The Beaufort Scale is an empirical measure for describing wind speeds.
The scale was created in 1806 by Sir Francis Beaufort an Irish born, British Admiral and Hydrographer. The scale consists of thirteen classes:-
|0||Calm – Smoke rises vertically||Less than 1|
|1||Light Air – Smoke shows direction||1-3|
|2||Light Breeze – Wind felt, weather vanes move||4-7|
|3||Gentle Breeze – Leaves in motion light flag extended||8-12|
|4||Moderate Breeze – Dust raised, small branches move||13-18|
|5||Fresh Breeze – Small trees in leaf sway||19-24|
|6||Strong Breeze – Large branched in motion||25-31|
|7||Moderate Gale – Whole trees in motion||32-38|
|8||Fresh Gale – Twigs break off, walking impeded||39-46|
|9||Strong Gale – Slight structural damage to buildings||47-54|
|10||Whole Gale – Trees uprooted, severe building damage||55-63|
|11||Violent Storm – Widespread damage to structures||64-75|
|12||Hurricane – Massive widespread damage to structures||Above 75|
How windy is it?
With sound we all know when an engine is noisy, but it is only by using a meter that we know how noisy. The same applies to wind speeds. Of course we dont need to set up an anemometer to know if it is windy, but often our estimation of wind speed is not very accurate and can be influenced by such things as chill factors etc.
This was a serious problem for seamen for many years as there were several systems for judging wind speeds in use. So at the beginning of the 19th century, Admiral Sir Francis Beaufort devised his famous scale. This covered 13 wind strengths based upon the number of sails a full rigged ship could carry. This scale was used for 100 years, when it was changed to make it more useful to landlubbers (and now aeromodellers).
The speeds are in m.p.h. which is how the forecasters show them on T.V. In full size aviation wind information is gven as “Wind Vectors”.
For example 270/15. So the wind is due west at 15 knots.
1 knot = 1 nautical mile per hour. A nautical mile is 6080 ft.
Thus a wind speed of 20 knots = about 23 m.p.h.