Beaufort scale

The scale was devised in 1805 by Irish-born Francis Beaufort (later Rear Admiral Sir Francis Beaufort), a Royal Navy officer, while serving on the HMS Woolwich. The scale that carries Beaufort's name had a long and complex evolution from the previous work of others (including Daniel Defoe the century before) to when Beaufort was Hydrographer of the Navy in the 1830s when it was adopted officially and first used during the voyage of HMS Beagle under Captain Robert FitzRoy, later to set up the first Meteorological Office (Met Office) in Britain giving regular weather forecasts. In the early 19th century, naval officers made regular weather observations, but there was no standard scale and so they could be very subjective – one man's "stiff breeze" might be another's "soft breeze". Beaufort succeeded in standardizing the scale.

Where v is the equivalent wind speed at 10 metres above the sea surface and B is Beaufort scale number. For example, B = 9.5 is related to 24.5 m/s which is equal to the lower limit of "10 Beaufort". Using this formula the highest winds in hurricanes would be 23 in the scale.

Today, hurricane-force winds are sometimes described as Beaufort scale 12 through 16, very roughly related to the respective category speeds of the Saffir–Simpson Hurricane Scale, by which actual hurricanes are measured, where Category 1 is equivalent to Beaufort 12. However, the extended Beaufort numbers above 13 do not match the Saffir–Simpson Scale. Category 1 tornadoes on the Fujita and TORRO scales also begin roughly at the end of level 12 of the Beaufort scale, but are independent scales – although the TORRO scale wind values are based on the 3/2 power law relating wind velocity to Beaufort force.

The wind speeds in different units are not mathematically equivalent; e.g. 12–19 km/h is not equivalent to 8–12 mph, and both are not equivalent to 7–10 knots. The reason is that the Beaufort scale is not an exact nor an objective scale. It was based on visual and subjective observation of a ship and of the sea. The corresponding integral wind speeds were determined later, but the values in different units were never made equivalent.

The scale is used in the Shipping Forecasts broadcast on BBC Radio 4 in the United Kingdom, and in the Sea Area Forecast from Met ?ireann, the Irish Meteorological Service. Met ?ireann issues a "Small Craft Warning" if winds of Beaufort force 6 (mean wind speed exceeding 22 knots) are expected up to 10 nautical miles offshore. Other warnings are issued by Met ?ireann for Irish coastal waters, which are regarded as extending 30 miles out from the coastline, and the Irish Sea or part thereof: "Gale Warnings" are issued if winds of Beaufort force 8 are expected; "Strong Gale Warnings" are issued if winds of Beaufort force 9 or frequent gusts of at least 52 knots are expected.; "Storm Force Warnings" are issued if Beaufort force 10 or frequent gusts of at least 61 knots are expected; "Violent Storm Force Warnings" are issued if Beaufort force 11 or frequent gusts of at least 69 knots are expected; "Hurricane Force Warnings" are issued if winds of greater than 64 knots are expected.

The most popular navigational text of the late 18th century was The New Practical Navigator by John Hamilton Moore of the Royal Navy, first published in 1772. To have exact tables to work from, Bowditch recomputed all of Moore's tables, and rearranged and expanded the work. He contacted the US publisher of the work, Edmund March Blunt, who asked him to correct and revise the third edition on his fifth voyage. The task was so extensive that Bowditch decided to write his own book, and to "put down in the book nothing I can't teach the crew." On that trip, it is said that every man of the crew of 12, including the ship's cook, became competent to take and calculate lunar observations and to plot the correct position of the ship. The New Practical Navigator was published in 1799, followed by a second edition in 1800.

By 1802, when Blunt was ready to publish a third edition, Nathaniel Bowditch and others had corrected so many errors in Moore's work that Blunt decided to publish it as the first edition of a new work, The New American Practical Navigator. The current edition of the American Practical Navigator traces its pedigree to that 1802 edition. Edmund M. Blunt continued to publish the book until 1833; upon his retirement, his sons, Edmund and George, assumed publication. The elder Blunt died in 1862; his son Edmund followed in 1866. The next year, 1867, George Blunt sold the copyright to the government for $25,000. The government has published Bowditch ever since. George Blunt died in 1878.

Each volume of the United States Coast Pilot contains comprehensive sections on local operational considerations and navigation regulations, with later chapters containing detailed discussions of coastal navigation; an appendix provides information on obtaining additional weather information, communications services, and other data. An index and additional tables complete the volume.

Information comes from field inspections, survey vessels, and various harbor authorities. Maritime officials and pilotage associations provide additional information. Each volume of Coast Pilot is updated regularly using the weekly United States Government's weekly Notice to Mariners.

Coast Pilot volumes provide more detailed information than the National Geospatial-Intelligence Agency-produced multi-volume Sailing Directions publication because Sailing Directions is intended exclusively for the oceangoing mariner

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