Wind, waves and swell

Sea state · Power & sail

Wave size depends on how hard the wind blows, for how long, and over how much open water. Swell arrives from distant storms. The wave period tells you how steep the sea will feel: at the same height, a shorter period is rougher and more dangerous.

What builds a sea

• Three things set the size of wind waves: wind strength, how long the wind blows, and the fetch, meaning the open stretch of water it blows across without changing direction. The longer the wind works, the higher the waves and the longer their period.
• Fetch explains a lot of local difference. A long ocean fetch builds big seas. In a sound or strait the fetch is short, so waves cannot grow as large, but they can still be short and choppy, especially where wind opposes the current.
• NOAA gives an example: a steady 30 kt wind for 24 hours over a 340-mile fetch produces a significant wave height of about 17 ft, with a small chance of single waves over 35 ft.

Swell

• Swell is wave energy that has left the storm that made it. It is smoother and more regular, with a longer period, and can travel thousands of miles almost unchanged, so big swell can arrive on a calm day.
• Longer waves travel faster, so the long-period swell from a distant storm tends to arrive first.
• When swell reaches shallow water it slows, steepens and can grow to about one and a half times its deep-water height before it breaks. That is why bars, shoals and river mouths can break in swell that looked harmless offshore.

Why the period matters

• The period is the time between one crest and the next passing a fixed point. At the same height, a shorter period means the waves are closer together and steeper.
• The NWS states it plainly: for small vessels, danger increases as the wave period decreases for a given wave height. Read height and period together, never height alone.
• A wave becomes unstable and breaks once its steepness (height compared with length) approaches about 1 to 10.
• NWS forecasters on the West Coast use a height-versus-period table as guidance for small craft advisories. It was built for those waters only, but its message applies everywhere.

How wind waves and swell combine

• Forecast 'seas' combine wind waves and swell, but not by simple addition. The NWS takes the square root of the sum of the squares: 6 ft of swell plus 8 ft of wind wave gives about 10 ft of combined seas, not 14 ft.
• When wave trains come from different directions the sea gets confused, and now and then crests line up into a much bigger wave. The NWS notes these are more likely when several swell systems meet. The forecast's wave detail lists each train, so you can see when that is likely.
• Current changes waves too. A current running against the waves shortens and steepens them; one running with them lengthens and eases them (see Wind against current).

General guidance for recreational boaters, researched from the sources listed and written in our own words. Your equipment manuals, local knowledge, the Navigation Rules and a qualified instructor come first.