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Question for the meteorologists??

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Question for the meteorologists??

Postby Random » Sun Oct 09, 2005 12:54 pm

Over the last few months I've spotted a trend when I've been out kitesurfing - the wind weakens noticeably at the turn of the tide. Often on marginal days it dies out completely but even on some of the more hectic days it drops quite markedly.

Any answers why?

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Postby consumer » Sun Oct 09, 2005 5:46 pm

thats funny because at my local spot the wind consistently gets lighter when the tide is running a particular direction, and heavier in the opposite,

this is actual wind, not observed while riding, but with a meter.

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Postby FlyingHigh » Mon Oct 10, 2005 12:58 am

I heard that same thing. The other sailors on the sailboat I race on said it was a myth. I talked to two physicists who study the ocean, only one models winds however and the other deals with currents I think, but both said it was a myth.

I was told most people just feel the change in wind strength from the tide change, 3 knot tide = 6 knots difference in wind speed. Also if the tide changes at a particular time each day then it might be easy to think wind changes are due to the tide, not some daily thermal effect.

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Postby Nico » Mon Oct 10, 2005 3:56 am

The change of wind strength will definitely be afected by tidal levels.
The bigger the tidal amplitude the bigger the effect will be.
The logic behind it is so simple, the higher the tide the less room to move for the wind, the faster it has to travel (same volume).
Nico

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Postby FlyingHigh » Mon Oct 10, 2005 6:06 am

Where did you hear that?

The change in volume from the tide is negligible compared to the thickness of the atmosphere, so I doubt it would have a measureable effect. Also I've never heard of it as "the wind gets stronger as the tide rises and weakens as the tide drops" just that the wind changes when the tide changes.

Maybe a large volume of water moving can effect the wind just as the wind can move the water, but I don't think there is a large enough fetch compared to the time/distance winds are often stable over.

I emailed the NOAA and Woods hole about it, i'll let you know what they say if they reply.

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Postby Nico » Mon Oct 10, 2005 8:22 am

Flying high, this is an old fishermans knowledge here, and you can pretty much bank on it.
There are however other aspects that come and stuff this theory around a bit, like the currents that are also created by the tidal changes, these have abverse effects on wind increase, but only at certain areas.
This subject(chaos theory) is too complicated to throw around on the forum, but I would also be interested about the 'experts' point of view as well.
Nico

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Postby JS » Mon Oct 10, 2005 7:23 pm

The logic behind it is so simple, the higher the tide the less room to move for the wind, the faster it has to travel (same volume).


Sorry, but this logic is incorrect. Reducing the volume (really cross-sectional area) of any passage, whether it's a hose, river, tunnel, valley or whatever, will slow down a fluid passing through if a constant pressure is acting against it, which is the opposite of what you are suggesting. The exception to this would be a very specific local constriction which could create a local venturi effect, but this would not be created by tide.

Regardless, the effect of this is utterly insignificant and I could present a dozen other principles which are just as important but still insignificant in the context of tide.

But I can immediately think of two factors that may have a measurable impact. They are:

1. There is shear, or friction between wind and water, and just as wind drags the surface layer of water with it (and converts wind energy into waves), the tidal current will drag against the air, but only to a very small degree. Therefore, a change in the direction of current could change the wind strength and direction a little.

2. Sometimes a change in tide introduces water of a different temperature into the weather system. It may be cool ocean water, warm bay water, replacement of cold river outflow with warm, or any number of other local possibilities. Even a small temperature change of the water surface where you are, or even many miles away, could have a significant effect on the wind where you are.

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Postby Nico » Tue Oct 11, 2005 4:33 am

Hi JS.
Thanks for your opinion, but you fail to convince me that the tidal rise does not create a (mild) ventury effect on the wind.
Maybe someone could report about an area with big tidal amplitude, to confirm or not this theory.
I agree with you that shear and thermal changes will also affect the wind, but I cannot write off years of coincidental personal observations, and foremost traditional firshermens knowledge. The last fishermans statement is based on other experiences that are not scientifically explaineable, but yet very real.
Nico

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Postby JS » Tue Oct 11, 2005 7:58 am

No problem if you're not convinced Nico. I was only trying to help.

But even if you're sold on the fishermans' thin-atmosphere explanation, just do the math. Divide the tidal variation by the thickness of the atmosphere and figure out what infinitesmal fraction of a percentage point you could possibly be talking about.

Best regards, James

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Postby lemur » Tue Oct 11, 2005 10:33 am

Here in Atlantic Canada's Bay of Fundy we have the highest tides in the world...10+ meters at times



Scots Bay (Station #300)
7 days Tidal Prediction
Reference : Chart Datum


2005-10-16 (Sunday)
Time Height
ADT (m) (ft)
05:32 0.6 2.0
11:43 12.1 39.7
17:57 0.5 1.6

2005-10-17 (Monday)
Time Height
ADT (m) (ft)
00:08 12.2 40.0
06:22 0.4 1.3
12:31 12.3 40.4
18:46 0.2 0.7


If the tides did effect the winds I would suspect that we would notice significant changes. This is not the case in either wind direction or speed. My understanding is that primarily the winds are affected by changes in pressure and the warming of land masses by the sun.

Perhaps at your local spot the best times to surf are dictated by the tides and this just happens to coincide with the warming of adjacent land mass.

Rob


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