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Old 03-26-2007, 07:28 PM   #1
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Hi

I'm thinking about this for a while, but can't figure it out.

Why wind blowing against the current produce extra rougth sea conditions?

I know the explanation, that velocity of the wind over the water is increased, but looking on pilot charts I can see, that for example Gulf Current speed is around 2 to 3kts in the fastest parts.

If the wind blows 3kts faster, it won't even make 1B more.

It is even more striking, when the wind blows hard. For example if it's 8B, what difference does it make if its 37 or 40kts?

My suspicion is, that current divides water into layers and the waves interfere with lower layers of water in similar way they interact with the shallow bottom. Of course it doesn't explain, why direction of wind does matter.

I got this idea when I read "We, the Navigators" by David Lewis, where he described, that native Pacific Islanders were able to detect the current on the open ocean while dead reckoning. He said, that they could tell, that there was a current, and its direction by the shape and movement of the waves.

Anyone has the answer to this mistery?
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Old 03-26-2007, 08:27 PM   #2
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I don't know if I can explain it, or even if my explanation is even adequate, but I'll try.

For example, a falling tide increases a river's flow out its mouth, and when winds are blowing against this strong current you can see the turbulence as the wind forces the waves to go in the opposite direction that the current is taking it. Somewhat like what rocks do to the smooth flowing water in white water rivers, forcing the water to pile up against the obstruction and make strong waves.

Though the Gulf Stream's current is 1.5, 2 knots, it's huge with thousands of miles of momentum behind the water. Wind blowing against the current is forming waves that are piled higher because the mass of water is running in the opposite direction, causing the water to "pile up" against the wind. 15 to 20 knots of wind generates a lot of force.

There are lots of places where knowledge of tides and wind direction are critical to the safe negotiation of a river mouth.

Somebody out there help me, I think I'm in over my head.
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Old 03-26-2007, 08:41 PM   #3
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Does this assist? It is the extreme though.

Copied from http://www.cruisingconnections.co.za/books2.htm

Extract from "SOUTHERN AFRICAN CRUISING NOTES"

by Tony Herrick


ABNORMAL WAVES:
It is a known fact that giant waves occur on the South African coast in the Agulhas current region, where southwesterly gales prevail against the southward flowing Agulhas current. Professor Mallory of Cape Town University analysed the recorded conditions that prevailed each time a number of ships were damaged by exceptional waves, and found that in all cases the dominant waves were always from the southwest.

The weather patterns play a major part in that the most dangerous period occurs when cells of low pressure are moving along the coast in a northeasterly direction. These lows are a regular feature of the eastern seaboard and it often happens that during their passage the wind can change from a near northeasterly gale to a southwesterly gale, sometimes in a matter of minutes. The southwest wind then reinforces the existing waves generated by a short choppy sea, which acts directly against the Agulhas current.

It is the interaction between the strong southwesterly wind and the strong south flowing current which at times can reach 6 knots that creates monstrous freak waves, of which the charts warn:
"Abnormal waves of up to 20 meters in height, preceded by deep troughs may be encountered in the area between the edge of the continental shelf and twenty miles to seaward thereof". The warning also describes the necessary evasive action to be taken under unfavourable conditions, namely, to stay clear of the areas seaward of the continental shelf. In other words, move inshore, inside the 200 meter line. This well established rule has given rise to the belief that the bottom topography plays a part in the generation of giant waves, but in fact this only plays an indirect role.

Please remember that the conditions along the southeast coast of South Africa are unique; the region can only be made safer through an understanding of the forces involved and by treating the seas with the respect they deserve, regardless of loss of time. "Do not have a deadline to meet at the other end". MANY South African sailors sail along this coastline all the time - simply prepare yourself with all the knowledge and information that is available for a safe passage. (Reference is made to a research paper - "Giant Wave - Anomalous Seas of the Agulhas Current" - by Ecxart H Schuman.)

Hope that helps answer your question.

.



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Old 03-26-2007, 10:17 PM   #4
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Thanks for anwsers.

JanneP, I have the same feeling about it, but since Newton we know, that it's nonsens.

Water on the surface doesn't know, that it's moving with the current. Sailors don't know it neither, that's why we cannot dead reckoning without information about currents (with exception of the Ancient Navigators of Pacific he he...). For the surface water it doesn't matter if the wind is 3kts stronger, or water itself is flowing 3kts against the wind. There is no other point of refference then air mass and the water.

Lighthouse, It's interesting information, but it only confirms, that when the wind is blowing against the current seas get rough. Waves are much bigger then it would be if there was no current and the wind blew 6kts faster. There is no mechanism.

I think, that it's really about vertical speed gradient inside the current. I've found the best explanation of water movement inside the wave here

http://www.coastal.udel.edu/faculty/rad/linearplot.html

Now if we imagine that on the bottom water is moving with the respect to the surface it could disturb this circulation, but I'm not sure in what way.

My other idea is about difference in salinity of the different layers in the current. Something of this nature:

http://www.nioz.nl/nioz_nl/281c18dbf72fc51...f49dddc1143.php

but again it wouldn't explain why the direction of wind matters.

Thanks, and I'm waiting for further discussion.
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Old 03-27-2007, 01:12 AM   #5
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I don't see what Newton has to do with it. It's not wind and water "flowing" in the same direction. Since it occurs at the mouths of rivers on an outgoing tide and onshore wind, it's not about water layers, either different speeds or density/salinity.

Try running water out of a hose and aiming the air stream from a strong fan at it - the wind is pushing against the water flow, and the water will overcome the wind, but will spray and splash and build up against the wind.

Offshore waves are wind-driven - the wind creates them, which is why you can tell the force of the wind by the state of the water/waves. The greater the wind, the higher the waves, and at some point the strength of the wind causes high enough waves to topple and break. And if the current/water is running in the opposite direction, the "speed" of the wind on the water is going to be greater - 15 knots of wind is going to be 17 knots of wind if the current is running at 2 knots against the wind. As with true and apparent wind on a sailboat, though except for downwind it's a vector value. 15 knots of wind, sailboat sailing downwind at 7 knots, apparent wind is 8 knots and the seas are going to feel pretty calm as you are running. Turn around and reach back the other way, into the wind at 6 knots (you're working hard) and the true wind of 15 knots is now going to be somewhere around 18 knots apparent and you're going to feel those waves quite a bit. The point is that the wind raises waves on the water, and when the water is flowing in the opposite direction the current-driven waves are pushed up by the wind-driven waves coming from the other direction.

Here's a description of a wave from http://www.school-for-champions.com/science/waves.htm

"Wave motion is defined as the movement of a distortion of a material or medium, where the individual parts or elements of the material only move back-and-forth, up-and-down, or in a cyclical pattern.

It appears as if something is actually moving along the material, but in reality it is just the distortion moving, where one part influences the next."

Here's an animation of a water wave: http://www.classzone.com/books/earth_scien...o=visualization

Yet water in the Gulf stream IS moving in the opposite direction from the wind engine.
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Old 03-27-2007, 01:58 AM   #6
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Not being a physicist, I can only speculate that friction between water and wind is increased when the movements oppose. Also energy cannot disappear, only alter in its effect. So increased energy of water against wind transforms lateral motive force into vertical force...and, for every action there is an equal and opposite reaction (Olivia Newton-John's 15th law in E#major) ergo, increases in the action of wind against wave, must cause a reaction of opposite intensity. As water cannot be compressed, it must go up and out.

Or at least, so it seems to me.

David.
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Old 03-27-2007, 04:23 AM   #7
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I don't quite understand the thing about waves on the river or the water hose. I'm talking about deep sea waves, not interferring with the bottom.

I understand, that for the surface water on the sea apparent wind will be vector difference of the true wind speed and its (the water) speed. Do you want to say JeanneP, that 6kts increase in the wind speed acting on the surface of the water in Aghulas current causes freak waves capable of sinking a tanker?

Sir Newton has something to say, because it was his idea, that there is no absolute point of reference, thus it doesn't matter if it is the wind blowing 6kts faster or the current going 6kts against the wind.

Also, I don't understand "current-driven waves". If there is no wind, there are no waves. Current alone cannot create waves I suppose. Maybe ripples on its borders, but that's because of friction between two masses of water.

I'm going to cross the gulf current from NY to Bermuda this autumn. I know the wornings: "if there is NE blowing just wait with departure, because the wind is going against the current and it creates rought seas".

May I say "hey, there is 25kts blowing, the current is 2-3kts, so the sea will be like 27-28kts of wind and no current. No big deal..." May I???

btw. this animation is misleading JanneP. The ball doesn't move right-left like it should. it should move in a circle like arrows show. It moves only up and down.

Thanks for answers
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Old 03-27-2007, 10:07 AM   #8
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You missed the critical factor with the ocean current. Swells. In the calmest conditions you still have a swell. This is caused by the current running over the ocean bottom, doesn't matter how deep the ocean is. A 2 knot current can easily make 8 foot swells, but without wind waves, you will judge the sea at only a foot or two. Few people can judge ocean wave and swell heights as they refer to how high the wave is, shape wise, versus how high the ocean rises and falls.

As the example given by Lighthouse, my experience is that the largest waves, worst sea state, is not caused by directly opposing wind and currents but by smaller vectored conditions. You can watch a wave and swell combine and make a much bigger wave, for a few moments, when the two patterns are about 45 degrees apart in vector relationship. These are the conditions I hate and some crusiers refer to as "lumpy" sea state. An even worse condition is when a front has just passed by, you have wind waves from the previous direction, new wave generation from the passing front, then the ocean swell...very bad news.
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Old 03-27-2007, 11:20 AM   #9
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I'm not sure why you think that the animation is incorrect. As is explained, the energy travels in a circular motion, but the water stays in essentially the same place, and the ball floats on top of the water, it is not part of the homogeneous water. My mentioning water from a hose and from outflowing rivers was meant to illustrate that regardless of friction or other forces, water moving in one direction and wind moving against it in the opposite direction creates waves that are higher than if there were no opposing wind. And, most likely as Auzzee mentioned, because water cannot be compressed the waves build up very quickly.

I would not recommend that you consider trying to cross the Gulf Stream in a norther. That you cannot understand the physics of what is happening within the Gulf Stream, and I cannot explain it, does not mean that it does not exist. It would be uncomfortable, and for some boats it has proven to be destructive and dangerous.

Here's an excerpt from "Florida Boating", http://www.vantagepointguides.com/how_to/f...ing_weather.htm

"Dangerous Northerly Winds in the Gulf Stream

It cannot be emphasized strongly enough. During strong northerly (NW/N/NE) winds, the Gulf Stream becomes unsafe for anything other than the largest naval ships."

Ignore at your peril.

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Old 03-27-2007, 07:00 PM   #10
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Interesting idea about current running over the ocean bottom but...

In general swell is known to be wind generated.

From: http://www.es.flinders.edu.au/~matto...lecture09.html

"One classification uses the forces which generate the waves. In ascending order of wave lengths we have:

- Meteorological forcing (wind, air pressure); sea and swell belong to this category.

- Earthquakes; they generate tsunamis, which are shallow water or long waves.

- Tides (astronomical forcing); they are always shallow water or long waves. "

and further:

"As an example, consider waves produced by a distant storm. In the open ocean such waves travel as deep water waves; their wave speed therefore depends on their wavelength λ. Thus long waves travel fastest and arrive at distant locations first. They are experienced as swell."

Because there are stable wind systems, like trade winds, we can experience sustained swell patterns.

Also, the Gulf Current is about 1000m deep, so it can interfere with the bottom only iside the continent shelf, close to Florida. North from Cape Hatteras current goes in the deep sea, and cannot interact with the bottom. Notice, that waves caused by the river current are visible only in very shallow, very fast rivers. 800m deep river going 3kts would have smooth surface if there was no wind.

It is known, that two wave systems crossing at some angle will create confused sea. Thats because of the interference of the waves. When two peaks crash, their peaks hights are added, and the valeys depth will be added too. So we dont have a rytm of the single system, but also there are those extra high peaks and extra deep troughs.

Now ocean swell is about 200m long, that gives us speed of 35kts ( speed[kts] =2.5*sqrt(length[m]) ). Again 3kts of current don't make a big difference. So the mistery remains, because, while wind blowing against swell will produce confused sea, the current doesn't create the swell itself.

JeanneP, I know, that norther winds creates big seas in the gulf stream. You agree, that much worse, then 3kts stronger wind alone. Thats why I don't accept the explanation, that it's getting rough, because apparent wind on the surface is stronger. I fully accept advice not to cross GS during norther.

thanks
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Old 03-27-2007, 09:06 PM   #11
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I'm sure the answer lies somewhere in the "String Theory", apparently everything else that physics can't explain is somehow answered by non-dimensional strings.

But seriously now, what is gravity?

Oh jeez...I'm thinking out loud again...sorry!
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Old 03-29-2007, 08:42 AM   #12
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String Theory KNOT for me. {Laughing at my Pun and myself, NOT the String Theory}

I really admire Sir Isaac Newton and his work

I am going to skip all the input and go back to the Piotreks' original question:

Why winds blowing against the current produce extra rough sea conditions?

It seems for the most part everybody agrees that it does, and has tried to explain WHY in different ways. In the explanation process many forces that affect the movement of the seas have been added, earthquakes, salinity, earth rotation, tides, swells, friction, vectors, etc. which do impact the movement of the ocean, but only complicate his simple question.

The original question only has two forces: moving water current and moving air current. They are in opposing directions.

The air current is invisible and you can not control it, so for demonstration purposes let's replace that force with something you can see, and control, say your hand.

Lets also substitute the sea current for a small river you are standing next to. Your feet on the river bank will provide a stationary point of reference.

1. HOLD THE WATER

Kneel down and hold your hand in the water current. Your hand provides an obstruction to the flow. The water "piles up" on the up stream side of your hand, and has a lower level or void on the down stream side. On a larger scale of the same demonstration, look at a large rock in the river.

2. WITH THE WATER FLOW

Now if you move your hand down stream at the same rate of flow as the water current, there is no impact. The high and low water areas on either side of your hand disappear or are equal.

3. PUSHING THE WATER - Down Stream

If you move your hand down stream twice as fast as the rate of flow of the water, the water becomes higher on the downstream side, and the void is on the upstream side of your hand.

4. PUSHING THE WATER - Up Stream

If you move your hand up stream (opposite the flow direction) at the same rate as the current flow, the water becomes even higher on the up stream side of your hand than in the first example of just holding your hand in the water.

If you have difficulty visualizing the effect your hand in the water has, replace it with something larger, an oar, a sheet of plywood, a barge.

Now letís assign values, so we can see the difference in results.

Flow Rate measured in (travel distance in a given time period) Meters per Second (mps).

Let us assume the river flow is 2 mps. That will remain the constant; always the same.

1. HOLD THE WATER

FORCE..........RATE........DIRECTION

River Flow....+ 2 mps....<<< Down Stream

Hand Flow...... 0 mps....>|< Stationary

Result.............+2 mps.....(Small Waves)

2. WITH THE WATER FLOW

FORCE........RATE........DIRECTION

River Flow...+ 2 mps...<<< Down Stream

Hand Flow... - 2 mps...<<< Down Stream

Result............0 mps...(No Waves)

EXPLANATION of values -2 mps +2 mps:

The river is moving at 2 mps. Your hand is moving at 2 mps.

Both are in relation to a stationary point.

In relationship to each other, they are moving at the same rate of speed, and in the same direction. They are moving together. The plus and minus cancel each other.

3. PUSHING THE WATER - Down Stream

FORCE..........RATE...............DIRECTION

River Flow....+ 2 mps............<<< Down Stream

Hand Flow.....(-2+2)...4 mps..<<< Down Stream

RESULT........+2 mps..............(Small Waves) (Same Net Result as #1)

EXPLANATION of values (-2+2) 4 mps:

The river is moving at 2 mps. Your hand is moving at twice that rate or 4 mps.

Both are in relation to a stationary point.

In relationship to each other, your hand exceeds the river flow by + 2 mps

4. PUSHING THE WATER - Up Stream

FORCE..........RATE......... DIRECTION

River Flow.....+ 2 mps..... <<< Down Stream

Hand Flow.....+ 2 mps..... >>> UP Stream

RESULT.........+ 4 mps..... (Big Waves)(Forces in Opposite Directions)

EXPLANATION of values +2 mps and +2 mps.

The river is moving at 2 mps. Your hand is moving at the same rate or 2 mps.

Both are in relation to a stationary point.

In relationship to each other, there is a combined effect of passing each other in opposite directions.

An Interesting Note is that your hand is moving 1/2 the rate in example #3, but the end result is twice as much.

Still focusing on the original question there are other factors that causes the larger waves when the wind is blowing against the sea current. The initial force of the wind will slow or stop the surface water, or hold the water back from where the current is going, but only briefly. Only to the point where the force of the current is greater than the wind. Than it comes rushing forward, pushed forward by water current, and pulled forward by gravity. Gravity keeps trying to pull water back to its' natural position. Once in motion (in wave form) it has momentum (Isaacs Law: An object in motion tends to stay in motion), and over shoots its' natural position. Gravity tries to pull it back again, meanwhile the wind is pushing it back and the current behind it is still coming forward. Those cycles keep repeating and the waves get progressively larger to some point, mostly dependant upon the wind speed and direction.

Did that help? I hope so.

It is a very good question, and most challenging to explain in understandable terms, especially in writing.
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Old 03-29-2007, 09:43 AM   #13
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@Aqua Man



Thanks for that!
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Old 03-29-2007, 03:59 PM   #14
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Aqua-Man,

You'll need to explain boundary layers in the next class.

Cheers,

Ken
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