ground swell

Swell originating from a distant open-ocean storm. The standards aren't exact, but a fully developed (or "significant") ground swell requires about 2,000 miles between the wave-generating storm and the coast. Ground swell is sometimes defined as having a period (the average time between the crests of two consecutive waves) of more than 15 seconds, while a locally generated wind swell can be defined as having a period of 11 seconds or less. Swells with a 12- to 15-second period originate from ...