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Discussion Starter #41
In my example a mean to say there is a NET 4 kt difference in apparent wind speed over the surface of the water. The Gulf stream is about 2kt. So, with my numbers a 20 kt wind out of the north would be 22 kt over the surface of the water. If the wind was out of the south, then this would be 18 kt over the surface of the water. However, the difference between 22 kt and 18 kt does not account for how nasty the water would be if out of the north. Thus my problem with being stuck thinking of this as a frame of reference scenario.
you hit the nail on the head!
 

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Discussion Starter #42
Relative speed has nothing to do with this. Wind creates a circular motion of the water particles. The opposing current affect the circular motion making the waves shorter and steeper.
Read more here https://gcaptain.com/high-wind-wave-events-crossing-gulf-stream-explained/
There is always circular motion of particles in waves, whether the current is in the same direction as the wind or opposite. So why does that make a difference?

I read the link you give and could not find an explanation there, either.
 

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Discussion Starter #43
Sometimes those free boat show seminars are pretty good. If you ever get a chance to attend one by Frank Bohren (Emeritus Professor of Marine Sciences at UConn), I highly recommend it. I've been to a couple of his presentations over the years at the Annapolis show, and he always has useful stuff to say. Here's his version of the chart you showed. It's from Oceanography and Seamanship by van Dorn. I'll get around to buying the book someday.

OK, maybe we are getting somewhere now! This looks like a good source. I have the book on order now. Thanks for the pointer!
 

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I see that I started a really interesting discussion. I am not going to double the length of the thread by replying to every contribution so far but let me address some of them.

As for your arguments here, Arcb, I agree with everything you say. Except that I don't think this addresses the main question that I have which is relativity. Why is ANYTHING (wave height, shape, subsurface friction, whatever) about a 20 knot wind against a 2 knot current different than a 22 knot wind in the absence of a current, or a 24 knot wind with the current? This could potentially make a difference in shallow water, as you say, but the Gulf Stream (for instance) is a kilometer deep https://en.wikipedia.org/wiki/Gulf_Stream I doubt very much that a wave, of a height of at a max a few tens of feet can 'feel' the bottom that is 4000' below.
The drive way/carpet wasn't intended to represent the bottom, it was intended to represent the opposing energy in the current, which is what shortens the wave length and results in steeper waves. I was trying to illustrate the problem in simple terms so it could be easily understood by people that may not necessarily have a strong background in meteorology/oceanagraphy/limnology.

Marks diagram explains the same thing as I explained but in mathematical terms. Really, aside from studying nautical science (which I did for 4 years prior to becoming a professional navigator) the best way to learn whats going on is just go out in a current in an opposing wind and observe. You can see the wave length shortening with your naked eye, and it has nothing to do with the bottom, as I mentioned, the bottom can cause standing waves, or it can cause a shortening of the wave length independent of the current, or the current can cause a shortening of the wave length independent of the bottom, or all these factors can combine. There need not be a bottom involved, however if there is the issue will be compounded by it.

Short of observation and attending classes there are good nautical texts on the subject. Some one mentioned Bowditch, which coincidentally is a standard text in any nautical science program. Its a good place to start with some reading. Internet forums are a challenging place at best to discuss complex hydrodynamic issues, I think the best that can be expected is suggestions on learning resources or you just get the Coles notes version ,which is what I was trying to give.
 

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I am not sure that width has much to do with it. ...

I do a lot of my sailing on a large local river. There are sections ... that get quite wide. The estuary in places is maybe 15 miles wide or more, but the current is still substantial ... When the wind blows contrary to the current the whole river gets rough, steep cappy waves pretty much bank to bank. The potential fetch in this section of river is about 180 miles, so it can get pretty rough.

Your example, @Arcb, helps clarify my thoughts. Yes, I get it that the steep waves are pretty much "bank to bank”, so my clumsy clause about the “relative width” does not capture the situation.

Here is a new formulation of the situation: whenever the wave train is generated in one body of water, and the current arises in a different body, and the two systems collide, the steep waves occur. Their separate behaviours are simple and easily understood; when they run into each other, different behaviour arises.

In your example the wave train got generated out in the ocean part where the river current does not exist, and the river current got generated on sloping land where the wind does not exist. Then they arrive at the same place and interact. The steep wave phenomenon happens there. It depends on creating two distinct momentum systems, and then having them meet and mingle.

But if they don’t *collide*, then the phenomenon does not arise. If a local wind creates a north-bound wave train atop a 300-mile-wide area of ocean already marching south, I will again claim that the steep wave action does not occur, because there is no *collision* going on; one momentum system is simply being created within another, and the waves will be perfectly normal-looking waves. They will have normal length, and (relative to the water) normal speed; relative to the *ground* they will move at the (vector) sum of the normal wave velocity and the underlying current velocity.

So the question has changed again, this time into: “Why is the meeting of a current and a separately-generated opposing wave train a problem?”
 

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Various people point out that our steepened-wave phenomenon should comply with the idea of relative motion, and I believe they are quite correct. But everything is a conceptual muddle until we figure out which two things are moving relative to each other. I have already abandoned the idea that they are
1) the wind
2) the current
because you could remove the wind suddenly and the phenomenon would persist.

Also, I have abandoned the notion that they are
1) the water
2) the ground
Also, I have abandoned the notion that they are
1) the waves
2) the ground
The thesis in my previous post is that the two germain concepts are
1) body of water where a wave train is developed
2) another body of water (without a wave train)
When these two things move toward each other, we get the steepened waves.

Normally, one would think of (1) as an ocean sitting still on the ground and (2) as a current moving over the ground. But that is only the common observation. If you consider the movement of the earth relative to space, you add in a vector of some 900 knots to both bodies — which make no difference at all. And in this vein you understand that movement with respect to the ground is also irrelevant. What matters is only that there are two bodies of water in motion relative to each other.
 

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Wind essentially causes a surface current on the water. The surface current is resisted by slower moving sub surface water. The friction of the subsurface water combined with the flow of the surface water creates a cyclical flow of energy. More wind on top results in faster surface current while sub surface current continues to create friction. The cyclicing energy causes waves to build in height.

A counter acting subsurface current results in greater subsurface friction counteracting the surface flow, resulting in steeper waves.
I think this is a description of some phenomena that occur in rivers, but I don’t believe it pertains to our open ocean problem.

Wind does cause a small surface current, which is resisted by the water below it. Yes.
But once you remove the wind, that small current is lost very quickly. I’d guess it would be gone in minutes. The waves, however, persist. Big time. (For days, and even weeks on the open Pacific.) And wherever those waves encounter a contrary current, they will steepen. So I don’t think it has anything to do with the wind. And therefore I don’t think that has anything to do with top-layer movements or subsurface friction.

It has to do with the cyclic momentum in the wave water suddenly being asked to travel at a newly-imposed rate.
 

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Wind creates a circular motion of the water particles. The opposing current affect the circular motion making the waves shorter and steeper.
Read more here https://gcaptain.com/high-wind-wave-events-crossing-gulf-stream-explained/
I think I finally grok your point, and I feel it is correct.

Also, thank you for the link. I quote from 2 paragraphs found therein:

Waves Moving Against the Current

When ocean … swell waves encounter a current moving in the opposite direction, the response will be for wave speed and length to decrease, wave period will not change, but wave heights will increase, resulting in taller, steeper waves.

When swell that originates elsewhere encounters a current, its wavelength and height change. When the current is flowing in the same direction as wave travel, wavelength increases while wave height decreases. Where currents oppose waves, wavelengths decrease, and wave heights increase.
Take note of these things:
1) The title has no mention of “wind”.
2) The second para makes specific mention of the fact that the swell originates elsewhere.
Both these details support ideas expressed elsewhere in this thread.

(full disclosure: the first ellipsis above is the place where I removed the word “wind” for clarity because the original sentence has a grammatical ambiguity. I believe he meant “ocean wind-waves, a.k.a. ocean swell-waves” and not “ocean wind, or ocean swell waves”.)
 

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Then there is barometric pressure and moon cycles to consider too plus other factors. A lot of forces and motions at play here in the big washing machine where if lets say forces accumulate to displace an inch of water for 1,000 miles there will also be forces in play when it levels back off again in a many times chaotic fashion. How many tons of force will there be behind even one inch of water that is displaced for a thousand miles or so by a storms passing, etc? What about water displaced by another storm front coming in or even bypassing that may be hundreds of miles away and the wake that creates as it goes by?

We view the bottom as stationary however that may not be 100% true as there is also movement there as testified to when the creeping plates of the sea floor ridge up and at times create a disruption.

So the earth is in motion in an elliptical orbit around the sun, the moon is pulling on the water at changing angels throughout the day, currents are in play, earths crust is moving, barometric pressure is changing and then there are changing winds too all where we want to sail our boats. Yep the water at times will not be smooth and there will be some waves.

Too many things in motion for any one cut and dry definitive answer.
 

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Discussion Starter #50
The drive way/carpet wasn't intended to represent the bottom, it was intended to represent the opposing energy in the current, which is what shortens the wave length and results in steeper waves. I was trying to illustrate the problem in simple terms so it could be easily understood by people that may not necessarily have a strong background in meteorology/oceanagraphy/limnology.

Marks diagram explains the same thing as I explained but in mathematical terms. Really, aside from studying nautical science (which I did for 4 years prior to becoming a professional navigator) the best way to learn whats going on is just go out in a current in an opposing wind and observe. You can see the wave length shortening with your naked eye, and it has nothing to do with the bottom, as I mentioned, the bottom can cause standing waves, or it can cause a shortening of the wave length independent of the current, or the current can cause a shortening of the wave length independent of the bottom, or all these factors can combine. There need not be a bottom involved, however if there is the issue will be compounded by it.

Short of observation and attending classes there are good nautical texts on the subject. Some one mentioned Bowditch, which coincidentally is a standard text in any nautical science program. Its a good place to start with some reading. Internet forums are a challenging place at best to discuss complex hydrodynamic issues, I think the best that can be expected is suggestions on learning resources or you just get the Coles notes version ,which is what I was trying to give.
I appreciate trying to explain it in simple terms but it does not make sense to me. As I said, I do not see how a water molecule can distinguish between being in a 20 knot wind added to a 2 knot current vs experiencing 22knot wind in still water. All these are _relative to the bottom_ which is is the frame of reference we always use but the point is that the frame of reference does not matter as long as you are consistent. We know this for a little over 100 years (Einstein etc).

That does not mean I doubt for one second that the phenomenon exists, the evidence based on the collective experience is overwhelming (your examples are very much to the point). So I must be missing something and I want to know what it is.

I am looking forward to reading the van Dorn book that TakeFive brought up, I should get it in my mailbox end of the week (was too cheap to pay for expedited shipping :)
 

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I appreciate trying to explain it in simple terms but it does not make sense to me. As I said, I do not see how a water molecule can distinguish between being in a 20 knot wind added to a 2 knot current vs experiencing 22knot wind in still water. All these are _relative to the bottom_ which is is the frame of reference we always use but the point is that the frame of reference does not matter as long as you are consistent. We know this for a little over 100 years (Einstein etc).

That does not mean I doubt for one second that the phenomenon exists, the evidence based on the collective experience is overwhelming (your examples are very much to the point). So I must be missing something and I want to know what it is.

I am looking forward to reading the van Dorn book that TakeFive brought up, I should get it in my mailbox end of the week (was too cheap to pay for expedited shipping :)
If the waves are formed outside the area where the current exists, they will have to change speed and wavelength when they enter an area with a current. If the current is contrary to the waves, the wavelength will shorten. If the current is in the same direction as the waves, the wavelength will increase.

It is sort of like motion causing the Doppler shift in sound or the red shift in light.

(The formulas that I earlier credited to Bowditch are also in Van Dorn 1974 on p 154 and the graph is on p 234. Willard Bascom, "Waves and Beaches" is also good.)

Bill
 

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There is always circular motion of particles in waves, whether the current is in the same direction as the wind or opposite. So why does that make a difference?

I read the link you give and could not find an explanation there, either.
This passage does not directly mention the particle movement patterns. My books on oceanography is In Norwegian so no use posting copies or quotes here.
Waves Moving Against the Current

When ocean wind or swell waves encounter a current moving in the opposite direction, the response will be for wave speed and length to decrease, wave period will not change, but wave heights will increase, resulting in taller, steeper waves. In some cases, this can even lead to waves breaking, resulting in greater energy against hulls. This type of event can occur in all seasons and may or may not be associated with the cold air outbreaks mentioned above. The region around the Agulhas Current is particularly prone to high waves resulting from this type of wind opposing wave-current interaction.
 

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Discussion Starter #53
If the waves are formed outside the area where the current exists, they will have to change speed and wavelength when they enter an area with a current. If the current is contrary to the waves, the wavelength will shorten. If the current is in the same direction as the waves, the wavelength will increase.

It is sort of like motion causing the Doppler shift in sound or the red shift in light.

(The formulas that I earlier credited to Bowditch are also in Van Dorn 1974 on p 154 and the graph is on p 234. Willard Bascom, "Waves and Beaches" is also good.)

Bill
OK, that does make sense. If there is an inhomogeneity somewhere in the system, with a wave train encountering a change in average speed, then I understand that some change in wave patterns can occur. In the area where a gradient in current speed exists, the wave shape can change (e.g. in the way you describe). However, as soon as the gradient is removed, ie the current is homogeneous again (the same everywhere), the relativity argument must be valid again: there is no difference in ANYTHING (wave shape, height, whatever) between a 20knot wind against 2 knot current and a 22knot wind over still water (=current zero).

This would mean that the observed phenomena occur only at locations where the current changes strength (or direction). Is that the case? Coming back to the Gulf Stream, then you should be fine if you are in the middle of it (where the current is the same everywhere around you) and the problems should only exist at its borders. Is that the case? I think it is not but am willing to be corrected.

(of course another possibility is that the current is everywhere non-homogeneous so there would be 'pockets' of current going in all directions everywhere, like in turbulent flow, but I believe that is not the case in the big ocean currents like the Gulf Stream)
 

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The trick is that wave trains travel at speeds from 5 knots to 50 knots (depending on their wavelength) and travel for distances that may include thousands of miles. The waves that you see in any one place were most probably generated quite some distance away.

The waves in the Gulf Stream are generally made somewhere else. When they enter the Gulf Stream they are altered, and when they leave they return to their previous form.

If they enter the Gulf Stream moving in the same direction as the Gulf Stream, their wavelength is increased and their height is reduced while they are in the area of the current. If they are moving in the opposite direction, their wavelength is reduced and their height is increased. After leaving the Gulf Stream (if they did not break) they return to their original length and height.

As Bowditch says, "A following current increases wavelengths and decreases wave heights. An opposing current has the opposite effect, decreasing the length and increasing the height. A strong opposing current opposing current may cause the the waves to break. The extent of the wave alteration is dependent upon the ratio of the still-water wave speed to the speed of the current."

Just like cars on the highway, if they slow down from a fast speed, they bunch up. When they speed back up they spread back out. I watch that happen at every wreck on the Interstate highway.

Bill
 

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Discussion Starter #55
The trick is that wave trains travel at speeds from 5 knots to 50 knots (depending on their wavelength) and travel for distances that may include thousands of miles. The waves that you see in any one place were most probably generated quite some distance away.

The waves in the Gulf Stream are generally made somewhere else. When they enter the Gulf Stream they are altered, and when they leave they return to their previous form.

If they enter the Gulf Stream moving in the same direction as the Gulf Stream, their wavelength is increased and their height is reduced while they are in the area of the current. If they are moving in the opposite direction, their wavelength is reduced and their height is increased. After leaving the Gulf Stream (if they did not break) they return to their original length and height.

As Bowditch says, "A following current increases wavelengths and decreases wave heights. An opposing current has the opposite effect, decreasing the length and increasing the height. A strong opposing current opposing current may cause the the waves to break. The extent of the wave alteration is dependent upon the ratio of the still-water wave speed to the speed of the current."

Just like cars on the highway, if they slow down from a fast speed, they bunch up. When they speed back up they spread back out. I watch that happen at every wreck on the Interstate highway.

Bill
Well, if these waves come from thousands of miles away, what does the local wind have to do with them? Why should it matter which direction it comes from?
 

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I've been wanting to jump into this discussion, but haven't had time. I'll have to keep it (sort of) short for now, and can provide more details later.

This whole concept of relative motion is classic Lagrangian frame of reference, where the "observer" is a moving particle of material (liquid or solid). This can lead to great simplifications of the equations of motion, where the relative fluid motion is all that matters, independent of the absolute motion of the frame of reference.

Unfortunately, those simplifications only apply when the frame of reference is non-accelerating, and inertial forces are weak relative to other forces such as viscous friction and gravity. Factoring the equations of motion into non-dimensional variables leads to dimensionless parameters such as the Reynolds number, Peclet number, Froude number. Of the many equations and dimensionless variables, the Navier-Stokes equation and Reynolds number are most relevant here, and with low Reynolds number the equations can usually be simplified to the more simple Stokes equations that can often be solved analytically without computers. Reynolds number is the ratio of inertial forces to viscous forces:



Unfortunately the geometry of the open seas has such a large characteristic length (typically the depth of the body of water) that any motion at all leads to a high Reynolds number, meaning that flow has a lot of inertia and very little viscous dissipation. This means that the energy of fluid motion has nowhere to go except to create eddies and waves, which are basically turbulence. Flow in narrow channels (or pipes) has a much smaller characteristic length, and if it's slow enough it will have a low Reynolds number leading to laminar flow, free of any eddies. In terms of energy transport, what happens is that the two major components of flow (momentum and vorticity) both diffuse to the rigid surface that encloses the liquid, taking energy away from the liquid and preventing turbulence and minimizing waves. But in the open seas, there is no such rigid surface nearby to absorb the energy, so the water churns away.

High Reynolds number flows require the full Navier-Stokes equations, which include nonlinear terms which are what lead to the eddies and turbulence. But Lagrangian frame of reference is almost impossible in this situation, requiring Eulerian frame of reference instead (where coordinate system is at rest). Wave action thus becomes dominated by non-linear effects that are more complicated than wind speed relative to water. In such a case, 10 kt wind against 3 kt current is very different than 16 kt wind with 3 kt current.

In low Reynolds number "creeping" flows, if the force that causes the flow is removed, the motion stops instantaneously (because there is no inertia). This is the situation where relative motion is all that matters.

In high Reynolds number flows, removal of forces (such as wind that's creating the waves) will eventually allow the seas to calm, but not instantaneously. Inertia causes the waves to propagate, sometimes for days and over thousands of miles, particularly in very deep seas where the is no solid surface to absorb the momentum or vorticity. That's why a storm in the North Atlantic can cause heavy swells in the Caribbean a week later.

As for the motion of the Earth, I think the surface actually moves about 1000 mph at the equator (not 24,000 mph). IIRC, the Earth's circumference is about 24,000 miles, and it spins once every 24 hours, so 24,000/24=1000.

The reason why Lagrangian frame of reference works on land (despite the high speed) is that the land mass of the Earth is a solid, and solid mechanics are different from fluid mechanics.
 

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I'll try to elaborate on my prior message with a few targeted responses.
As I said, I do not see how a water molecule can distinguish between being in a 20 knot wind added to a 2 knot current vs experiencing 22knot wind in still water...the point is that the frame of reference does not matter as long as you are consistent. We know this for a little over 100 years (Einstein etc).
The frame of reference does matter, because it cannot be accelerating. If you were to pick a tiny element of water as your frame, it would be moving around in circles and getting faster and slower. Both are examples of acceleration, so the simplifications of only considering relative motion cannot completely describe all the energetics that are going on.

I'm not sure that Einstein has proven what you are claiming. Don't forget that the fundamental concept of special relativity is that speed of light appears to be the same regardless of frame of reference. That's a totally different thing from what we're discussing, but it is a good example of a situation where traditional Newtonian mechanics breaks down. It's also another example where "common sense" can lead us astray.
 

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Well, if these waves come from thousands of miles away, what does the local wind have to do with them? Why should it matter which direction it comes from?
The question is, "How local is local?".

This year coming back to the U.S. from the Abacos in the Gulf Stream off St Augustine, we ran into a strong evening summer thunderstorm. NOAA warned us to expect 50 knots. We got about 55 knots for a half hour mostly from the north. While it was not pleasant, none of the waves were excessively steep or breaking. Their tops were blowing off, but not much more. They were locally generated by a thunderstorm with a diameter of maybe 20 miles. (The tale is one or two paragraphs in my wife's blog that she writes for her friends and our grandchildren. Irish Eyes to the Bahamas June 26, 2018)

On the other hand, a winter cold front extends well outside the Gulf Stream and, although "local", with its north and northwest winds it generates waves outside the Gulf Stream that when they enter the Gulf Stream become steep, pointed, breaking, monsters stacked on top of the more locally generated waves.

Bowditch has a table (Table 3303 on p825 of vol 1 of my 1984 edition) giving the height and period of waves given the wind speed and fetch over which the wind is blowing.

Time and fetch matter. Large seas need high wind speeds, long fetches, and time.

Dangerous seas have large heights and short periods.

Bill
 

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Well, if these waves come from thousands of miles away, what does the local wind have to do with them? Why should it matter which direction it comes from?
Yes, the issue is more the direction of swells and waves relative to the current. Swells are generally created some where else, not local, or at a different point in time. When the swells from far away interact with the current the wave length is shortened resulting in steeper swells, regardless of wind direction. The wind direction doesnt really matter in this case.

Then there are wind driven waves, these are caused by the wind happening now, in a given location and may later become swells. These waves also get steeper in a contrary current.

The common denominator in these two situations is the direction of waves/swell relative to current, not the wind.

So yes, the wind direction can be a bit of a red herring, what matters is the direction the waves are traveling relative to the current, not so much the wind.
 

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OK, that does make sense. If there is an inhomogeneity somewhere in the system, with a wave train encountering a change in average speed, then I understand that some change in wave patterns can occur. In the area where a gradient in current speed exists, the wave shape can change (e.g. in the way you describe). However, as soon as the gradient is removed, ie the current is homogeneous again (the same everywhere), the relativity argument must be valid again: there is no difference in ANYTHING (wave shape, height, whatever) between a 20knot wind against 2 knot current and a 22knot wind over still water (=current zero).

This would mean that the observed phenomena occur only at locations where the current changes strength (or direction). Is that the case? Coming back to the Gulf Stream, then you should be fine if you are in the middle of it (where the current is the same everywhere around you) and the problems should only exist at its borders. Is that the case? I think it is not but am willing to be corrected.

(of course another possibility is that the current is everywhere non-homogeneous so there would be 'pockets' of current going in all directions everywhere, like in turbulent flow, but I believe that is not the case in the big ocean currents like the Gulf Stream)
In my thinking: once the waves enter the contrary current, they are forced to slow down and in so doing grow steeper. They'd then stay that way until they are stretched out again by some other change in speed. So in the middle of the gulf they'd still be steep, not just at the boundary. The analogy that makes sense to me is cars in traffic. Imagine you have cars lined up on a freeway going 60mph spaced 200ft apart (and some imaginary ideal driving environment). They enter a slow area and slow to 30 mph. As they do so they will be forced closer. In the slow area, they will now be traveling 30 mph and are now spaced 100ft apart, and will stay that way as long as the speed is maintained. Once they get out of the slow area and return to 60 mph they stretch out to 200ft spacing again. The cars are wind driven waves, the slow area is the contrary current. So the two relative quantities being compared are the vector of the (wind driven) surface waves originating outside a current, and the water current, causing a change in speed of the waves. If the current were going with the wind the opposite would happen and they'd be flattened out.
 
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