My son asked me this question, and I don't know the answer.

I'm not used to not knowing answers, so this bothers me:

Why do the waves keep rolling into the beach, even when the tide's going out?

And furthermore, why is it that waves are always rollin IN onto every beach in the world, no matter if it's on the north, east, west, or south side of an island for example?

Any ideas, theories, suggestions, wild guesses?

Redstone

back and forth....;)

How old is your son? He has a neat, curious mind - that's cool. I also like the fact that you want to tell him the correct answer - I always felt it was important to answer my kids' questions, rather than brush them off as so many people do. And if I didn't know, I'd find out. I ended up learning a lot that way.

I don't know the answer but it sure sounds like you're a good dad, Redstone, with a neat kid. :thumbsup:

being brushed off when he asks a question. The only problem is that now I'm working on transitioning him into finding the answers for himself...as you can imagine, he likes the old way of Dad explaining everything to him much better.

Redstone

And I don't mean Keith.

and that causing the tides to rise and fall. But his question is: If the moon is pulling the water away from the beach, thereby causing the tide to fall, why are the waves still rolling in?

Damn if I can figure it out.

Redstone

Sorry, all I can muster.

Science, biology...not my forte.

My only thought is that the waves have to roll in somewhere. Geez, that's pretty damn lame.

??

There is some kind of weather in different parts of the ocean at all times. Undersea earthquakes. This builds up waves and they proceed out in all directions until they reach land. :shrug: Just a guess though.

Volcanoes, steam vents, temperature gradients, currents-- the waters of the world are far from just sitting there quietly, waiting for the fisherman. Storms, wind, water spouts, whirlpools-- the world is far more alive and in motion than we can understand.

"The winds cause waves on the surface of the ocean (and on lakes). The wind transfers some of its energy to the water, through friction between the air molecules and the water molecules. Stronger winds (like storm surges) cause larger waves. You can make your own miniature waves by blowing across the surface of a pan of water. "

whereas,

"Tides are periodic rises and falls of large bodies of water. Tides are caused by the gravitational interaction between the Earth and the Moon. The gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon. Another bulge occurs on the opposite side, since the Earth is also being pulled toward the moon (and away from the water on the far side). Since the earth is rotating while this is happening, two tides occur each day. Isaac Newton was the first person to explain tides scientifically."

Here's where I got all that:

http://www.enchantedlearning.com/subjects/ocean/index.shtml

that doesn't explain why:

1) The tide is going out, AND there's a wind blowing from the land out to sea. The waves are STILL rolling IN. (This is not an uncommon set of circumstances.)

2) You have an island. It's not a large one, maybe a mile in diameter. And it's a nice flat island, maybe ten feet above sea level, so you can feel the wind blowing all across the island. The wind is blowing from the West. That would explain waves crashing onto the beach on the west side of the island, but why do the waves still roll IN on the eastern side of the island?

I'm damned confused, still.

Redstone

but I think (and don't take my word for it because I am no scientist of any kind) it's because we only see waves breaking against the shore that are moving that direction...waves that might be created by wind blowing away from shore will break on distant beaches somewhere else. And the waves we might see on an island break on all sides partly because we only notice them when they're breaking (no matter from what direction) and because waves from different directions all hit the island, not just the waves that are moving the same direction as the prevailing wind.

Jeez, that answer just by itself is pretty confusing...

And also, the wind itself is a pretty confusing thing to start thinking about....

into a still pool of water, the ripples move outward. It's a basic principle of the physics of motion and water.

Try to picture in your mind a bathtub filled with water. In the middle of the tub, you've got large rock that rests on the bottom of the tub and pokes up out of the water.

If you dropped a small pea into the water to the west of the rock, the ripples would bounce off the east edge of the tub and come back weaker. But Earth is a sphere and has no 'walls' off which to bounce.

Now, you have a child, and perhaps remember from your own youth the joy of slipping back and forth to 'slosh' the water in a tub. Let that serve as analogy for the tidal action. We have a constant 'sloshing' east-west in the tub. Now here's the important part, when we talk about the ocean, the pea is not analogous to the type of wind we're discussing. It's more like taking a toilet plunger to the south end of the tub while the water is sloshing east-west.

When you've got all that motion going on, one can easily invision the waves on the rock in the tub, but think on it, that up and down motion of the toilet plunger, even amidst the sloshing east/west is causing the water to creep up evenly on all sides of the tub, and all sides of the rock. The level to which it will creep is determined by the strength of the plunging (wind and subduction quakes) and the volume of the tub (water content of the ocean in relationship to the land).

and I do appreciate you taking the time to write it out at such length.

And "sloshing" takes the wind out of the equation, making it that much more plausible. Fits right in to most theories of fluid dynamics as well.

I think you get the prize on this one.

Thanks again,

Redstone

generated constantly, eventually, some wave (which has tremendous amounts of power and can travel exceptionally long distances) is going to reach a point somewhere where it will break, no matter what the direction.

It would seem to me (just guessing) there is insufficient water/power along the coast to generate obvious wave patterns (remember that the waves that come in to the shore are actually very large masses of water that aren't particularly noticeable (in many cases) until the depth of the water decreases and the waves mound up and become obvious. Aside from the fact that there isn't that much water/energy, I'd say that the fact that heading away from the shore the depth increases would make any waves less noticeable too.


Again though, I'm just guessing.

Edit: minor clarification of a thought.

as close to the middle as you can get it. Watch how long the ripples last.

Throw in two stones - far apart, yet still close to the center. Watch how those ripples interact. Then try a few more stones at the same time.

The tidal force of the moon is exponentially stronger, in much deeper H2O, then add sealife movement. The ocean is in constant motion.

The shelf forces H2O up, cold mixes w/ warm and movement occurs. The calmest of beaches are those that are miles away from the shelf. Most are not.

This answer worked for me and my nephew, with the added benefit of him fearing my magical powers. Valid until he hits the first grade or so.

there is a giant Kitchen Aid Mixer, and it never stops.

how's every old thing with you? Well, I hope.

Redstone

We meet again, Redstone............

Been a while, huh? Summer holidays, online fatigue, and all that. All is grand here.

I hope it's even better with you.

Leftie

causes currents where it hits deeper, cooler water. The currents cause waves to come in. Even when the tide goes out, the warm and cool water mix, causing water currents.

or something like that.

..grav. force between the earth and the moon.

area, it creates waves that you can see....

You have ONE smart kid there!! He's a thinker and we need more thinkers in this world!

well done Redstone.


aA

You know how boats move out to deep water when a tidal wave is coming. Throughout most of its lifetime the tidal wave is pretty small, maybe only a 5 or 10 foot swell on the surface of the water, but when it gets to shallow water it rises up into something destructive. Look up "tidal wave" on wikipedia if you want a more detailed explanation of the mechanics of this.

By its very nature a shoreline is preceded by a rising seafloor, so breaking waves never go "out", they come in getting taller the closer they come until eventually they break. Minus the influence of the moon, wind and such, there are all kinds of waves and swells going in all different directions on the surface of the ocean. The rising seafloor before a shoreline acts as kind of a filter. Only those swells moving towards shore get taller and break as a wave. You just don't notice the others because they don't do anything particularly interesting.

Bonus question: In some science fiction story about a world with no moon, is it scientifically accurate to say the shorelines don't have incoming breaking waves?

"'Shut up,'" he explained."
:rofl:

that I wrote back in the 1980s...I didn't now anyone else knew it!

It's a timelessy pithy comment, yes?

Redstone

and they travel huge distances - 100s/1000s of miles until they hit land. Every ocean basin is going to have numerous active storm systems at any given time, with swells propagating outward from the storm in all directions, so basically every coastline is going to be receiving a swell from some direction at any given time.

As for the island question, even if the swell is only from one direction, as the waves encounter the shoaling water around the island there will be refraction (wrapping around) and diffraction that directs some wave energy into the protected leeward side of the island.

I'm still trying to answer "mom what keeps the sun up there? What if it falls?" That was when he was 5. He's 23 now and I still don't know. I guess I'll have to ask him.

mine is also 9 and it makes me learn things I may have forgotten or never learned. Kids are so cool that way!

The explaination can be a very easy one. No need for complexity.
The incoming tide isn't created by waves washing up on the beach.
The tides are primarily from gravity.
Waves are energy. The ocean waves are similar to radio waves. They don't carry water with themselves. They move through the water,as a radio wave moves through the air.
The waves washing up on the beach are simply the energy expending itself in shallow water. The energy can no longer continue to travel.
It actually isn't washing water in to create a high tide.

Tides and waves must be looked at as unrelated events.

would be the edge of the ocean just sitting there--creeping slowly in and out every 12 hours according to the tides, of course, but no slooshing back and forth. The surface of the ocean would have to be absolutely still--the way you don't get waves in a bowl of water sitting on a table, but bumping the table just the slightest bit gets the water moving, even just a little.

The surface of the ocean is never going to be absolutely still, because of the waves caused by wind, variations in the temperature of the water in different parts and at different depths of the ocean, etc. Small ripples in deep water amplify as they hit shallow water, and become waves.

So, the waves are unrelated to the tide, although the average point where they hit the beach is. The waves occur because a relatively static, slowly-creeping-tide alternative isn't possible in a world with variable weather/wind.

I must admit, this question had me stymied too at first. Neat, curious kid you've got there!

Or something like that. It's the best way I can summarize it without explaining what I don't really know to begin with.

water surface waves are very akin to soud waves, and you can readily hear noises made around corners.

This is why when (for example) a peninsula pointing due west into waves will have waves rolling in on all three sides--not just it's west facing point. The waves will literally bend into the land.

As for waves rolling in AGAINST the wind, first you need to remember that the water in a wave doesn't actually go anywhere--it just moves up and down, but basically remains in place, so waves near shore sort of "slosh around" a bit. Additionally (particularly in oceans) there is generally more than wave pattern occuring in any one location at any given time. In ocean's wave can pretty much propagate until they hit something, so if there is a storm hundreds of miles away its wave will still reach you. Look to the famous Oahu North Shore--most of those giant waves are coming from near Siberia.

On a sheltered body of water--such as a lake-- it is a bit clearer as thre is less chance for many wave patterns to be superimposed on one another. And as a chap who has spent many days slugging it out against the wind in a canoe I can attest that without complicating factors waves DO come out from shore in some cases.

I hope that helped. :)

Oceanography--paying off at last! ;)