Why is a groundswell called a "ground" swell? Isn't it just a strong windswell from farther away?
asked by Larry
Answer
Leading surfing meteorologist and Surfline forecaster Sean Collins responds:
Well, you're pretty close Larry, but it's slightly more complicated than just a simple naming convention.
Windswells are swells that are generated by local winds within a few hundred miles of the coast. As a result, the swell periods are short (four to 10 seconds between crests) and the waves tend to stack up on each other with very consistent sets. Also, a windswell's energy doesn't extend very deep -- maybe only 100 feet or so -- so it can only travel a few hundred miles before dissipating because most of the wave energy is concentrated closer to the ocean surface.
Groundswells are swells that are usually generated by winds much farther away. These swells are created by strong winds over long distances -- we refer to this distance as the "fetch" -- and over longer periods of time. As a result, more energy is transferred into the water during the generation of the waves, which create longer swell periods. Waves with longer swell periods can travel great distances without losing the swell energy like windswells do. Furthermore, swell energy in groundswells can also extend down to around 1,000 feet deep. This allows greater interaction with the ocean floor when the groundswells move into shallow water. For example, this process of refraction will allow groundswells to wrap into a spot -- up to 180 degrees or more -- while windswells wrap very little if at all.
