Several factors influence the shape and quality of breaking waves. These include the bathymetry of the surf break, the direction and size of the swell, the direction and strength of the wind and the ebb and flow of the tide. Swell is generated when wind blows consistently over a large area of open water, called the wind's fetch. The size of a swell is determined by the strength of the wind, the length of its fetch and its duration. So, surf tends to be larger and more prevalent on coastlines exposed to large expanses of ocean traversed by intense low pressure systems.