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Beaufort wind force scale

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Category: Weather Weather
Content source: SKYbrary About SKYbrary
Content control: SKYbrary About SKYbrary
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Tag(s) Weather Risk Management

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 scale Mean wind speed Limits of wind speed Wind descriptive terms Probable wave height Probable maximum wave height Seastate
0 0kts <1kts Calm - - 0
1 2kts 1-3kts Light air 0.1m 0.1m 1
2 5kts 4-6kts Light breeze 0.2m 0.3m 2
3 9kts 7-10kts Gentle breeze 0.6m 1.0m 3
4 13kts 11-16kts Moderate breeze 1.0m 1.5m 3-4
5 19kts 17-21kts Fresh breeze 2.0m 2.5m 4
6 24kts 22-27kts Strong breeze 3.0m 4.0m 5
7 30kts 28-33kts Near gale 4.0m 5.5m 5-6
8 37kts 34-40kts Gale 5.5m 7.5 6-7
9 44kts 41-47kts Strong (Severe) gale 7.0m 10.0m 7
10 52kts 48-55kts Storm 9.0m 12.5m 8
11 60kts 56-63kts Violent storm 11.5m 16.0m 8
12 - >63kts Hurricane >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 scale Limits of wind speed Wind descriptive terms Probable wave height Probable maximum wave height
13 72-80kts Hurricane >14.0m >16.0
14 81-90kts Hurricane >14.0m >16.0
15 91-99kts Hurricane >14.0m >16.0
16 100-109kts Hurricane >14.0m >16.0
17 110-119kts Hurricane >14.0m >16.0

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