The size of the waves depends on the fetch, the distance that the wind has blown over the water and the strength and duration of that wind.

If the wind dies down, the wave formation is reduced, but already-formed waves continue to travel in their original direction until they meet land.

The waves reach their maximum height when the rate at which they are travelling nearly matches the speed of the wind. In open water, when the wind blows continuously as happens in the Southern Hemisphere in the Roaring Forties, long, organised masses of water called swell roll across the ocean.

A strong blow over the ocean causes larger waves as the moving air pushes against the raised ridges of water.

The friction between air and water caused by a gentle breeze on a pond causes ripples to form.

Wind blowing over the surface of a body of water forms waves that are perpendicular to the direction of the wind.

Additional contributions, as much as one quarter of the total, come from water sources on land, such as melting snow and glaciers and extraction of groundwater for irrigation and other agricultural and human needs.

Most of this rise can be attributed to an increase in the temperature of the sea due to climate change, and the resulting slight thermal expansion of the upper 500 metres (1,600 ft) of water.

For at least the last 100 years, sea level has been rising at an average rate of about 1.8 millimetres (0.071 in) per year.

At the last glacial maximum, some 20,000 years ago, the sea level was about 125 metres (410 ft) lower than in present times (2012).