What the wave height means, and what it doesn't
The words on a beach page here are not the words on the shipping forecast, and that is deliberate. The official scale was built in 1921 to judge whether a ship was safe.
The words this site uses
Every beach page shows one word for the sea, worked out from the forecast wave height in metres. Flat is under 0.2m. Small waves is under 0.5m. Bumpy is under 1m. Big waves is under 1.5m. Very big waves is under 2.5m. Dangerous is under 4m. Extreme is anything above that.
None of those words appears in any published scale, and that is on purpose. They are this site's own, chosen so that nobody can read them as the forecast and get a different answer from the one the forecast is giving.
Why they are not the shipping forecast's words
There is an official scale. WMO Code Table 3700, which the Met Office publishes for the Shipping and Inshore Waters forecasts and Met Éireann publishes for Irish waters, runs Calm, Smooth, Slight, Moderate, Rough, Very rough, High, Very high, Phenomenal.
It is the Douglas sea scale, devised in 1921 by a Royal Navy hydrographer to estimate the roughness of the sea for navigation. It shows: six of its ten words describe seas above 2.5m, which no bather is in, and everything under 1.25m — the entire range in which a swimmer actually decides — gets three words.
This site used to borrow three of those words. It said Slight at 0.3m, where the official scale says Smooth, and it said Rough at 1.5m, where every official source says Moderate and reserves Rough for 2.5m and above. That is worse than an invented word, because it looks authoritative and disagrees. The words were replaced in September 2026.
So each beach page now prints both: this site's word for the swim, and underneath it the official term for the same water, named and attributed. Above 2.5m this site is deliberately the louder of the two, because the official scale is judging a boat and this one is not.
There is no bather's sea state scale to use instead. The WHO, the RNLI, the RLSS and Water Safety Ireland were all checked when this was decided, and every bather-facing instrument is a flag with no metres on it. The WMO itself says it does not recommend sea state for forecasts and warnings, and recommends wave height instead.
What a wave height actually is
Significant wave height is not the height of the waves you will meet. It is the average height of the highest third of them, which means the largest waves in any given sea are roughly double the number on the page. A 1m reading is a sea with occasional 2m waves in it.
It is also modelled rather than measured. There is no buoy at your beach. The figure comes from a marine model on a grid several kilometres across, so two beaches near each other often share one reading, and a bay sheltered by a headland can be far calmer than the number suggests. It is a forecast of the open water near the coast, not a measurement of the water in front of you.
The number shown is the day's maximum, not the height at the moment you look.
Wind waves and swell are not the same sea
Beside the height, each page gives the wave period in seconds — the gap between one wave and the next — and names what kind of sea it is. Under eight seconds it says wind waves: short, steep water raised by the wind blowing here, now. Over ten it says swell: long, ordered waves that were made by weather somewhere else and have travelled.
The distinction matters more than the height. Two metres of long swell and two metres of short wind chop are different bodies of water to swim in. Swell arrives in sets, pulls harder, and is what breaks heavily on a beach that looked calm between sets.
Wind waves and swell are Open-Meteo's own names for the two parts of the sea it models, which is why this site uses them rather than words of its own.
What none of it tells you
It says nothing about rip currents, which are the thing that most often needs a lifeguard, and which can run on a day the sea looks small. It says nothing about the shape of the seabed, sandbars, or where the water is deep.
It is not a surf forecast. It has no idea which tide a spot works on.
And it is not advice about whether you should swim. A red flag, a lifeguard, or a sign at the beach beats every number on this site.
Where this comes from (6 sources)
- World Meteorological Organization · WMO Code Table 3700, its bands, and the WMO's own statement that it does not recommend sea state for forecasts and warnings and recommends wave height instead.
- Met Office · The sea state terms used in the Shipping and Inshore Waters forecasts, and the heights attached to them.
- Met Éireann · The same bands as published for Irish waters, with significant wave height named as the quantity.
- NOAA National Data Buoy Center · Significant wave height is the average of the highest third of the waves, and the largest waves are about double it.
- Open-Meteo · The marine model behind the wave height, wave period, wind wave and swell figures on each beach page, and the names of those variables.
- RNLI · That rip currents are a hazard independent of wave height, and what to do in one.