Not all waves are local. Swell is a series of long-wavelength waves that have traveled out of the area where they were generated by wind, often arriving as smooth, regular undulations.

Swell can persist after the wind has dropped and can come from a completely different direction from the local wind sea. It affects ship motion, especially roll, and can make harbor entrances and cargo operations dangerous even on otherwise calm days. Surf on exposed coasts is often swell-driven.

Forecasts distinguish wind sea from swell and give direction and period. Masters use that information when choosing anchorages or timing bar crossings. A long, heavy swell can roll a ship uncomfortably even when the wind is light.

• Long-period waves from distant wind generation
• May arrive from a different direction than local wind
• Affects roll, surf, and harbor operations
• Reported separately in marine forecasts