Beaufort wind force scale

Definition

The Beaufort wind force scale, usually referred to as just the Beaufort scale, was developed over 200 hundred years ago and is an empirical measure for describing wind intensity based on observed sea conditions. Vestiges of it are still used in marine forecasts around the world.

History

The Beaufort Scale was developed by Sir Francis Beaufort of the U.K. Royal Navy in 1805. He related sea conditions to estimated wind speeds in terms that other mariners could relate to such as “gale, storm, and hurricane”. For example, “sea like a mirror” corresponded to “calm winds”, whereas “moderately high waves; edges of crests begin to break into the spindrift” corresponded to “gale force winds”. The scale was derived to help the sailing ships of that time operate under a variety of wind and sea conditions. The basic scale had 13 categories from 0 (calm winds) to 12 (hurricane force winds).

Because it was impossible to ascertain accurate wind speeds on a sailing vessel in those days, there were no specific wind speed values given. Later, as the science of meteorology progressed, estimated wind speeds for the various categories were added as were possible wave heights. A land-based Beaufort scale was also developed in 1874.

Today, the Beaufort Scale can be used to estimate wind speeds when no instrumentation is available. Conversely, if a wind speed forecast is provided, corresponding outside conditions can be anticipated. The U.K. Met Office, the U.S. National Weather Service, and other national weather services still use the terms “gale’, “storm”, and “hurricane force wind warnings” in their marine forecasts.

Specifications and equivalent speeds

Beaufort wind scaleMean wind speedLimits of wind speedWind descriptive termsProbable wave heightProbable maximum wave heightSeastate
00kts<1ktsCalm--0
12kts1-3ktsLight air0.1m0.1m1
25kts4-6ktsLight breeze0.2m0.3m2
39kts7-10ktsGentle breeze0.6m1.0m3
413kts11-16ktsModerate breeze1.0m1.5m3-4
519kts17-21ktsFresh breeze2.0m2.5m4
624kts22-27ktsStrong breeze3.0m4.0m5
730kts28-33ktsNear gale4.0m5.5m5-6
837kts34-40ktsGale5.5m7.56-7
944kts41-47ktsStrong (Severe) gale7.0m10.0m7
1052kts48-55ktsStorm9.0m12.5m8
1160kts56-63ktsViolent storm11.5m16.0m8
12->63ktsHurricane>13m-9

Extended 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. Other regions use the Saffir-Simpson Hurricane Wind Scale to further categorize tropical cyclone wind speeds.

Beaufort wind scaleLimits of wind speedWind descriptive termsProbable wave heightProbable maximum wave height
1372-80ktsHurricane>14.0m>16.0
1481-90ktsHurricane>14.0m>16.0
1591-99ktsHurricane>14.0m>16.0
16100-109ktsHurricane>14.0m>16.0
17110-119ktsHurricane>14.0m>16.0

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