A blue-green ocean surface with waves and sea foam.

Beaufort wind force scale

The Beaufort scale, which is used in Met Office marine forecasts, is an empirical measure for describing wind intensity based on observed sea conditions.

Specifications and equivalent speeds
Beaufort wind scale Mean Wind Speed Limits of wind speed Wind descriptive terms Probable wave height Probable maximum wave height Seastate Sea descriptive terms
Knots ms-1 Knots ms-1 in metres1 in metres2
0 0 0 <1 <1 Calm - - 0 Calm (glassy)
1 2 1 1-3 1-2 Light air 0.1 0.1 1 Calm (rippled)
2 5 3 4-6 2-3 Light breeze 0.2 0.3 2 Smooth (wavelets)
3 9 5 7-10 4-5 Gentle breeze 0.6 1.0 3 Slight
4 13 7 11-16 6-8 Moderate breeze 1.0 1.5 3-4 Slight - Moderate
5 19 10 17-21 9-11 Fresh breeze 2.0 2.5 4 Moderate
6 24 12 22-27 11-14 Strong breeze 3.0 4.0 5 Rough
7 30 15 28-33 14-17 Near gale 4.0 5.5 5-6 Rough-Very rough
8 37 19 34-40 17-21 Gale 5.5 7.5 6-7 Very rough - High
9 44 23 41-47 21-24 Strong gale3 7.0 10.0 7 High
10 52 27 48-55 25-28 Storm 9.0 12.5 8 Very High
11 60 31 56-63 29-32 Violent storm 11.5 16.0 8 Very High
12 -   64+ 33+ Hurricane 14+ - 9 Phenomenal

Notes

  1. These values refer to well-developed wind waves of the open sea.
  2. The lag effect between the wind getting up and the sea increasing should be borne in mind.
  3. The official term is Strong gale, however, the Met Office uses the descriptive term Severe gale

To convert knots to mph multiply by 1.15, for m/s multiply by 0.514.