Definition
Waves that have travelled out of their generating area and become regular, long-crested and long-period, typically 8 to 20 seconds or more. Longer waves travel faster in deep water, so swell sorts itself by period and the longest arrives first, which is why a rising long swell can precede a distant storm by a day or more. Swell direction may differ greatly from the local wind, producing an uncomfortable cross sea.
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