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Beaufort Scale for Wind Speed Estimation

The Beaufort Wind Scale is named after Sir Francis Beaufort, an admiral in the British Navy. He developed the scale in 1805 in order to estimate wind speed by noting how sails move in the wind. It turned out to be a great help and was later adapted for use on land.


Force 0

Strength: Calm
Speed: Less than 1 mile per hour (mph), less than 2 kilometers per hour (kph)
Observations: Tree leaves don't move, smoke rises vertically, sea is calm


Force 1

Strength: Light Air
Speed: 1-3 mph, 2-6 kph
Observations: Tree leaves don't move, smoke drifts slowly, sea is lightly rippled


Force 2

Strength: Slight Breeze
Speed: 4-7 mph, 7-11 kph
Observations: Tree leaves rustle, flags wave slightly, small wavelets or scale waves


Force 3

Strength: Gentle Breeze
Speed: 8-12 mph, 12-19 kph
Observations: Leaves and twigs in constant motion, small flags extended, long unbreaking waves


Force 4

Strength: Moderate Breeze
Speed: 13-18 mph, 20-29 kph
Observations: Small branches move, flags flap, waves with some whitecaps


Force 5

Strength: Fresh Breeze
Speed: 19-24 mph, 30-39 kph
Observations: Small trees sway, flags flap and ripple, moderate waves with many whitecapes


Force 6

Strength: Strong Breeze
Speed: 25-31 mph, 40-50 kph
Observations: Large branches sway, flags beat and pop, larger waves with regular whitecaps


Force 7

Strength: Moderate Gale
Speed: 32-38 mph, 51-61 kph
Observations: Whole trees sway, large waves ("heaping sea")


Force 8

Strength: Fresh Gale
Speed: 39-46 mph, 62-74
Observations: Twigs break off trees, moderately high sea with blowing foam


Force 9

Strength: Strong Gale
Speed: 47-54 mph, 75-87 kph
Observations: Branches break off trees, shingles blown from roofs, hight crested waves


Force 10

Strength: Whole Gale
Speed: 55-63 mph, 88-101 kph
Observations: Some trees blown down, damage to buildings, high churning white sea


Force 11

Strength: Storm
Speed: 64-74 mph, 101 kph-119 kph
Observations: Widespread damage to trees and buildings, mountainous waves


Force 12

Strength: Hurricane
Speed: 75 mph or greater, 120 kph or greater
Observations: Severe and extensive damage

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF