Definition
Measured in seconds, and the key to wave speed and length in deep water. Wavelength in metres is approximately 1.56 times the period squared, and wave speed in knots is roughly 3 times the period. A 10 second swell therefore has a wavelength near 156 metres and travels at about 30 knots. Short periods relative to height mean a steep, uncomfortable sea; long periods mean an easy, rolling one.
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