In short
- Wind waves are made by the local wind. They are short, steep and come from roughly the same direction as the wind. Swell was made by distant wind and arrives long, smooth and regular.
- The quickest tell is the period: wind sea usually runs under about 8 seconds, swell often 10 seconds or more.
- Heights do not simply add. Combined significant height is the square root of the sum of the squares: 1.2 m of swell and 0.9 m of wind sea make 1.5 m, not 2.1 m.
- When the two come from different directions the sea gets confused and peaky. Read the directions as well as the heights.
Where each one comes from
Wind raises waves through three things: wind speed, fetch, which is the distance of open water the wind blows over, and duration, how long it has been blowing. Increase any of them and the waves grow taller and longer, up to the limit the other two allow. A 20 knot breeze over a three mile bay builds a short chop and stops there; the same breeze across a hundred miles of open water for a day builds a proper sea.
While the wind is still blowing on them, those waves are the wind sea. They are a jumble of heights and lengths, mostly steep, with crests that break into white horses in a fresh breeze. They travel roughly with the wind.
Once waves run out from under the wind that made them, they become swell. The shorter waves die away and the longer ones travel on, sorting themselves by length as they go: longer waves move faster, so a new swell often announces itself with long-period forerunners before the bulk of it arrives. Swell can cross an ocean in a few days and lose surprisingly little height on the way. It arrives as long, rounded, regular ridges, often from a direction that has nothing to do with today's wind.
- 1A: swell. One wavelength is marked: long and regular, from weather far away.
- 2B: wind sea. Short, uneven waves raised by the breeze where you are.
- 3C: what you actually see. The swell lifts and drops the boat while the chop rides on its back.
| Kind of sea | Typical period | How it feels on a boat |
|---|---|---|
| Wind chop on a bay or sound | 2 to 4 s | Quick, slapping motion; spray upwind, little real lift |
| Coastal wind sea | 4 to 7 s | Steep and hard upwind; pounding and slamming in a fast boat |
| Short or mixed swell | 7 to 10 s | Noticeable lift and roll, still a little steep |
| Ocean groundswell | 11 to 16 s and more | Slow rise and fall; easy in deep water, large on shoals and bars |
These are typical ranges, not hard borders. A long fetch and a strong wind can make a wind sea of 9 or 10 seconds, and a swell that has travelled only a short way may still be in the 7 to 8 second range. The guide to wave height and wave period explains why period changes the feel of a given height so much.
Telling them apart in a forecast
A single "seas 4 to 6 ft" hides the split. Look for the parts:
- Separate lines. Many NWS coastal waters forecasts add a wave detail line, such as "SE 4 ft at 11 seconds and S 3 ft at 5 seconds". The long-period part is the swell, the short-period part the wind sea.
- The period. If a forecast or a buoy shows only one period, a long one says swell dominates, a short one says wind sea.
- The direction. Wind sea comes from roughly the wind direction. A wave direction that is far from the wind's is swell.
- Buoys. NDBC buoys report swell height, swell period, wind wave height and wind wave period separately. Checking one up-coast before you leave tells you what is actually arriving.
On the water, the tell is the rhythm. Swell lifts the whole boat slowly and regularly, the horizon disappearing and returning every ten seconds or so. Wind sea is quick, irregular and steep, and you feel it as slapping and pitching rather than lifting.
How swell and wind sea add up
Wave energy adds, and energy goes with the square of height. So the combined significant height is the square root of the sum of the squares of the parts:
Combining two seas
1.2 m of swell and 0.9 m of wind sea: 1.2 squared is 1.44, 0.9 squared is 0.81. Add them for 2.25, and the square root is 1.5 m.
Two equal seas of 1.0 m make about 1.4 m, not 2 m. The bigger part always dominates: a 2 m swell with a 0.5 m chop on it is still a 2.06 m sea.
That is the number a forecast gives as total seas, and it is useful for comparing days. For deciding how a trip will feel, the split matters more. A 1.5 m total made mostly of 12 second swell is a comfortable passage; the same 1.5 m made mostly of 5 second wind sea is a hard slog to windward.
The two also change on different clocks. Wind sea builds within hours once the breeze fills in and starts to lie down within a few hours of it dropping, leaving a lumpy leftover sea that slowly smooths into swell. Swell does not care about today's wind at all. It arrives, peaks and fades over a day or more as the distant storm that sent it moves on.
Crossing seas
When swell and wind sea come from different directions, their crests cross. Where they meet in step, they stack into short-lived pyramids taller than either; where they cancel, there are odd flat holes. The sea looks confused, the boat's motion becomes irregular, and steering in it takes more work.
- 1Swell, travelling in the direction of the arrow. Long, widely spaced crests.
- 2Wind sea from a different direction. Short, tightly spaced crests.
- 3Where the two sets of crests cross, peaks build and collapse, and the motion becomes irregular.
Crossing seas are common in a few predictable places: just after a front, when the new wind sea builds across the old one; near headlands, where swell bends round the point while the wind blows straight; and on coasts with a sea breeze blowing across a groundswell. Changing course by a few tens of degrees often helps more than slowing down, because it changes which sea you take on the bow.
Swell over bars and shoals
A wave starts to feel the bottom when the depth falls below about half its wavelength. For short wind waves that only happens in shallow water. A 12 second swell, though, is about 225 metres long in deep water, so it begins to feel the seabed in roughly 110 metres, well out on the continental shelf.
As the water shoals, the swell slows, its crests close up and it grows taller. Over a bar or a shallow bank it can steepen enough to break, even when the sea outside looks gentle. That is why a modest long-period swell is the thing to watch at a harbour entrance, and why an ebb running out over the bar makes it worse. Our guide to wind against tide covers that combination.
Worked example: a beam swell and a head sea
Heading south along a coast
The forecast reads "Seas 5 ft. Wave detail: E 4 ft at 11 seconds and S 3 ft at 5 seconds." Four squared is 16, three squared is 9, the root of 25 is 5, so the total checks out.
You're heading south. The 5 second wind sea is right on the bow: short and steep, so expect pitching and spray. The 11 second swell comes from the east, on the beam, rolling the boat slowly from side to side. Together they make a corkscrew motion that tires a crew far more than 5 feet sounds.
If the southerly is a daytime sea breeze, the wind sea will ease in the evening while the swell stays. Leaving later puts the swell on the beam with the chop gone; leaving early, before the breeze fills, does the same at the other end of the day.
The same total height can be easy or hard depending on which part is which and where each one hits the boat. Read the split, put it against your course, and the forecast starts to describe the trip you'll actually have.
