Sunday, March 15, 2009

Estimating wind without an anemometer (wind guage)

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