What is the sound of one hand cla....Oh, forget it.
 
[ QUOTE ]
Of course mystique is quite right. Pitch does change. Wash my mouth out...


[/ QUOTE ]

No it does not!

Imagine two people listening to a police car siren, one is in front of the police car one is behind. They hear different frequncies but the same siren is making the noise! Someone in a car following at the same speed hears a constant note!
 
Perhaps I'm talking nonsense, but here's the way I look at it.
What hair I have left grows forward, if I comb it back it sticks up, if I comb it forward it lies flat.
 
Yes, that was my position until I thought more deeply about it.

Now think about the stationary listener in front of the car with the siren. The only way they can hear a higher pitch is for higher pitch sounds to be reacher their ear. Its not an illusion. In turn, the only way the higher pitch sounds can reach their ear if for the sounds moving forwards from, and in direction as, car to be of higher pitch. Similar argument for listener behind.

I think that sounds radiating in direction at right angles to direction of movement (ie up, down and out to side) will be at nominal frequency of siren
 
I sailed on the Humber where fast tidal streams and shallow water are the rule.
I didn't need to know why it was rough, just that it would be.

Life's difficult enough without trying to work out if the doppler effect applies. /forums/images/graemlins/wink.gif
 
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