Swells

A swell starts under a windy storm hundreds or even thousands of miles away. The wind pushes on the water for long enough that the surface organises into rolling lines of energy that radiate outward. Once those lines escape the storm they keep moving in straight, organised rows until they hit shallow water near a coast.

That trip is what separates a clean groundswell from local wind chop. The longer the energy has had to travel and organise, the more powerful and predictable the surf will be when it arrives.

Out at sea, swell height is small, often just a few feet from trough to crest. Once that same energy hits a shallow sandbar or reef, it has to compress into a fraction of the depth, so it stands up much taller before it breaks. The breaking wave you actually ride is called the face, and it can be twice the offshore swell height.

WaveWise reports face height because that is the number that matters in the lineup. The deep-water swell number is upstream; the face is what you paddle for.

Swell direction names the compass point the energy is coming from. A west-facing beach lights up on west swell. The same beach can be flat on a strong north swell, because the energy goes past it instead of into it.

Headlands, islands and reefs can also bend swell as it wraps around them. That is why a spot can fire on one specific angle and disappoint on the next.

The forecast often shows a primary swell plus one or two secondary swells. When two swells from different directions arrive together, their crests overlap in confusing ways and the lineup gets bumpy and hard to read.

The cleanest sessions tend to be when one swell clearly dominates. If two are about the same size from different directions, expect a messier face even if the height number looks great.