By definition, swell period is the time required for one complete wave length to pass a fixed point, and it is given in seconds. Nearly all the swell you’re likely to ever see surf from will range from 4 to 22 seconds, but for lots of spots that high end swell never happens. Usually the time is measured between two successive wave crests, but measuring from trough to trough would yield the same result. Swell period is a defining characteristic of a swell and as such it must be known in order to make accurate surf forecasts.
Swell period determines how fast a swell will propagate across the open ocean. The speed at which a swell travels through the sea is given in knots, or nautical miles per hour, (1 nautical mile equals 1.1508 statute miles), and it is calculated by multiplying the swell period by 1.5. So a swell with a period of 20 seconds will travel at 30 knots, while swell with a 10 second period travels at 15 knots.
Winds on water make waves. The stronger the winds are and the longer their duration, the more momentum they transfer into the sea and the deeper that kinetic energy penetrates. Swell period is a measure of that acquired momentum and it determines how far a swell will be able to travel in the open ocean. Short-period swell, (11 seconds or less) will usually decay within a few hundred miles, while long-period swell, (above 14 seconds), is capable of far greater journeys. That’s why powerful groundswell generated in the Southern Ocean can wind up as waves nearly half a world away in the Alaskan Gulf.
We see swell on the surface of the sea but its kinetic energy may extend downward to far greater depths. In that regard, swells are like icebergs, with only a small fraction of their true dimensions visible on the surface while the rest is hidden underwater. Just how far underwater a swell’s motion goes depends on swell period. That depth in feet is calculated by squaring the swell period then multiplying the product by 2.56. In the case of a swell with 20 second period, the math is 20 x 20 = 400, 400 x 2.56 = 1,024 feet. So that swell would begin to drag on the seafloor once it moved into waters of that depth. As the sea became shallower, the drag would increase. In contrast, a swell of six second period would not start to feel the resistance of the seabed until it entered seas that were 92 feet deep. Interference with the seafloor can change a swell’s direction, in some instances up to 180 degrees. That wrap is much more pronounced in long-period swells than in short-period ones. So knowing a swell’s period can be critical in forecasting if the swell will head straight into the coast or whether it will take a detour along the way.
Swell period performs its final role once a swell reaches the surf zone and shoaling kicks in. While closing in on the shore, the leading edge of a swell dragging along the seafloor slows down more than the trailing edge does. The resultant backup gives the incoming water nowhere to go but up, so wave height increases. The longer the swell period, the more water gets pushed upward. A 3-foot wave with a 10-second swell period may only grow to be a 4-foot breaking wave, while a 3-foot wave with a 20-second swell period can jack up to be a 15-foot breaking wave given the right ocean floor bathymetry. That is a fivefold increase from the deep-water swell height and a prime example of why anyone who ignores the importance of swell period does so at their own peril.
Since swell period plays such a crucial part in producing the surf you see, Surfline provides abundant and easily accessible swell period information. On that chart will be 16 days worth of swell period and swell height forecasts with each swell colored coded for your convenience.