Waves generated by distant storms can travel thousands of kilometers across the ocean as long, regular undulations known as swell. Unlike local wind waves, swell has a longer period and a smoother appearance, and it can arrive even when the local wind is calm.

As waves leave the storm that created them, shorter components dissipate or fall behind, leaving the longer, faster waves to propagate as swell. Swell carries energy across entire ocean basins and delivers it to distant coastlines as surf. Surfers and coastal engineers pay close attention to swell forecasts because wave height, period, and direction determine break quality and coastal impact.

Satellite altimeters and wave buoys measure swell height and period. Numerical wave models track the generation, propagation, and dissipation of swell across the global ocean. On some beaches, the arrival of long-period swell from a storm days earlier and thousands of kilometers away is a regular occurrence.

A calm sunny day can still bring powerful surf when distant swell reaches the shore.