Quezi

Strong wind today in virtual land (graphic by fdecomite - CC-BY)

The Beaufort scale measures approximate wind speed, by visual observation. Such empirical scales had been in use for centuries at least, before Admiral Sir Francis Beaufort developed his scale in 1805 by adding visual descriptions that could be used by sailors to estimate the winds.

Subsequently the scale was revised several times, and in 1906 was extended for land use.

Although the Beaufort scale is not accurate enough for meteorological record-keeping, it is useful for estimating wind speeds from a moving vehicle, or in the distance such as at the top of a hill. The Beaufort terms (such as “gale force”) are still in use for weather reporting.

Force 0, Calm (zero to 1 knot, or nautical mile per hour): On land it’s calm, and smoke rises vertically. On the sea, it’s like a mirror.

Force 1, Light Air (1 to 3 knots): On land, the direction of wind is shown by smoke drift, but not by wind vanes. On the sea, there are ripples with the appearance of scales, but no foam crests.

Force 2, Light Breeze (4 to 6 knots at ground, 4 to 7 knots at 10 meters above ground): On land, wind is felt on the face, leaves rustle and wind vanes move. At sea there are small wavelets, still short but more pronounced. Crests have a glassy appearance and do not break.

Force 3, Gentle Breeze (7 to 10 knots at ground, 8 to 12 knots at 10m): Leaves and small twigs are in constant motion, and the wind extends a light flag. At sea there are large wavelets. Their crests begin to break. There’s foam of glassy appearance, and perhaps scattered “white horses”.

Force 4, Moderate Breeze (11-16 knots at ground, 13-18 knots at 10m): The wind raises dust and loose paper, small branches are moved. The sea has small waves, becoming larger, with fairly frequent “white horses”.

Force 5, Fresh Breeze (17-21 knots at ground, 19-24 knots at 10m): Small trees in leaf begin to sway, crested wavelets form on inland waters. At sea there are moderate waves, taking a more pronounced long form. Many white horses are formed.

Force 6, Strong Breeze (22-27 knots at ground, 25-31 knots at 10m): Large branches are in motion, whistling is heard from telephone wires, and umbrellas are used with difficulty. Large waves begin to form; the white foam crests are more extensive everywhere, with probably some spray.

Force 7, Near Gale (28-33 knots at ground, 32-38 knots at 10m): Whole trees are in motion, inconvenience is felt when walking against the wind. The sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind.

Force 8, Gale (34-40 knots at ground, 39-46 knots at 10m): The wind breaks twigs off trees and generally impedes progress. The sea has moderately high waves of greater length, the edges of crests begin to break into spindrift. The foam is blown in well-marked streaks along the direction of the wind.

Force 9, Severe Gale (41-47 knots at ground, 47-54 knots at 10m): Slight structural damage occurs, with chimney-pots and slates removed. At sea there are high waves and dense streaks of foam along the direction of the wind. Crests of waves begin to topple, tumble and roll over. Spray may affect visibility.

Force 10, Storm (48-55 knots at ground, 55-63 knots at 10m): Seldom experienced inland; trees are uprooted and considerable structural damage occurs. At sea there are very high waves with long overhanging crests. The resulting foam, in great patches, is blown in dense white streaks along the direction of the wind. On the whole the surface of the sea takes on a white appearance. The “tumbling” of the sea becomes heavy and shock-like. Visibility is affected.

Force 11, Violent Storm (56-63 knots at ground, 64-72 knots at 10m): Very rarely experienced inland, accompanied by widespread damage. The sea has exceptionally high waves which may for a time obscure the view of small and medium-size ships. The sea is completely covered with long white patches of foam lying along the direction of the wind. Everywhere the edges of the wave crests are blown into froth. Visibility is affected.

Force 12, Hurricane (64-71 knots at ground, 73-83 knots at 10m): The air is filled with foam and spray. The sea is completely white with driving spray. Visibility is very seriously affected.

One knot equals approximately 1.852 kilometers per hour.

The Beaufort scale has been supplemented by the Enhanced Fujita Scale for use at the higher speeds encountered in hurricanes.