In the lyrics of Billy Joel's Storm Front, we hear of the Beaufort Scale, "We've got a low pressure system and a northeast breeze. We've got a falling barometer and rising seas. We've got the cumulonimbus and a possible gale. We've got a force nine blowing on the Beaufort scale."
Admiral Sir Francis Beaufort developed a scale that allows sailors to judge wind speed by observing its effect on the state of the sea and performance of Royal Navy vessels in 1806. The scale helped reduce the subjectivity of varied reports coming from different weather observers. The scale was adapted for use on land in the early 1900's.
A benefit of the scale is that a sailor can take a forecast wind speed and compare it to the Beaufort Scale, which allows the sailor to picture the sea conditions expected prior to leaving port. Photographs related to each of the sea states associated with the Beaufort can be seen a page hosted by Western Michigan University.
A description for each number of the Beaufort Scale is listed below. Estimated wind speed is shown as an equivalent speed at 10 meters above the sea surface. National Weather Service weather warnings for the appropriate scale numbers are listed in the notes.
Beaufort Scale Number 0 – Calm
- Wind Speed: 0-1 Knots
- Description of Seas: Sea like a mirror
- Sea Disturbance Number: 0
- Probable Wave Height: 0 meters
Beaufort Scale Number 1 – Light Air
- Wind Speed: 1-3 Knots (1-5 kph)
- Description of Seas: Ripples with the appearance of scales are formed, but without foam crests
- Sea Disturbance Number: 0
- Probable Wave Height: 0.05-0.10 meters
- Notes: Yachts just have steerage way
Beaufort Scale Number 2 – Light Breeze
- Wind Speed: 4-6 Knots (6-11 kph)
- Description of Seas: Small wavelets, still short, but more pronounced. Crests have a glassy appearance and do not break
- Sea Disturbance Number: 1
- Probable Wave Height: 0.10-0.15 meters
- Notes: Wind fills sails of yacht, which then may move at 1-2 knots
Beaufort Scale Number 3 – Gentle Breeze
- Wind Speed: 7-10 Knots (12-19 kph)
- Description of Seas: Large wavelets. Crests begin to break. Foam of glassy appearance. Perhaps scattered white horses
- Sea Disturbance Number: 2
- Probable Wave Height: 0.6 meters
- Notes: Yachts start to careen and travel at 3-4 knots
Beaufort Scale Number 4 – Moderate Breeze
- Wind Speed: 11-16 Knots (20-28 kph)
- Description of Seas: Small waves, becoming larger; fairly frequent white horses
- Sea Disturbance Number: 3
- Probable Wave Height: 1.0 meters
- Notes: Good working breeze for yachts; carry all sail with good list
Beaufort Scale Number 5 – Fresh Breeze
- Wind Speed: 17-21 Knots (29-38 kph)
- Description of Seas: Moderate waves, taking a more pronounced long form; many white horses are formed. Chance of some spray
- Sea Disturbance Number: 4
- Probable Wave Height: 2.0 meters
- Notes: Small Craft Warning 18-33 knots, Yachts shorten sail
Beaufort Scale Number 6 – Strong Breeze
- Wind Speed: 22-27 Knots (39-49 kph)
- Description of Seas: Large waves begin to form; the white foam crests are more extensive everywhere. Probably some spray
- Sea Disturbance Number: 5
- Probable Wave Height: 3.0 meters
- Notes: Small Craft Warning 18-33 knots, Yachts with double reef in mainsail; care required when fishing
Beaufort Scale Number 7 – Near Gale / Moderate Gale
- Wind Speed: 28-33 Knots (50-61 kph)
- Description of Seas: Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind
- Sea Disturbance Number: 6
- Probable Wave Height: 4.0 meters
- Notes: Small Craft Warning 18-33 knots, Yachts remain in harbor, those at sea lie to
Beaufort Scale Number 8 – Gale / Fresh Gale
- Wind Speed: 34-40 Knots (62-74 kph)
- Description of Seas: Moderately high waves of greater length; edges of crests begin to break into spindrift. The foam is blown in well-marked streaks along the direction of the wind
- Sea Disturbance Number: 6
- Probable Wave Height: 5.5 meters
- Notes: Gale Warning 34-47 knots, All yachts make for harbor if possible
Beaufort Scale Number 9 – Severe Gale / Strong Gale
- Wind Speed: 41-47 Knots (75-88 kph)
- Description of Seas: High waves. Dense streaks of foam along the direction of the wind. Crests of waves begin to topple, tumble and roll over. Spray may affect visibility
- Sea Disturbance Number: 6
- Probable Wave Height: 7.0 meters
- Notes: Gale Warning 34-47 knots
Beaufort Scale Number 10 – Storm / Whole Gale
- Wind Speed: 48-55 Knots (89-102 kph)
- Description of Seas: Very high waves with long over-hanging crests. The resulting foam, in great patches, is blown in dense white streaks along the direction of the wind. On the whole the surface of the sea takes on a white appearance. The 'tumbling' of the sea becomes heavy and shock-like. Visibility affected
- Sea Disturbance Number: 7
- Probable Wave Height: 9.0 meters
- Notes: Storm Warning 48-63 knots
Beaufort Scale Number 11 – Violent Storm / Storm
- Wind Speed: 56-63 Knots (103-117 kph)
- Description of Seas: Exceptionally high waves (small and medium-size ships might be for a time lost to view behind the waves). The sea is completely covered with long white patches of foam lying along the direction of the wind. Everywhere the edges of the wave crests are blown into froth. Visibility affected
- Sea Disturbance Number: 8
- Probable Wave Height: 11.0 meters
- Notes: Storm Warning 48-63 knots
Beaufort Scale Number 12 – Hurricane
- Wind Speed: 64+ Knots (118+ kph)
- Description of Seas: The air is filled with foam and spray. Sea completely white with driving spray; visibility very seriously affected
- Sea Disturbance Number: 9
- Probable Wave Height: 11.0+ meters
- Notes: Hurricane Warning 64+ knots
Howtoons has an awesome cartoon graphic describing and showing the Beaufort Scale.
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