Beaufort Scale

The Beaufort Scale

Admiral Sir Francis Beaufort of the British navy developed a system in 1805 to estimate the wind strengths without the use of instruments. It is still in use today.

 

Beaufort Scale

Wind Speed (m/s)

Wind Speed (Km/h)

Wind Speed (mph)

Description

Conditions

0

<0.5

<1

<1

Calm

Calm. Smoke rises vertically.

1

0.5-1.5

1-5

1-3

Light air

Wind motion visible in smoke.

2

2-3

6-11

4-7

Light breeze

Wind felt on exposed skin. Leaves rustle.

3

4-5

12-19

8-12

Gentle breeze

Leaves and smaller twigs in constant motion.

4

6-8

20-29

13-18

Moderate breeze

Dust and loose paper raised. Small branches begin to move.

5

9-10

30-39

19-24

Fresh breeze

Branches of a moderate size move. Small trees begin to sway.

6

11-13

40-50

25-31

Strong breeze

Large branches in motion. Whistling heard in overhead wires. Umbrella use becomes difficult. Empty plastic garbage cans tip over.

7

14-17

51-61

32-38

Moderate Gale

Whole trees in motion. Effort needed to walk against the wind. Swaying of skyscrapers may be felt, especially by people on upper floors.

8

18-20

62-74

39-46

Fresh Gale

Twigs broken from trees. Cars veer on road.

9

21-24

75-87

47-54

Strong Gale

Larger branches break off trees, and some small trees blow over. Construction/temporary signs and barricades blow over. Damage to circus tents and canopies.

10

25-28

88-101

55-63

Storm

Trees are broken off or uprooted, saplings bent and deformed, poorly attached asphalt shingles and shingles in poor condition peel off roofs.

11

29-33

102-118

64-73

Violent Storm

Widespread vegetation damage. More damage to most roofing surfaces, asphalt tiles that have curled up and/or fractured due to age may break away completely.

12

>33

>118

>73

Hurricane

Considerable and widespread damage to vegetation, a few windows broken, structural damage to mobile homes and poorly constructed sheds and barns. Debris may be hurled about.

 

Most wind turbines operate at the following wind speeds:

Cut-In Wind Speed : Scale – 3, Gentle Breeze

Operate Wind Speed : Scale – 4 to 7, Moderate Breeze to Moderate Gale

Stall Wind Speed : Scale – 7, Strong Breeze

Cut-Out Wind Speed : Scale – 8, Moderate Gale

In Hong Kong, the wind speed range of Tropical Cyclone Warning Signals are shown for your reference:

Beaufort Scale: The Standby Signal No. 1This is a stand-by signal, indicating that a tropical cyclone is centred within about 800 km of Hong Kong and may affect the territory.

Beaufort Scale: The Strong Wing Signal Number 3Strong wind is expected or blowing generally in Hong Kong near sea level, with a sustained speed of 41-62 km/h (kilometres per hour), and gusts which may exceed 110 km/h, and the wind condition is expected to persist. Winds are normally expected to become generally stronger in Hong Kong within 12 hours after the issue of this signal. Winds over offshore waters and on high ground may reach gale force.

 

Beaufort Scale: The Number 8 Northwest Gale or Storm Signal   Beaufort Scale: The Number 8 Southwest Gale or Storm Signal   Beaufort Scale: The Number 8 Northeast Gale or Storm Signal   Beaufort Scale: The Number 8 Southeast Gale or Storm Signal

Gale or storm force wind is expected or blowing generally in Hong Kong near sea level, with a sustained wind speed of 63-117 km/h from the quarter indicated and gusts which may exceed 180 km/h, and the wind condition is expected to persist.

 

Beaufort Scale: The Increasing Gale or Storm Signal Number 9 Gale or storm force wind is increasing or expected to increase significantly in strength.

 

Beautfort Scale: The Hurricane Signal Number 10Hurricane force wind is expected or blowing with sustained wind speed reaching upwards from 118 km/h and gusts that may exceed 220 km/h.