Significant Wave Height
Abbreviation: Hs
Statistical measure of sea state: average height of the highest one-third of waves observed.
In practice
Forecast values from numerical models (ECMWF, GFS) underpin every weather routing decision. SWH > 5 m typically prompts speed reduction or route change.
Regulatory detail & full definition
Significant wave height (Hs, also written SWH) is a statistical measure of sea state defined as the average height of the highest one-third of the waves in a record; it approximates the wave height a trained observer would report by eye. It is the standard wave-height value in all marine forecasts (NOAA, ECMWF, the Met Office) and in ship-routing and stability assessments. Because it is an average of the largest third, individual waves are higher: roughly 1 in 10 waves reaches about 1.7× Hs, and rare rogue waves can approach or exceed 2× Hs.
Authoritative source: WMO Sea State Code ↗
Related
Source: WMO Sea State Code; IMO MSC.1/Circ.1228 (Operational guidance to masters in adverse weather)
High confidence
Editorial confidence: how we grade.