What is the Beaufort Scale?

The Beaufort Scale is an Internationally recognised scale used by sailors to measure wind strength. Why is this important? Well, first of all, the strength, direction and time over which wind blows will have a dramatic effect on the sea state. Three days of Gales from the South West will make the English Channel a very unpleasant and potentially hazardous place to be, especially during Spring Tides.

But what is a Gale? Well, that’s where Admiral Sir Francis Beaufort’s Scale for the measurement of wind comes in. You see, whilst Admiral Lord Fitzroy had been issuing weather forecasts for shipping for some time, one man’s strong breeze might be another man’s Gale, dependent on their experience, constitution and size and type of vessel! In order to better define a wind state objectively, a scale was invented which could best identify the likely effect of a wind’s speed on the surrounding environment. 

A Gale was defined under this Scale to be a Force 8 on the Beaufort Scale, with ‘moderately high waves with breaking crests forming spindrift. Well-marked streaks of foam are blown along wind direction. Considerable airborne spray’. as opposed to a Hurricane, which is Force 12 and is described as ‘Huge waves. Sea is completely white with foam and spray. Air is filled with driving spray, greatly reducing visibility’.

Importantly, the wind strength is defined by reference to the sea state, so most sailors will quickly identify as Force 5 as the Beaufort Scale explains it to be 

FORCE 5:

  • At Sea, moderate waves of some length. Many whitecaps. Small amounts of spray.
  • On land, branches of a moderate size move. Small trees in leaf begin to sway.

The average day sailor probably considers a Force 3 to 4 to be perfect and a Force 5 to be a bracing days sailing, whereas the novice may feel rather battered by the experience of a Force 5, especially if sailing into it! In fact, a Force 6 is often referred to as ‘a yachtsman’s gale’.

Nowadays, with the advent of accurate wind instruments, many sailors also refer to wind speed in terms of knots. The easy way to make an approximate transfer from knots to the Beaufort Scale is by this simple calculation:

EXAMPLE:

True Wind Speed = 20 kts
To convert to Beaufort Scale, divide by 5 and add 1. So, 20/5 = 4 + 1 = Force 5

This works well as an approximation although each Beaufort Force is in fact a range of wind speeds with the rule of thumb set out above usually falling between the maximum and minimum parameters. It should be noted that gusts 40% higher than the forecast wind speed should be expected. This can make a solid Force 6 feel considerably more challenging, with gusts up to 38 - 40 kts!

Beaufort Number Description Wind Speed Wave Height Sea Conditions
0 Calm < 1 knot 0 m Flat.
1 Light air 1 - 3 knots 0 - 0.2 m Ripples without crests.
2 Light breeze 4 - 6 knots 0.2 - 0.5 m Small wavelets. Crests of glassy appearance, not breaking.
3 Gentle breeze 7 - 10 knots 0.5 - 1 m Large wavelets. Crests begin to break; scattered whitecaps.
4 Moderate breeze 11 - 16 knots 1 - 2 m Small waves with breaking crests. Fairly frequent whitecaps.
5 Fresh breeze 17 - 21 knots 2 - 3 m Moderate waves of some length. Many whitecaps. Small amounts of spray.
6 Strong breeze 22 - 27 knots 3 - 4 m Long waves begin to form. White foam crests are very frequent. Some airborne spray is present.
7 High wind 28 - 33 knots 4 - 5.5 m Sea heaps up. Some foam from breaking waves is blown into streaks along wind direction. Moderate amounts of airborne spray.
8 Gale 34 - 40 knots 5.5 - 7.5 m Moderately high waves with breaking crests forming spindrift. Well marked streaks of foam are blown along wind direction. Considerable airborne spray.
9 Strong gale 41 - 47 knots 7 - 10 m High waves whose crests sometimes roll over. Dense foam is blown along wind direction. Large amounts of airborne spray may begin to reduce visibility.
10 Storm 48 - 55 knots 9 - 12.5 m Very high waves with overhanging crests. Large patches of foam from crests give the sea a white appearance. Considerable tumbling of waves with heavy impact. Large amounts of airborne spray. Reduced visibility. 
11 Violent storm 56 - 63 knots 11.5 - 16 m Exceptionally high waves. Very large patches of foam, driven before the wind, cover much of the sea surface. Very large amounts of airborne spray severely reduce visibility.
12 Hurricane force > 64 knots > 14 m Huge waves. Sea is completely white with foam and spray. Air is filled with driving spray, greatly reducing visibility.

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