From a light breeze to a strong gale, wind speed can be described in numerous ways. All of which are categorized on the Beaufort Wind Force Scale.
Developed in 1805 by Sir Francis Beaufort, an officer in the UK’s Royal Navy, the scale is an empirical measure of wind speed. It relates wind speed to observed conditions at sea and over land instead of using precise measurements. Simply put, it allows a person to estimate wind speed with visual clues.
Initially, it was only used at sea and was based on the effect the wind had on the sails of a frigate – the most common type of ship in the British Navy at the time. By the mid-1800s, the scale was adapted to also reflect a certain number of anemometer rotations – a device that measures wind speed.
In the early 20thcentury, most ships transitioned to steam power and the scale descriptions were changed to reflect the state of the sea instead of the sails. Around the same time, the scale was extended to land observations. For example, the amount of leaf, branch, or whole tree movement is a visual indicator of the force of the wind.
Today, the scale has 13 categories (0 -12), with 0 representing calm winds and 12 being hurricane force. It is in use in several countries around the globe.
In the US, when winds reach force 6 or higher, the NWS begins issuing advisories and warnings for different environments. For marine areas, force 6-7 winds would prompt a small craft advisory, force 8-9 would warrant a gale wind warning, and a wind reaching force 10-11 would call for a storm warning. Force 12 would constitute a hurricane-force wind warning. On land, winds expected to reach force 6 or higher would cause a high wind warning to be issued.
If the winds are connected to a tropical cyclone, they would be measured on the Saffir-Simpson scale. The same type of special circumstances would also hold for a tornado, which would be measured on the Enhanced Fujita Scale.
The Beaufort Wind Force Scale. Credit: Isle of Wight Weather Ctr