Swell

Long, regular waves generated by distant storms; swell height and period drive surface conditions, surge, and whether a dive day is workable.

Swell is the train of long, smooth waves that travels far from the storms that created it, distinct from the short choppy wind waves generated locally. Divers care about two numbers in the forecast: swell height, and swell period, the seconds between crests. A long-period swell of 12 seconds or more carries much more energy than its height suggests and produces strong surge at depth.

Swell determines whether entries and exits are safe. For shore dives it defines the surf zone you must cross, and for boat dives it governs how rough the ride and the pickup will be. A 2 m short-period chop can be miserable but diveable, while a 2 m long-period ground swell can close out a shallow reef entirely and turn a swim-through into a piston.

Reading the swell forecast the night before is part of good dive planning: many operators call sites in the morning based on it. Note the conditions in your log, since knowing that a site works on a 1 m south swell but not a 1.5 m west swell is hard-won local knowledge. When the forecast sits on the edge, let the skipper make the call.