The time in seconds between consecutive wave crests passing a fixed point. Longer periods mean more powerful waves.
Wave period is arguably more important than wave height for predicting surf quality. Short period (under 10 seconds) means the waves were generated by local wind and haven't had time to organize — they'll be weak, closely spaced, and messy.
Long period (14+ seconds) means the swell traveled from a distant storm, waves are well-organized, and they carry much more energy. A 4-foot swell at 16 seconds will produce significantly larger and more powerful waves at the beach than a 6-foot swell at 8 seconds.
This is because long-period waves reach deeper into the ocean, carrying energy that releases as the wave hits shallow water. When reading a surf forecast, always look at period alongside height.
Example usage
"Don't be fooled by the 3-foot forecast — it's at 17 seconds. Those waves will be solid when they hit the reef."
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