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Force 5

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(Redirected from Force Five)

noun - Force 5 or Force Five is a measure of wind speed based mainly on observed sea conditions (on land it is categorized by the physical effects it has on vegetation and structures) of the Beaufort Wind Force Scale.

Conditions[edit]

Under Force Five conditions, the wind speed is between 29-38 km/hour or 18-24 mph or 16-20 knots or 8.0-10.7 m/s with a wave height of 2-3 meters and moderate waves of some length and many whitecaps with a small amount of spray. On land, the conditions cause branches of a moderate size to move and small trees in leaf begin to sway.

Usage[edit]

The swimmers crossed the channel under Force 5 conditions.

Beaufort Scale[edit]

The Beaufort Wind Force Scale was created in 1805 by Sir Francis Beaufort, a British admiral. 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 a top administrator in the Royal Navy in the 1830s when it was adopted officially and first used during Charles Darwin's voyage on HMS Beagle. 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.

The initial scale of thirteen classes (zero to twelve) did not reference wind speed numbers, but related qualitative wind conditions to effects on the sails of a man-of-war, then the main ship of the Royal Navy, from "just sufficient to give steerage" to "that which no canvas sails could withstand." At zero, all his sails would be up; at six, half of his sails would have been taken down; and at twelve, all sails would be stowed away.

The scale was made a standard for ship's log entries on Royal Navy vessels in the late 1830s and was adapted to non-naval use from the 1850s, with scale numbers corresponding to cup anemometer rotations. In 1916, to accommodate the growth of steam power, the descriptions were changed to how the sea, not the sails, behaved and extended to land observations. Rotations to scale numbers were standardized only in 1923. George Simpson (meteorologist), Director of the UK Meteorological Office, was responsible for this and for the addition of the land-based descriptors. The measure was slightly altered some decades later to improve its utility for meteorologists. Today, many countries have abandoned the scale and use the metric-based units m/s or km/h instead, but the severe weather warnings given to public are still approximately the same as when using the Beaufort scale.

The Beaufort scale was extended in 1946, when Forces 13 to 17 were added. However, Forces 13 to 17 were intended to apply only to special cases, such as tropical cyclones. Nowadays, the extended scale is only used in Taiwan and mainland China, which are often affected by typhoons.

Wind speed on the 1946 Beaufort scale is based on the empirical formula: v = 0.836 B3/2 m/s where v is the equivalent wind speed at 10 meters 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 scale and TORRO scales also begin roughly at the end of level 12 of the Beaufort scale but are indeed independent scales.

Note that wave heights in the scale are for conditions in the open ocean, not along the shore.

External links[edit]