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)
Observations: Tree leaves don't move, smoke rises vertically, sea is calm
Force 1
Strength: Light Air
Speed: 1-3 mph
Observations: Tree leaves don't move, smoke drifts slowly, sea is lightly rippled
Force 2
Strength: Slight Breeze
Speed: 4-7 mph
Observations: Tree leaves rustle, flags wave slightly, small wavelets or scale waves
Force 3
Strength: Gentle Breeze
Speed: 8-12 mph
Observations: Leaves and twigs in constant motion, small flags extended, long un-breaking 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
Observations: Small trees sway, flags flap and ripple, moderate waves with many whitecaps
Force 6
Strength: Strong Breeze
Speed: 25-31 mph
Observations: Large branches sway, flags beat and pop, larger waves with regular whitecaps
Force 7
Strength: Moderate Gale
Speed: 32-38 mph
Observations: Whole trees sway, large waves ("heaping sea")
Force 8
Strength: Fresh Gale
Speed: 39-46 mph
Observations: Twigs break off trees, moderately high sea with blowing foam
Force 9
Strength: Strong Gale
Speed: 47-54 mph
Observations: Branches break off trees, shingles blown from roofs, high crested waves
Force 10
Strength: Whole Gale
Speed: 55-63 mph
Observations: Some trees blown down, damage to buildings, high churning white sea
Force 11
Strength: Storm
Speed: 64-74 mph
Observations: Widespread damage to trees and buildings, mountainous waves
Force 12
Strength: Hurricane
Speed: 75 mph or greater
Observations: Severe and extensive damage
