Wave Explorer

Significant wave height is not the height of the biggest wave, and that is the single most misread number in marine forecasting. It is roughly the mean height of the largest third of the waves — so about one wave in a thousand will be near twice it. A 4 m forecast is a sea with 8 m waves in it. The colour table here bands the range a forecast is actually written against rather than smoothing it into a rainbow, because the thresholds are where the decisions are.

The sea is not one thing. What a buoy measures is a wind sea being driven right now, with one or more swell trains running through it from storms that may be thousands of kilometres away and days in the past. Height alone cannot tell those apart — 2 m of 6-second wind chop and 2 m of 17-second groundswell are the same number and completely different water. So the field picker is banded: the combined sea, the wind sea under it, and the swell trains crossing it, each with its own height, period and direction. Where two swell trains of comparable size cross, the sea is far more confused than either height suggests, and comparing Swell 1 and Swell 2 is how you see it coming.

Period is the tell. A long period means the waves have outrun the storm that made them and sorted themselves into clean sets; a short one means the wind is still on them. Long-period swell also carries far more energy per metre of height and refracts hard onto a coast, which is why a 2 m 18-second forecast matters to a harbour and a 2 m 6-second one does not. ECMWF and AIFS publish something no other source here does — the sea state's height split by period band, ten seconds through thirty — so you can watch a single groundswell arrive band by band rather than inferring it from a peak-period chart.

Height and wind speed have separate unit controls, and that is deliberate rather than fussy. A single metric/imperial switch pairs metres with km/h and feet with mph — and the unit every marine forecast on earth is actually written in, the knot, is in neither. So the two are independent and both default to the marine convention: metres and knots. Take feet without losing knots, or km/h with either. Nothing is re-fetched and no pixel changes when you switch: the picture is the same forecast, relabelled.

Direction is where it is coming FROM, the same convention as wind, on every model here. The colour wheel bands it into the sixteen compass sectors rather than a linear ramp, so opposite headings look nothing alike. The arrows and streamlines run the other way — along the direction of travel — because that is what a propagating swell actually does.

Data © ECMWF, NOAA/NCEP and Environment and Climate Change Canada. Forecast Waves is not affiliated with any of them, and is not a source of marine warnings — see About.