Beaufort Wind Scale
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The Beaufort scale of wind force was developed in 1805 by British navy officer and hydrographer Francis Beaufort. It is not based on instrumental measurement but on observation alone.
1-3 knots 2-5km/h (1-3mph). Sea lightly rippled. Direction shown by smoke drift but not by wind vanes.
4-6 knots 6-11 km/h (4-7 mph). Small wavelets; crests do not break. Wind felt on face; leaves rustle; wind vanes moved by wind; flags wave slightly.
7-10 knots 12-19 km/h (8-12 mph). Large wavelets; crests begin to break; possible scattered white horses. Leaves and twigs in constant motion; light flags extended.
11-16 knots 20-29 km/h (13-18 mph). Small waves becoming longer; fairly frequent white horses. Raises dust and loose paper; small branches moved; flags flap.
17-21 knots 30-39 km/h (19-24mph). Moderate waves taking a more pronounced long form; many white horses; the chance of some spray. Small trees in leaf begin to sway; crested wavelets form on inland waters.
22-27 knots 40-50 km/h (25-31mph) . Large waves form; white foam crests more extensive; probably some spray. Large branches in motion; whistling heard in telephone and overhead power lines; umbrellas used with difficulty.
28-33 knots 51-62 km/h (32-38 mph). Whole trees in motion; inconvenience felt when walking against the wind. Sea heaps up; foam blown in streaks along direction of wind; spindrift begins to be seen.
34-40 knots 63-75 km/h (39-46 mph). Moderately high waves; edges of crests break in to spindrift; foam is blown in well marked streaks along the direction of the wind. Twigs break off trees; progress generally impeded.
41-47 knots 76-87 km/h (47-54mph). High waves; sea begins to roll; spray affects visibility. Branches break off trees; slight structural damage, roofing dislodged.
48-55 knots 88-102 km/h (55-63 mph). Very high waves with long overhanging crests; sea takes on white appearance; heavy rolling of the sea. Seldom experienced inland; trees uprooted; considerable structural damage.
56-63 knots 103-117 km/h (64-72mph). Exceptionally high waves;small and medium sized ships occasionally lost from view behind waves; edges of wave crests blown into froth. Rarely experienced inland; widespread damage.
64 knots or more 118 km/h or more (75 mph or more). Air filled with foam and spray; sea completely white with driving spray; visibility seriuosly affected. Very rarely experienced inland; severe and extensive damage.
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