Beaufort, Wind Strength and Marine Weather Forecasts

Aufgewühlte Ostsee mit brechenden Wellen und Schaum

Brechende Wellen auf der aufgewühlten Ostsee.

Beaufort, Wind Strength and Marine Weather Forecasts

The Beaufort scale assigns wind speeds to grades 0 through 12: grade 0 denotes calm, while grade 12 denotes hurricane-force winds. A marine weather forecast distinguishes wind information from sea state. Beaufort describes wind strength; wave height and wave period characterize the waves.

Mean wind and gusts

The Beaufort rating refers to the sustained mean wind. Gusts are brief increases in wind speed; during showers, according to the DWD, they can be up to two Beaufort grades stronger than the mean wind. For Bft 4, the DWD sea-state table gives approximately 20 to 28 km/h, or 11 to 16 knots; for Bft 5, around 29 to 38 km/h, or 17 to 21 knots; and for Bft 6, approximately 39 to 49 km/h. Each grade represents a range of speeds.

Waves and forecast period

The significant wave height H₁/₃ is the average height of the highest third of waves over an observation period. Individual waves may be higher. The wave period is the time between successive wave crests. Wind speed, the duration of the wind’s influence, and fetch—the uninterrupted stretch of water over which the wind blows—affect the wind waves that develop; swell from other areas may also be present.

The DWD’s guideline values for fully developed wind waves give a significant wave height of about one metre for Bft 4 and around two metres for Bft 5. These values relate the Beaufort grade to sea state. Wind strength and wave height remain separate measurements: the Beaufort scale describes the wind, while H₁/₃ is the average height of the highest third of the waves.

A marine weather forecast applies to named sea areas and a specified forecast period. Climate averages, by contrast, are based on longer observation records; 30-year reference periods are generally used for climate comparisons. The seasonal overview describes typical conditions, while the specific forecast gives the wind and sea state for the stated period and location. Fog on the coast can also reduce visibility.

Photo: Karin Höll