Sea State: Wave Height, Wave Period, and Significant Wave Height

Sea-state forecasts may report wave height, wave period, and significant wave height separately. These values describe different properties of the wave field and can only be meaningfully compared when they use the same definition, unit, and time period.

Height and period measure different things

Wave height is measured from the trough to the next crest. Wave period is the time between two successive wave crests passing a fixed point.

What “significant” means

Significant wave height is the average height of the highest third of waves during a measurement period. Individual waves can be higher. For twelve waves, the four highest are averaged; values of 4, 4, 5, and 7 metres yield a figure of 5 metres.

Distinguishing wind waves from swell

Wind waves and swell are different components of the wave field. The DWD sea-state table gives approximate significant wave heights for fully developed wind waves without swell superimposed. Which components a forecast reports depends on the definition of the particular product. The marine weather report and Beaufort scale indicate wind strength; it should not be confused with wave height. On a cruise, rough seas may also make it relevant to prepare for seasickness.