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Beaufort Scale - www.webstation.com.au - WebStation Australia
Beau- fort No Description Cyclone category Average wind speed (Knots) Average wind speed (kilometres per hour) Specifications for estimating speed over land Specifications for estimating speed over water
0 Calm   Less than 1 less than 1 Calm, smoke rises vertically. Sea like mirror
1 Light Air   1 - 3 1 - 5 Direction of wind shown by smoke drift, but not by wind vanes. Ripples with the appearance of scales are formed, but without foam crests
2 Light breeze   4 - 6 6 - 11 Wind felt on face; leaves rustle; ordinary wind vane moved by wind. Small wavelets, still short, but more pronounced; crests have a glassy appearance and do not break
3 Gentle breeze   7 - 10 12 - 19 Leaves and small twigs in constant motion; wind extends light flag. Large wavelets; crests begin to break; foam of glassy appearance; perhaps scattered white horses
4 Moderate breeze   11 - 16 20 - 28 Raises dust and loose paper; small branches moved. Small waves, becoming longer; fairly frequent white horses
5 Fresh breeze   17 -21 29 - 38 Small trees in leaf begin to sway; crested wavelets form on inland waters. Moderate waves, taking a more pronounced long form; many white horses are formed (chance of some spray)
6 Strong breeze   22 - 27 39 - 49 Large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty. Large waves begin to form; the white foam crests are more extensive everywhere (probably some spray)
7 Near gale   28 - 33 50 - 61 Whole trees in motion; inconvenience felt when walking against the wind. Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind
8 Gale Category 1 34 - 40 62 - 74 Breaks twigs off trees; generally impedes progress. Moderately high waves of greater length; edges of crests begin to break into the spindrift; the foam is blown in well-marked streaks along the diection of the wind
9 Strong gale Category 1 41 - 47 75 - 88 Slight structural damage occurs (chimney pots and slates removed). High waves; dense streaks of foam along the direction of the wind; crests of waves begin to topple, tumble and roll over; spray may affect visibility
10 Storm Category 2 48 - 55 89 - 102 Seldom experienced inland; trees uprooted; considerable structural damage occurs. 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 a white appearance; the tumbling of the sea becomes heavy and shock-like; visibility affected
11 Violent storm Category 2 56 - 63 103 - 117 Very rarely experienced; accompanied by widespread damage. Exceptionally high waves (small and medium sized ships might be for a time lost to view behind the waves); 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 affected
12 Hurricane Category 3,4,5 64 and over 118 and over Severe and extensive damage. The air is filled with foam and spray; sea completely white with driving spray; visibility very seriously affected

An average wind is averaged over 10 minutes, and is read at a height of 10 metres above open flat ground. Values for gusts are higher than average wind values.

WIND STRENGTH DEFINITIONS AND DAMAGE POTENTIAL

Cyclone Category System

CATEGORY 1
Negligible house damage. Damage to some crops, trees and caravans. Craft may drag moorings.
A Category 1 cyclone's stongest winds are GALES with gusts to 125 km/h.
These winds correspond to Beaufort 8 and 9 (Gales and strong gales).

 
CATEGORY 2
Minor house damage. Significant damage to signs, trees and caravans. Heavy damage to some crops. Risk of power failure. Small craft may break moorings.
A Category 2 cyclone's strongest winds are DESTRUCTIVE winds with gusts of 125 -170 km/h.
These winds correspond to Beaufort 10 and 11 (Storm and violent storm).

 
CATEGORY 3
Some roof and structural damage. Some caravans destroyed. Power failures likely.
A Category 3 cyclone's strongest winds are VERY DESTRUCTIVE winds with gusts of 170 - 225 km/h.
These winds correspond to the highest category on the Beaufort scale, Beaufort 12 (Hurricane).

 
CATEGORY 4
Significant roofing loss and structural damage. Many caravans destroyed and blown away. Dangerous airborne debris. Widespread power failures.
A Category 4 cyclone's strongest winds are VERY DESTRUCTIVE winds with gusts of 225 - 280 km/h.
These winds correspond to the highest category on the Beaufort scale, Beaufort 12 (Hurricane).

 
CATEGORY 5
Extremely dangerous with widespread destruction.
A Category 5 cyclone's strongest winds are VERY DESTRUCTIVE winds with gusts of more than 280 km/h.
These winds correspond to the highest category on the Beaufort scale, Beaufort 12 (Hurricane).

 
As well as indicating the expected maximum wind gusts near the centre of the cyclone, warning messages will usually indicate the strength of the maximum wind gusts expected over particular areas in any of the following terms:
GALES with gusts to 125 km/h
DESTRUCTIVE winds with gusts above 125 km/h
VERY DESTRUCTIVE winds with gusts above 170 km/h

Extract of information from Bureau of Meteorology web site - http://www.bom.gov.au/