EASY READ The Earth
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What causes tides and waves?
Sea levels rise and fall twice a day. This pattern is known as the tides. When the wind blows across the surface of the sea, it turns the water over and over in circles. These are waves. The stronger the wind and the greater the distance over which it blows, the larger the waves.
Tides
Tides are caused by gravity: the attraction that a large object (in this case, the Moon) has over another object. As the Earth spins, the ocean waters on the side of the Earth closest to the Moon bulge outwards. This results in a high tide. The rest of the Earth has a low tide.
When the Sun and Moon line up, the Sun’s gravity increases the pull. This results in very high and low tides, called spring tides. When the Sun and Moon are at right angles, the tidal range, the difference between high and low tide, is at its lowest. These are neap tides.
Tidal bore
In narrow estuaries, the tidal range is greater than it is in the open sea. The body of water rushing in with the tide builds into a rolling wall of water, called a tidal bore. They can reach a height of 1.5 metres (5 feet) in the Severn Estuary in England. The world's largest tidal bore is on the Qiantang River in China. It can reach up to 9 metres (30 ft) in height, and travel at up to 40 km/h (25 mph).
Waves
Waves can travel for many kilometres across the open sea before breaking on the seashore. As a wave approaches the shore, its lower part drags along the sea bed, slowing it down. The top part continues moving quickly. It eventually topples over or “breaks”.
Consultant: Ian Fairchild
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