Asked by: Benny Matthews, Oxford
They're mainly due to the wind. The surface of a calm patch of water becomes stirred up by the frictional effect of the wind blowing over it, forming ripples. These ripples increase the amount of 'grip' the wind has on the surface, increasing the amount of energy it can transfer and causing the water to form waves. Coming into a beach, they rise up until collapsing into surf.
Robert is a science writer and visiting professor of science at Aston University.