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Glossary

Beaufort Scale

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

Description: Calm

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

Description: Hurricane

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

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