A Wind Scale: Beaufort

The Beaufort Wind Scale categorizes wind speed up to hurricane strength.

The Beaufort Wind Scale categorizes various wind speeds up to hurricane strength (74 miles per hour) and is used in weather forecasting. The scale is named after its creator, Admiral Sir Francis Beaufort of the British Navy, who developed it in the early 1800's. It is still used heavily today, primarily with maritime forecasts.

The 13-point scale, from 0 to 12, defines breezes, gales and hurricane force winds and also gives information on how various wind speeds affect land and sea.

The number assigned to a wind speed category is called the "Beaufort Number". This number also indicates the wind's characteristics and possible damaging effects.

A Beaufort Number (BN) of "0" means that the wind is blowing at less than one mile per hour. This is defined as "calm". Smoke rises vertically and the sea is still.

The Beaufort Number 1 signifies a wind speed of one to three mph. This is considered a "light air". At sea, you could expect to see ripples in the water, but no waves large enough to form foam crests. No significant effects on land occur at this stage. Smoke slants and is carried by the wind.

BN 2, with winds four to seven mph, defines a light breeze. Still no significant effects can be noticed on land, but at sea, the ripples turn into small wavelets.



BN 3 signifies winds of eight to 12 mph. This is a gentle breeze, creating small whitecaps on the ocean water. The wind is now strong enough to extend flags.

BN 4 is a moderate breeze, with a wind speed of 13 to 18 mph. This wind is strong enough to lift small leaves and twigs off the ground and move small branches on trees. This wind also creates small waves in the ocean with numerous whitecaps.

BN 5, with winds of 19 to 24 mph, is a fresh breeze. This wind produces moderate waves out in the ocean with some spray. Small trees begin to sway.

BN 6, with winds of 25 to 31 mph, is a strong breeze. Large waves form in the ocean with abundant white caps and a lot of spray. On land, large branches on trees sway. This type of wind is also associated with the average thunderstorm or squall line on land.

BN 7 signifies winds of 32-38 mph. This is defined as a moderate gale. The wind is strong enough to make entire trees sway and it begins to become difficult for a person to walk around outside, against the wind. In the ocean, large waves are splashed around and the spray is thicker and feels like large raindrops. The term "gale" is only used in marine meteorology. On land, this wind speed would be considered a strong wind, the type most usually associated with severe thunderstorms and tropical cyclones. This is also the time when a gale warning is put up. This is done by the posting of a square flag is all black with a red square in the middle.

BN 8, a fresh gale with winds of 39 to 46 mph, also signifies tropical storm force winds. Twigs and small branches are broken off trees. Out on the water, larger and higher waves develop. To warn sailors of this potentially hazardous wind, a tropical storm flag is put up. This flag is very similar to the gale warning flag, only the colors are reversed.

BN 9, with winds of 47 to 54 mph, is a strong gale. The sea begins to roll and dense streaks of foam and spray are blown about, reducing visibility. On land, slight structural damage may occur. The wind is strong enough to blow off portions of slated roofs.

BN 10 is a whole gale, with wins 55 to 63 mph. At this point, the waves are very high with overhanging crests. The sea begins to look whiter because of all the foam, and visibility is reduced even more. On land, the wind is strong enough to break large branches off trees and knock down small to medium-sized trees.

BN 11 signifies winds 64 to 72 mph. These are storm winds that create exceptionally high waves. The ocean is now covered with white foam patched. On land, widespread moderate damage could be expected. Strong tropical storms with winds of this strength have been known to create waves of six to 10 feet.

BN 12, with winds above 73 mph, signifies hurricane-force winds. At sea, the air is filled with foam and the ocean appears completely white from foam and spray, greatly reducing visibility. On land, the winds are violent and widespread destruction can be expected. Trees and power lines are downed. Twigs and other small, light objects become projectiles, causing damage to property.

Winds faster than 73 mph, hurricane force, are measured on the Saffir-Simpson Scale.

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