The scale describing wind force devised by Admiral Sir Francis Beaufort in 1808, in which winds are graded by the effect of their force (originally, the amount of sail that a fully-rigged frigate could carry).
Beaufort number: 0
Wind speed (km/h): <1
Wave Height (metres): 0
Sea Conditions: Flat
Land conditions: Calm. Smoke rises vertically.
Beaufort number: 1
Description: Light air
Wind speed (km/h): 1.1 - 5.5
Wave Height (metres): 0 - 0.2
Sea Conditions: Ripples without crests.
Land conditions: Wind motion visible in smoke.
Beaufort number: 2
Description: Light breeze
Wind speed (km/h): 5.6 - 11
Wave Height (metres): 0.2 - 0.5
Sea Conditions: Small wavelets. Crests of glassy appearance, not breaking.
Land conditions: Wind felt on exposed skin. Leaves rustle.
Beaufort number: 3
Description: Gentle breeze
Wind speed (km/h): 12 - 19
Wave Height (metres): 0.5 - 1
Sea Conditions: Large wavelets. Crests begin to break; scattered whitecaps.
Land conditions: Leaves and smaller twigs in constant motion.
Beaufort number: 4
Description: Moderate breeze
Wind speed (km/h): 20 - 28
Wave Height (metres): 1 - 2
Sea Conditions: Small waves with breaking crests. Fairly frequent white horses.
Land conditions: Dust and loose paper raised. Small branches begin to move.
Beaufort number: 5
Description: Fresh breeze
Wind speed (km/h): 29 - 38
Wave Height (metres): 2 - 3
Sea Conditions: Moderate waves of some length. Many white horses. Small amounts of spray.
Land conditions: Branches of a moderate size move. Small trees begin to sway.
Beaufort number: 6
Description: Strong breeze
Wind speed (km/h): 39 - 49
Wave Height (metres): 3 - 4
Sea Conditions: Long waves begin to form. White foam crests are very frequent. Some airborne spray is present.
Land conditions: Large branches in motion. Whistling heard in overhead wires. Umbrella use becomes difficult. Empty plastic garbage cans tip over.
Beaufort number: 7
Description: High wind, Moderate gale, Near gale
Wind speed (km/h): 50 - 61
Wave Height (metres): 4 - 5.5
Sea Conditions: Sea heaps up. Some foam from breaking waves is blown into streaks along wind direction. Moderate amounts of airborne spray.
Land conditions: Whole trees in motion. Effort needed to walk against the wind. Swaying of skyscrapers may be felt, especially by people on upper floors.
Beaufort number: 8
Description: Gale, Fresh gale
Wind speed (km/h): 62 - 74
Wave Height (metres): 5.5 - 7.5
Sea Conditions: Moderately high waves with breaking crests forming spindrift. Well-marked streaks of foam are blown along wind direction. Considerable airborne spray.
Land conditions: Some twigs broken from trees. Cars veer on road. Progress on foot is seriously impeded.
Beaufort number: 9
Description: Strong gale
Wind speed (km/h): 75 - 88
Wave Height (metres): 7 - 10
Sea Conditions: High waves whose crests sometimes roll over. Dense foam is blown along wind direction. Large amounts of airborne spray may begin to reduce visibility.
Land conditions: Some branches break off trees, and some small trees blow over. Construction/temporary signs and barricades blow over. Damage to circus tents and canopies.
Beaufort number: 10
Description: Storm, Whole gale
Wind speed (km/h): 89 - 102
Wave Height (metres): 9 - 12.5
Sea Conditions: Very high waves with overhanging crests. Large patches of foam from wave crests give the sea a white appearance. Considerable tumbling of waves with heavy impact. Large amounts of airborne spray reduce visibility.
Land conditions: Trees are broken off or uprooted, saplings bent and deformed. Poorly attached asphalt shingles and shingles in poor condition peel off roofs.
Beaufort number: 11
Description: Violent Storm
Wind speed (km/h): 103 - 117
Wave Height (metres): 11.5 - 16
Sea Conditions: Exceptionally high waves. Very large patches of foam, driven before the wind, cover much of the sea surface. Very large amounts of airborne spray severely reduce visibility.
Land conditions: Widespread damage to vegetation. Many roofing surfaces are damaged; asphalt tiles that have curled up and/or fractured due to age may break away completely.
Beaufort number: 12
Wind speed (km/h): ? 118
Wave Height (metres): ? 14
Sea Conditions: Huge waves. Sea is completely white with foam and spray. Air is filled with driving spray, greatly reducing visibility.
Land conditions: Very widespread damage to vegetation. Some windows may break; mobile homes and poorly constructed sheds and barns are damaged. Debris may be hurled about.
Scale now reads up to Force 17 determining varying strengths of hurricane:
13 Bft > 72-80 kts
14 Bft > 81-89 kts
15 Bft > 90-99 kts
16 Bft > 100- 108 kts
17 Bft > 109- 118 kts