Literally it's the difference in seconds between the peak of one wave in the set and the next. It's very very useful for us. There is a direct relationship between the period of a swell and the speed at which it travels, and a direct relationship between this speed and the quality and size of the surf on the beach. This is literally an index of the quality of a swell for the surfer. As a very, very rough guide the following might be useful:
| 1-6 Seconds | Weak local wind swells, lumpy, confused and without power. Rarely create surfable waves on the beach. |
| 7 - 8 Seconds | Relatively local wind swell which just might produce something ridable. These will normally be associated with onshore winds at the beach. If you can get this sort of swell with an offshore wind you might just score some gutless surf (and it could be big - just no power). |
| 9 - 10 Seconds | Not a good swell but starting to show some of the qualities of having travelled a little distance to the beach. Probably just enough power to create something surfable at many spots if large enough, but likely to be lumpy and lacking power. |
| 11-12 Seconds | A reasonable swell. This sort of swell can be good enough for most UK spots to show a surfable wave. With enough size and decent local winds it could amount to a good session. |
| 13+ Seconds | From here it just keeps getting better. Assuming some size to the swell and decent local winds it'd be a rare spot that didn't start to show good surf, and because these swells have travelled some distance to the beach the sets should start to be well defined and have a good shape. These swells will also travel twice as fast or more than weak wind swells and this will mean more power down the line for the surfer. The higher the period after about 13 seconds the closer you get to a perfect long distance swell. |
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