Beaufort scale

The Beaufort scale is an empirical measure for the intensity of the wind based mainly on sea-state or wave conditions. Its full name is the Beaufort wind force scale. It should be noted that the wave heights given pertain to the conditions found at open sea.

Beaufort number	Wind speed at 10 m above sea level (knots)	Description	Wave height (m)	Sea conditions	Land conditions
0	less than 1	Calm	0	Flat.	Calm.
1	2	Light air	0.1	Ripples without crests.	Wind motion visible in smoke.
2	5	Light breeze	0.2	Small wavelets.	Wind felt on exposed skin. Leaves rustle.
3	9	Gentle breeze	0.6	Large wavelets.	Leaves and smaller twigs in constant motion.
4	13	Moderate breeze	1	Small waves.	Dust and loose paper raised. Small branches begin to move.
5	19	Fresh breeze	2	Moderate (1.2 m) longer waves. Some foam and spray.	Smaller trees sway.
6	24	Strong breeze	3	Large waves with foam crests and some spray.	Large branches in motion. Umbrella use becomes difficult.
7	30	Near gale	4	Sea heaps up and foam begins to streak.	Whole trees in motion. Effort to walk against the wind.
8	37	Gale	5.5	Moderately high waves with breaking crests forming spindrift. Streaks of foam.	Twigs broken from trees.
9	44	Severe gale	7	High waves (2.75 m) with dense foam. Wave crests start to roll over. Considerable spray.	Light structure damage.
10	52	Storm	9	Very high waves. the sea surface is white and there is considerable tumbling. Visibility is reduced.	Trees uprooted. Considerable structural damage.
11	60	Violent storm	11.5	Exceptionally high waves.	Widespread structural damage.
12	64 and higher	Hurricane	14+	Huge waves. Air filled with foam and spray. Sea completely white with driving spray. Visibility very greatly reduced.	Massive and widespread damage to structures.

The scale was created by the British naval commander Sir Francis Beaufort around 1805. The initial scale did not have wind speeds, but listed a set of qualitative conditions from 0 to 12 by how a naval vessel would act under them - from 'just sufficient to give steerage' to 'that which no canvas could withstand'. The scale was made a standard part of log entries for Royal Navy vessels in the late 1830s.

The scale was adapted to non-naval use from the 1850s, with the Beaufort numbers being tied to cup anemometer rotations. The rotations to number was standardised only in 1923 and the measure was slightly altered some decades later to improve its utility for meteorologists. Today, hurricanes are sometimes numbered 12 through 16 using the Saffir-Simpson Hurricane Scale, with a Category 1 hurricane bearing Beaufort number 12, a Category 2 hurricane, Beaufort 13, and so on.