The Beaufort wind scale was developed and proposed in 1806, by Admiral Sir Francis Beaufort (1774-1857). Born in Ireland, Beaufort joined the Royal Navy aged 16, and quickly rose through the ranks. He began keeping a meteorological journal, recording amongst other things the effects of wind and storms. Even more so than today, in the days of sailing ships wind and weather were of great interest to mariners. All sailors had experience of different types of wind, and its effects, but there was no standard for communication. What was the difference between a strong breeze and a light wind, or a strong gale and a storm?. Beaufort's idea was to introduce a standard nomenclacture, which could be used and understood throughout the navy, and would apply globally.
Originally, Beaufort developed his scale based on the amount of sail a ship of the line would carry in certain conditions, and not on any actual windspeed measurements. Based on the numbers 0 to 12, Beaufort's was a qualitative scale, based on observation and experience. Thus win force 0 indicated a flat sea with little or no wind, and a ship would require full sail to move. At wind force 6, with strong winds and large waves, the ship would only need half its sails. Wind force 12 was indicitive of hurricane force winds, and a ship would have no sail for fear of damage. By the late 1830s the scale was the standard for log entries in the Royal Navy, and by the 1850s had been adopted by all British shipping.
With the advent of steamships, a scale based on the amount of canvas required for certain conditions was obsolete. Eventually, cup anemometers were used to measure windspeeds. Beaufort's scale numbers were still used, but each number was correlated with a windspeed range, and a description of the sea conditions that would accompany such a wind. On this scale, force 0 indicates a windspeed of less than a knot (1 nautical mile per hour), with a 'flat sea, like a mirror; calm wind'. Force 6, windspeeds of 22-27 knots, 'large waves form, and white foam crests; some spray; strong breeze; average wave height 13.5 ft'. Force 12, windspeeds in excess of 68 knots, 'hurricane winds; average wave height 45 ft'. It should be noted that the sea conditions described would be those expected in an open sea. Near to the coast, conditions could be very different, dependent on the topography of the coastline and seabed. In 1874 the scale was adopted for use in weather telegraphy, by The International Meteorological Committee.
The scale was also extended to land observations, and the windspeeds given in miles per hour. For example, on the land scale, force 2 is a wind of 4-7 mph, and is described as 'light winds; wind is felt on face and rustles leaves'. All this was standardized in 1923 by George Simpson, director of the British Meteorological Office, although additional amendments have been made throughout the ensuing years. In 1946, the scale was extended for use in the tropics, and numbers 13-17 were introduced to describe events such as tropical cyclones. The scale is still used today in the shipping forecast, broadcast twice daily by BBC Radio 4, for mariners in British waters. Internationally, the Beaufort scale is used, but with adaptations relevant to a particular countries maritime conditions. Both on land and at sea, windspeeds are usually quoted in kilometers or miles per hour.
The scale used by the British Met.Office can be seen here http://www.metoffice.gov.uk/weather/marine/guide/beaufortscale.html
This article was written with reference to the above site and Britannica online.