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What causes tides and waves?

Stormy waves in the open oceanStormy waves in the open oceanSea 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.


The gravitational pull by the Sun and MoonThe gravitational pull by the Sun and Moon


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.

A rocky islet at low tide in the Bay of Fundy A rocky islet at low tide in the Bay of Fundy 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.

Diagram of spring and neap tidesDiagram of spring and neap tides

A tidal boreA tidal bore

Tidal bore

The highest tidal range is found in the Bay of Fundy, between Nova Scotia and New Brunswick, Canada. The greatest average spring tide range is 14.5 m (48 feet).

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