Significant wave height
The wave field is a combination of waves of different height, length and directions. The waves are measured typically for 15–30 minutes. The variables describing the waves, like the significant wave height, are calculated from these measurements.
Wave height is the vertical difference between the wave through and the wave crest.
The significant wave height is a useful way to describe the sea state. It has been defined to approximate the wave heights visually estimated by experienced mariners. The significant wave height corresponds also the average of one third of the highest waves in a measured wave record when the sea is deep from the wave´s standpoint.
Precisely, the significant wave height is calculated from the energy spectrum of the measured waves. The wave spectrum tells how the energy is distributed to different wave lengths and directions. The square root of the energy spectrum´s variance is multiplied by four to get the significant wave height.
The highest individual wave in a wave field can be nearly two times higher than the significant wave height. If the highest individual wave can´t be measured directly, it can be estimated by calculating from the significant wave height.