Waves in the sea are formed when wind blows across the sea and whips the surface into ripples.
Water particles are dragged a short way by the friction between air and water, which is known as wind stress.
If the wind continues to blow long and strong enough in the same direction, moving particles may build up into a ridge of water. At first this is a ripple, then a wave.
Waves seem to move but the water in them stays in the same place, rolling around like rollers on a conveyor belt.
The size of a wave depends on the strength of the wind and how far it blows over the water (the fetch). If the fetch is short, the waves may simply be a chaotic, choppy ‘sea’. If the fetch is long, they may develop into a series of rolling waves called a swell.
One in 300,000 waves is four times bigger than the rest.
The biggest waves occur south of South Africa.
When waves move into shallow water, the rolling of the water is impeded by the sea-bed. The water piles up, then spills over in a breaker.
A wave over 40 m high was recorded by the USS Ramapo in the Pacific in 1933.