That’s why powerful groundswell generated in the Southern Ocean can wind up as waves nearly half a world away in the Alaskan Gulf. 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. 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. 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. That is plenty of time to think up an excuse for why you won’t be at work/school then. So if a storm that is 2,100 nautical miles away generates swell with a period of 14 seconds, that surf will show up at your shore in 2,100nm/(14 x 1.5)knots = 100 hours or 4 days and 4 hours. If you want to know how long it will be before waves from a distant storm reach your beach, you divide that distance by the swell speed, (swell period x 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. 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. Swell period determines how fast a swell will propagate across the open ocean.
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