Significant wave height

It is usually defined as four times the standard deviation of the surface elevation – or equivalently as four times the square root of the zeroth-order moment (area) of the wave spectrum.

The significant wave height (Hs) may thus refer to Hm0 or H1/3; the difference in magnitude between the two definitions is only a few percent.

SWH is used to characterize sea state, including winds and swell.

The original definition resulted from work by the oceanographer Walter Munk during World War II.

where Hm represents the individual wave heights, sorted into descending order of height as m increases from 1 to N. Only the highest one-third is used, since this corresponds best with visual observations of experienced mariners, whose vision apparently focuses on the higher waves.

Most easily, it is defined in terms of the variance m0 or standard deviation ση of the surface elevation:[6]

In case of a measurement, the standard deviation ση is the easiest and most accurate statistic to be used.

The RMS wave height, which is defined as square root of the average of the squares of all wave heights, is approximately equal to Hs divided by 1.4.

The maximum ever measured wave height from a satellite is 20.1 metres (66 ft) during a North Atlantic storm in 2011.

The Ocean Prediction Center and the Tropical Prediction Center's Tropical Analysis and Forecast Branch (TAFB) issue these forecasts.

RSMCs use wind-wave models as tools to help predict the sea conditions.