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Friday, January 7, 2011
Waves and tides
The Earth’s layer of water making up the oceans, often referred to as the hydrosphere, will according to the laws of gravity finds its own level. The Earth’s gravity however, varies over its surface according to the density of the rocks beneath; hot, low density rock, rising in a convection current through the mantle, will lead to a weaker gravitational pull and hence less water above it, where as thick, cold, dense rock will have greater gravitational pull and so will have more water above it. This accounts for variations of several meters revealed by the satellite surveys.
The stronger influence on the height of the oceans is the gravitational pull of the Moon, which causes a bulge of ocean that is pulled towards it. A similar bulge appears on the opposite side of the globe, as if in a falling away response, which is caused by the Earths spin. The result is two high tides during the course of a day separated by about 12 hours as the Earth completes its orbits. Since the moon is also in orbit, the time of high tide varies by just under an hour each day. The sun also exerts a gravitational influence on the tides when it pulls in the same direction as the Moon the tides are at their largest and are known as spring tides. When the sun pulls at right angles to the Moon it has a diluting effect and the tidal range is less; this causes neap tides the lowest high tides.
The surface of the ocean is constantly stirred by the wind, producing waves of transverse nature. A wave may travel for many Kilometers, and each particle of the water within it moves in a small circular motion. As the wave comes into shallow water of the shore, there is a change in speed, with the bottom of the wave slowing down at a greater rate than the top water. The top part in effect over takes the bottom, causing the wave to break. Depending on the angle at which the wave hits the shallow water, there can also be a change in direction of the wave since refraction will occur. The return flow of the water down the beach becomes a current or undertow that can be a danger to the swimmers or surfers.