Naked Science Forum
General Science => Question of the Week => Topic started by: thedoc on 15/09/2009 17:56:27
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While on holiday in Wales and looking out over Cardigan Bay I was wondering what dictates the frequency of the waves?
Asked by Gary
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We put this question to Stephen Salter, retired Professor of Engineering Design at Edinburgh University:
[img float=right]http://www.thenakedscientists.com/forum/copies/RTEmagicC_wave_01.jpg.jpg[/img]The frequency of ocean waves depends on the wind speed, the time that the wind has been blowing and the length of the sea that it is been blowing over which is what we call the fetch. We actually prefer to talk about the period of waves, which is the inverse of frequency because people like to think about numbers greater than one rather than thinking about small decimals. If you started with the wind blowing over calm water, the waves start with small heights and short lengths but these steadily increase. In deep waters, the waves with the longer lengths travel more rapidly and the growth continues until the speed of the waves is about the same as the speed of the wind when it can’t put any more energy in. And we describe this as a fully-developed sea. There’s actually been a mixture of periods and people are quarrelling about how you define the period in the mixture of them. Real seas often have a spectrum with more than one peak, showing that the waves are coming from more than one place or maybe that the wind speed changed while they were growing. Periods in seconds of the same sort of order as wind speeds measured in meters per seconds, a bit more or a bit less depending on how you define your period. And most sea waves have periods in the range of 5 to 15 seconds with the longer ones coming when you had a really fast wind blowing for a long time over a long bit of sea.
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The Moon, The Sun, The Wind.
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It is quite complex. Waves themselves are mostly created by wind blowing across the surface of the sea, a small ripple in the water will get enlarged and enlarged by the wind until it forms a wave. Basically any wavelength will get amplified by the wind, but energy will get put into short waves more quickly than long ones, so ripples on a pond tend to have a short wavelength.
However short wavelengths feel the greatest losses due to friction, so if the wind is blowing in the same direction for a long time the short wavelengths tend to get overwhelmed by the longer wavelengths.
Also long wavelengths will travel faster than shorter wavelengths, so if there is a storm in the middle of the ocean, the first waves to get to you will be the long wavelengths, and then the waves will get shorter and shorter. In the UK our weather comes from the atlantic, so a large long swell can often mean that a storm is coming our way.
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Good answer! But what impact does the depth of the water have? As the waves approach shore the shallower water will make them speed up won't it? Will this have the effect of making them bunch together?
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If the depth of the water gets shallower than about the wavelength of a wave it will slow down and get taller. This will affect long wavelength waves first, so might have some bunching effects over a few waves, but this will only be happening for a few tens of metres at the most, so will be a far smaller effect than the separation in the open ocean.
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I believe that the moon & sun affects wave size and frequency.
During full moons and new moons; the gravitational pull of the moon and the sun are working together so this causes bigger waves, higher high tides and lower low tides.
and during the 1st and 3rd quarter of the moon. the gravitational pulls work against each other so the tides are not as strong.
Some sea turtles can time their mating season with the moon during 1st and 3rd quarters so laying eggs will be much easier.
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Tides are a form of wave (there are 2 tidal waves which circle the earth once a day) they have a very long wavelength so you just see the water going in and out.
The depth of water will affect how the waves that are made travel, particularly near the shore - so tides will affect how destructive the waves can be etc. And tidal streams can produce waves when going around a headland in the same way water flowing in a stream can produce ripples as it goes over a rock. But the biggest effect is the wind.
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Great Cthulhu.