Significant wave height (Hs)

Significant wave height $H_s$ is the mean of the highest one-third of wave heights in a sea state, close to what a trained observer reports by eye, and equal to about $4\sqrt{m_0}$ where $m_0$ is the zeroth spectral moment (the area under the wave energy spectrum). It is the primary scalar describing sea severity, paired with a characteristic period in spectra like JONSWAP and Pierson-Moskowitz, and it sets operability limits and design wave loads.