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Author Topic: How a wave is formed  (Read 3952 times)

dennis

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on: July 11, 2010, 10:55:39 PM
To effectively paint waves we need to look at how they are formed and the water they consist of.

Let's look at the water itself first. Water has two important properties / features that play a part in the formation of waves.

Properties of water

1) If we pour water into a glass we notice that the water curls up against the edges of the glass. This is because of the type of bond between the water particles. It makes them 'stick' together, we call it surface tension. To relate this to a wave - if I move a section of water, it 'pulls' the water around it along with it.

2) The next feature of water is caused by gravity. If you have a 1km wide dam of water that is 1cm deep and a glass of water 10cm high, the glass of water will be exerting more downward pressure than the dam. The reason for this is that it is the height of the water and not the volume of the water that gravity is pulling down on.

An easy way to demonstrate this is to put your hand face up on a table. Place two bricks on your hand so that the bricks are next to each other. You will feel a certain weight. Now take one of the bricks and place it on top of the other and you will suddenly find that those same two bricks feel much heavier.

If we had to put on a diving suit and dive down into the sea we will find that the sea is putting more and more pressure on us as we move downward. This makes sense now as we know that it is the height of the water that is exerting the pressure on us. Let's break this pressure down into layers as we move toward the seabed :


Please realise that we are breaking these pressures into layers for demonstration purposes only, in the sea the pressure gradually increases, but the effect is the same.

But how does this effect a wave I hear you ask?

It is easier to push a light object over the floor than a heavy one, the same goes for water. The 'lighter' water at the top moves faster than the 'heavier' water lower down. To get an idea of where we are headed toward, take your hands and hold them parallel to the floor, one hand above the other. Now move both your hands to the side (in the same direction), but move your top hand faster than the bottom one. Didn't that start to look like the action a wave makes? So we're on the right track then.

What causes a wave?

Somehow we have to get the water in the sea moving. There are a multitude of factors that cause this, these include:

The pull of the sun and moon - they cause tides.
The wind causes swells on the water surface.
Earth tremors cause swells on the surface and currents underneath.
Temperature changes cause currents.
Rivers running into the sea cause swells and currents.
The slope of the seabed also gets the water moving as we will see shortly.

Time to start forming our wave

To simplify our demonstration we are going to have the wind cause our wave. Let's take a look at what happens:

1) As the wind blows over the surface of the water, because of friction it pulls the water along with it. Now the two features of water that we discussed come into effect. The surface tension of the water pulls the water next to it along. At the same time the water below the surface says "I'm at a higher pressure, i can't move as fast as you. But I also have surface tension, so slow down and move along at my speed". The water on the surface then gets 'pulled' back. This is what causes the swells we see when the wind blows over water.

2) As these swells are moving up and down and in the direction of the wind, the swells are causing swirls under the surface of the water. Take a look at the picture below to get an idea of what happens. The interesting thing is that even if the wind stops blowing, the swirls underneath the surface continue to move forward. If the wind continues to blow then the swirls slowly get 'wound up' as the swells move over them. Here you can think of a mouse in a wheel. The mouse(swirl under the surface) starts off at a walk, the wheel (swells) is lighter and has less friction so it moves a little faster, the mouse has to run to keep up, this effect snowballs until the mouse tumbles (wave breaks).


As you can see we have this underwater swirl that is gradually growing as it moves toward the beach. These swirls build up over thousands of kilometers sometimes before they reach our shoreline.

3) What happens when this swirl nears the shore is a combination of two things:

3a) The first is that the depth of the sea dramatically reduces.

3b) The second is that water from the previous wave is moving back down the shoreline into the sea.


4) As far as the deeper water is concerned, the minute it reaches the shore, it is as good as hitting a brick wall. It now has nowhere to go but up.

5) As this mass of water is moving up the shore, the water from the previous wave is storming back to the sea. Now these two masses of water start to fight with each other. The one wants to go shore-wards, the other wants to go seaward.


If we had to put a heavy weight boxer in the ring with a feather weight, who do you think is going to win? Yup, the heavy weight is way too big and strong for the feather weight. The same is going to happen to the smaller mass of water going back to the sea.

This smaller mass doesn't go without a fight though, it helps the upward movement of this large mass of water until the larger mass starts to go over the smaller mass.


6) What happens next is that as the wave pushes up and over, the mass of water starts to spread. In other words it thins out and starts to loose momentum. When the water thins out like this we can sometimes see the sunlight shining through it, and that makes for a dramatic painting.


7) As the wave loses it's momentum, gravity takes over again and the wave comes crashing down.

Whew, aren't you exhausted for the waves' part?

Foam

There is another important factor to painting the sea and that is the foam that is created. Let's take a look at the trauma the foam has to endure.

Why is foam caused in the first place?

There are a lot of impurities in the sea like salts, chemicals, dead plants, decomposed fish, you name it. With the rapid movement of the water these impurities cause the water to form bubbles.

These bubbles drift merrily on the water, sticking to each other, bobbing up and down and generally minding their own business. Our first reaction is to think that these bubbles are drifting towards the shoreline too, but that is not the case.

If you remember from our discussion on the formation of waves, you will recall that the surface of the water is relatively still. It is the swirls underneath the surface that are heading shoreward. The foam just bobs up and down on the water until a wave is formed.

What happens then is that the foam moves up with the forming wave until it gets to the top. Here it 'bodysurfs' on the wave until the wave comes crashing down. This movement of the foam up the wave helps us define the shape of the wave when we are painting it.

Because of the force of the upward movement of the wave and the foam being so light, some of the foam gets shot up into the air. This is where the amateur painter makes a huge mistake. He tends to paint this foam spraying forward in front of the wave. This isn't true. As the foam gets thrown up out of the wave, it loses momentum as it is now lighter than the wave. The wave then moves faster than the foam and the foam gets left behind.


The final part of this lesson is on the lighting of your wave when you paint it.

As your wave curls over it throws a shadow onto itself. The easiest way to demonstrate this is to hold your hand in the air with the fingers pointing to the sky. Now curl your fingers over so that they look like the shape of a breaking wave. As you are curling your fingers over look at how your fingers throw a shadow onto themselves and your hand. Now turn your hand from side to side and notice how the shadow also moves. Notice that the bottom of your hand (closest to your wrist) is in full light. Next time you are painting a wave curl your hand over like this to get the shadow of your wave correct.

« Last Edit: January 19, 2014, 08:40:17 PM by nolan »
You are what you THINK about - Napoleon Hill


MICKEY

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Reply #1 on: July 15, 2011, 10:55:49 PM
awesome !
MICKEY


Trudie Niehaus

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  • Trudie Niehaus
Reply #2 on: July 16, 2011, 03:05:23 AM
Jislaaik ... thank you, this is a very good "lesson".
Trudie Niehaus
Trudie Niehaus


 

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