A swell with a swell period over 11 seconds between successive waves. As a rule, the harder the wind blows in a storm, and the longer it blows over a longer distance of ocean, the bigger the swell will be and the longer the swell period will be between successive waves. The longer the swell period, the deeper the swell energy extends below the ocean surface, which interacts more with the ocean floor, or the "ground" so to speak. This is contrary to a windswell, which has a shorter swell period, and is always generated by local winds with brief duration and over a limited distance of ocean. Groundswells with longer swell periods can wrap (refract) greatly into many spots due to deeper interaction with the ocean floor compared to shorter period wind swells.