Not all winds are of equal height. This is partly because the wind seldom blows at a constant speed. When the wind dies down, wave length is maintained but wave height decreases gradually. Where there is no barriers in their path, waves crests may carry over vast distances. When waves created in different places come together, they produce a confused sea state. Boats may be completely overwhelmed by a storm at sea – getting swamped, smashed, or both, by the huge quantity of water falling on them.
The full force of a wave is not realized until it reaches the shore. As it sweeps in towards land, contact with the sea-bed slows down the lower part, but the wind keeps the top moving on. The wave becomes steeper, begins to overhang, and finally crashes down. On steep shores, waves do not have time to slow down. Suddenly, their way is barred and they smash against the rocks with tremendous force – 100 tonnes per square metre is not unusual.
Winds exceeding Force 12 (over 117km/h) can devastate anything that lies in their path. They are known as hurricanes in the Atlantic,typhoons in the North Pacific and cyclones in the Indian Ocean and around Australia. These great storms start when scattered cloud clusters of tropical thunderstorms are gathered together into a whirling spiral by the Earth’s rotation. At the centre of the spiral is a column lo low pressure – the eye.
Because of the low pressure, air is sucked into the spiral with great force, resulting in violent winds. These set up huge waves at sea, which have devastating effects. Hurricanes may persist for up to ten days, and although the path they will follow can be predicted to some extent, unexpected twists and turns are common.
A water spout is another example of the sea being whipped up. This is rather like a liquid tornado. Warm air rising from the sea creates a central column of low pressure which draws up a swirling rising wall of water. When a low pressure system – depression – passes quickly across the sea, the water level suddenly drops, then rises. A great swell is created, knows as a storm surge, which can flood huge areas as it hits land.
Just as terrifying as rough seas, are tsunamis, caused by earthquakes and volcanic eruptions in the ocean. Small earthquakes occur along the ocean ridges, where the Earth’s crust in thin and hot. The largest ones are along the lines of collision of the plates.
The quake lifts the ocean floor, which buckles and collapses, and the shock tremors suddenly move the whole mass of water above, right from the ocean floor to the surface. As the tremors radiant outwards, the tsunami sweeps across the ocean at a terrifying speed of up to 720 km/h. At first, tsunamis are very small and may not be noticed in the open sea. However, as they come to shallower water, they are slowed down by the drag of the sea-bed and the waves build into a vast wall of water with an awesome power for destruction. They hit the coastline with a tremendous force. Boats are thrown high up on the shore, land is filled with sea water and houses destroyed.