Beaufort Scale is a scale used at sea to estimate the wind speed based on observations of the sea surface and the force of wind. This scale was introduced by Irish hydrographer Francis Beaufort of the royal navy in 1805. As this scale is based on the observations of the sea surface and force of wind , Beaufort used the word 'wind force' instead of 'wind speed'.
This scale consists of 13 levels (0-12) of  Beaufort wind forces which were experienced by Beaufort as a sailor. Later the scale was accepted internationally as it was accepted by the World Meteorological Organization and , it was standardized and Beaufort numbers were attached to the probable wind speeds. The Beaufort wind force therefore the wind speed can be estimated only by observing the sea surface.
Beaufort scale wind speed characteristics of sea surface



The 13 levels (0-12) are as follows :

  • Force 0(Calm Wind) : The sea is seen like a mirror and the smoke rises vertically. The wind speed will be less than 1 knots.                                                                                                 
  • Force 1(Light wind) :  At this level slowing ripples will start to form on the surface . The direction of exhaust smoke will show the direction of wind. Wind speed can be between 1-3 knots.                                                                                                              
  • Force 2(Light breeze) : The wind is felt on the face and it makes the wind vane show the direction of wind . Small but more pronounced wavelets are formed. Wind speed will be between 4-6 knots.                                                                                                  
  • Force 3(Gentle breeze) : Formation of large wavelets and the crest of waves will break . White horses will be seen in a scattered form. Flags will start to flutter onboard. Wind speed between 7-10 knots is experienced.                                                          
  • Force 4 (Moderate breeze) : white horses( These are foam like appearances on the sea surface or breaking waves) are formed frequently. Waves will start to get a little longer in this level. Wind speed between  11-16 knots .                                                     
  • Force 5 (Fresh breeze) : Low chances of water spraying onboard and the waves get very longer. Formation of white horses increases and becomes rapid . Wind speed from 17- 21 knots.                                                                                                                            
  • Force 6 (Strong breeze) : Large waves will start to form and probability of a water spray increases . The wind speed between 22-27 knots.                                                            
  • Force 7(Near Gale force) : Difficulty in walking against the wind is seen. Spindrifts( spray blown from the crests of waves by the wind) begins to be seen. White horses are seen in streaks in the direction of wind . Wind speed  between 28-33 knots.                      
  • Force 8(Gale) : Waves of moderate height and great length are formed(5.5 -7 m) . Spindrifts are formed extensively. Wind speed between 34-40 knots.                                                           
  • Force 9(Severe Gale) : High and long waves are observed( 7-10 m). Dense streak of foam is seen in the direction of the wind. The sea will start to roll resulting the ship to roll. Spray increases and it reduces the visibility . Wind speed between 41-47 knots .            
  • Force 10(Storm) : The whole surface of the sea becomes white and rolling of the sea becomes violent. The visibility is more reduced than force 9. Very high and long waves(9-12.5m) are seen . The wind speed is 48-55 knots.                                                                     
  • Force 11(Violent Storm) : The visibility deteriorates much more and the presence of exceptionally higher waves (11.5 m to 16 m)  with overhanging crests are found. Whole sea surface is seen as white . Wind speed is between 56-63 knots.                            
  • Force 12(Hurricane) : The most violent wind force is wind force 12. The air will start to be present with foam and spray. Sea will be completely white and the visibility will be seriously affected. Wave height increases more than 14 m . The wind speed will exceed 64 knots.