The Beaufort Wind Force Scale is a scale of nautical wind force with twelve steps, plus five added solely for use with hurricane-force winds. The original standardised measure was based upon the amount and type of sail a well-conditioned man of war ship could or should carry in specific wind conditions, with Force 12 (hurricane) winds being defined as when the ship could show no canvas: this version was approved for general use throughout the British Navy in 1838. Land descriptors were added in the 1850s.
In 1903, a revised formula was introduced:
V = 1.87 x square root (B^3)
B represents the Beaufort scale number
V represents the corresponding wind speed 30 ft above the surface of the sea (mph)
As naval transporation became increasingly less dependent upon wind power, the sail descriptors were altered to sea descriptors (1906). Five additional hurricane-force steps were added in 1946, though these are rarely used, most countries preferring to use the Saffir-Simpson scale for hurricanes.
The following descriptors are currently standardised:
BEAUFORT FORCE 0 - CALM
Winds under 1 knot (0 kph)
Open Sea: The surface of the water is smooth or mirror-calm.
Land: Smoke rises vertically and appears undisturbed by the wind. Tree leaves do not move.
BEAUFORT FORCE 1 - LIGHT AIR
Winds of 1-3 knots (1-6 kph)
Open Sea: The water surface has scaly ripples without crests.
Land: Wind direction is indicated in the drift of smoke. However, the wind is not strong enough to move a weather vane.
BEAUFORT FORCE 2 - LIGHT BREEZE
Winds of 4-6 knots (7-11 kph)
Open Sea: Small wavelets are observed with crests that take on a glassy appearance but still do not break.
Land: Wind can be felt on the face or exposed skin. Tree leaves rustle and weather vanes begin to move. Light flags may wave slightly.
BEAUFORT FORCE 3 - GENTLE BREEZE
Winds of 7-10 knots (12-19 kph)
Open Sea: Large wavelets are observed. Crests begin to break and there are scattered whitecaps.
Land: Leaves and twigs are in constant motion while the wind will extend light flags.
BEAUFORT FORCE 4 - MODERATE BREEZE
Winds of 11-16 knots (20-29 kph)
Open Sea: Small waves of 1-4 feet in length are observed and whitecaps are more numerous.
Land: The wind will raise dust, leaves, and loose paper. Small branches in trees begin to move.
BEAUFORT FORCE 5 - FRESH BREEZE
Winds of 17-21 knots (30-38 kph)
Open Sea: Moderate waves of 4-8 feet are observed with many whitecaps and some spray and foam.
Land: Smaller trees in leaf begin to sway.
BEAUFORT FORCE 6 - STRONG BREEZE
Winds of 22-27 knots (39-50 kph)
Open Sea: Larger waves of 8-13 feet are observed. Whitecaps are now common with more foam and spray.
Land: Large tree branches move, the wind whistles in overhead wires, and the use of an umbrella becomes difficult.
BEAUFORT FORCE 7 - NEAR GALE
Winds of 28-33 knots (51-61 kph)
Open Sea: The surface heaps up with waves of 13-20 feet and white foam streaks can be observed off breakers.
Land: Whole trees are moving in the wind while walking becomes affected by the wind.
BEAUFORT FORCE 8 - GALE
Winds of 34-40 knots (62-74 kph)
Open Sea: Waves are moderately high at 13-20 feet and of greater length. Breaking crests begin to form spindrift and foam is blown in streaks.
Land: Whole trees are in motion with twigs being broken from them. Walking in the wind becomes very difficult and cars veer on the road due to wind.
BEAUFORT FORCE 9 - STRONG GALE
Winds of 41-47 knots (75-86 kph)
Open Sea: High waves consistently 20 feet or more are observed. The sea begins to roll and has dense streaks of foam while considerable spray may reduce visibility.
Land: Light structural damage will occur such as the removal of slate or shingles from roof tops. Tree branches break due to wind force.
BEAUFORT FORCE 10 - STORM
Winds of 48-55 knots (87-101 kph).
Open Sea: Very high waves of 20-30 feet with overhanging crests are observed. The sea is white with densely blown foam and is rolling heavily. Visibility is lowered.
Land: Trees are broken or uprooted. Structural damage is considerable.
BEAUFORT FORCE 11 - VIOLENT STORM
Winds of 56-63 knots (102-120 kph)
Open Sea: Waves are 30-45 feet in height and the sea is covered with white foam patches. Visibility is further reduced.
Land: Structural damage is widespread and considerable.
BEAUFORT FORCE 12 HURRICANE
Winds in excess of 64 knots (120+ kph)
Open Sea: Waves exceed 45 feet, the air is filled with foam and the sea is completely white with driving spray. Visibility is greatly reduced.
Land: Structural damage is severe and extensive.