Period
The most underrated number
Period is the seconds between one wave crest and the next. It is the single best predictor of how much actual power the surf will have.
What period measures
Imagine you stand on a buoy and time how long it takes between one wave passing under you and the next. That number, in seconds, is the period. Longer period means the wave has more space behind it, more water moving with it, and more punch when it breaks.
Short period (8 seconds and under)
Short period swell is usually wind chop generated close to the coast. The crests are bunched together, the energy is weak, and the surface looks choppy. The waves often closeout or crumble without forming a real face.
You can still learn in short-period surf if it is small and forgiving, but expect to work harder for fewer rideable waves.
Long period (13 seconds and up)
Long period is groundswell that has travelled from a distant storm. The crests are far apart, the lulls between sets feel quiet, and when a set arrives it stands up fast in shallow water.
Long period deserves respect. It produces stronger currents, can wrap into spots that look flat on short-period days, and feels significantly bigger than the height number suggests.
Same height, very different sessions
A 3 ft swell at 7 seconds usually breaks knee-high and weak. A 3 ft swell at 16 seconds can stand up shoulder-high with real punch. Same height number, completely different day.
When comparing forecasts, do not just sort by height. Sort by what the combination of height and period is going to do at your spot.