As the water moves, it forms eddies and small ripples. The ripples make the water's surface rough, giving the wind a better grip. The ripples, starting at a minimum wave speed of 0.
Giant Waves Quickly Destroy Arctic Ocean Ice and Ecosystems
The surface still looks glassy overall but as the wind speed increases, the wavelets become high enough to interact with the air flow and the surface starts to look rough. The wind becomes turbulent just above the surface and starts transferring energy to the waves. When trying to be more precise about waves, difficulties arise: how do we measure waves objectively? When is a wave a wave and should be counted?
Scientists do this by introducing a value E which is derived from the energy component of the compound wave. In the left part of the drawing is shown how the value E is derived entirely mathematically from the shape of the wave. Instruments can also measure it precisely and objectively. The wave height is now proportional to the square root of E.
The sea state E is two times the average of the sum of the squared amplitudes of all wave samples. The right part of the diagram illustrates the probability of waves exceeding a certain height. Fifty percent of all waves exceed the average wave height, and an equal number are smaller. The highest one-tenth of all waves are twice as high as the average wave height and four times more powerful. Towards the left, the probability curve keeps rising off the scale: one in waves is three times higher and so on.
Click here for a larger version of this diagram.
Wave - Wikipedia
When the wind blows sufficiently long from the same direction, the waves it creates, reach maximum size, speed and period beyond a certain distance fetch from the shore. This is called a fully developed sea. Because the waves travel at speeds close to that of the wind, the wind is no longer able to transfer energy to them and the sea state has reached its maximum. In the picture the wave spectra of three different fully developed seas are shown. The bell curve for a 20 knot wind green is flat and low and has many high frequency components wave periods seconds.
As the wind speed increases, the wave spectrum grows rapidly while also expanding to the low frequencies to the right.
Note how the bell curve rapidly cuts off for long wave periods, to the right. Compare the size of the red bell, produced by 40 knot winds, with that of the green bell, produced by winds of half that speed. The energy in the red bell is 16 times larger! Important to remember is that the energy of the sea maximum sea condition increases very rapidly with wind speed, proportional to its fourth power. The amplitude of the waves increases to the third power of wind speed. This property makes storms so unexpectedly destructive. The biggest waves on the planet are found where strong winds consistently blow in a constant direction.
Waves here average 7m, with the occasional waves twice that height! Directly south of New Zealand, wave heights exceeding 5m are also normal. The lowest waves occur where wind speeds are lowest, around the equator, particularly where the wind's fetch is limited by islands, indicated by the pink colour on this map. However, in these places, the sea water warms up, causing the birth of tropical cyclones, typhoons or hurricanes, which may send large waves in all directions, particularly in the direction they are travelling.
6.1: Free Waves
Waves entering shallow water As waves enter shallow water, they slow down, grow taller and change shape. At a depth of half its wave length, the rounded waves start to rise and their crests become shorter while their troughs lengthen. Although their period frequency stays the same, the waves slow down and their overall wave length shortens. The 'bumps' gradually steepen and finally break in the surf when depth becomes less than 1.
Note that waves change shape in depths depending on their wave length , but break in shallows relating to their height! How high a wave will rise, depends on its wave length period and the beach slope. It has been observed that a swell of m height in open sea, with a period of 21 seconds, rose to 16m height off Manihiki Atoll, Cooks Islands, on 2 June, Such swell could have arisen from a 60 knot storm. The photo shows waves entering shallow water at Piha, New Zealand. Notice how the wave crests rise from an almost invisible swell in the far distance.
As they enter shallow water, they also change shape and are no longer sinus-like. Although their period remain the same, their distance between crests and their speed, diminish.
Not quite visible on this scale are the many surfies in the water near the centre of the picture. They favour this spot because as the waves bend around the rocks, and gradually break in a 'peeling' motion, they can ride them almost all the way back to the beach. Note that a sandy bottom is just another medium, potentially capable of guiding gravity waves. It is about 1.
Yet, neither does it behave like a liquid, nor entirely like a solid. It resists downward and sideways movements but upward movements not as much. So waves cannot propagate over the sand's surface, like they do along the water's surface, but divers can observe the sand 'jumping up' on the leading edge of a wave crest passing overhead when the water particles move upward. This may help explain why sand is so easily stirred up by waves and why burrowing organisms are washed up so readily.
Spilling breakers are a familiar sight on most beaches. They arise from long waves breaking on gently sloping beaches. There are several rows of breakers. Plunging breakers can occur on steeply sloping beaches. There is only one row of breakers. Surging breakers surge over steeply sloping but not vertical beaches or rocks. Waves break one at a time. Photos Van Dorn, Wave groups Part of the irregularity of waves can be explained by treating them as formed by interference between two or more wave trains of different periods, moving in the same direction. It explains why waves often occur in groups.
The diagram shows how two wave trains dots and thin line interfere, producing a wave group of larger amplitude thick line. Such a wave group moves at half the average speed of its component waves. The wave's energy spectrum, discussed earlier, does not move at the speed of the waves but at the group speed. When distant storms send long waves out over great distances, they arrive at a time that corresponds with the group speed, not the wave speed. A group of waves with half the period 7s would take twice as long, and would arrive a day later.
Harris, Most wave systems at sea are comprised of not just two, but many component wave trains, having generally different amplitudes as well as periods. This does not alter the group concept, but has the effect of making the groups and the waves within them more irregular.
doggy-train.com/images/vibarojuv/mobile-spy.html Anyone having observed waves arriving at a beach will have noticed that they are loosely grouped in periods of high waves, alternated by periods of low waves. Some objects, such as darker colored objects, absorb more incident light energy than others. For example, black pavement absorbs most visible and UV energy and reflects very little, while a light-colored concrete sidewalk reflects more energy than it absorbs. Thus, the black pavement is hotter than the sidewalk on a hot summer day. Photons bounce around during this absorption process and lose bits of energy to numerous molecules along the way.
This thermal energy then radiates in the form of longer wavelength infrared energy.