Beaufort Wind Scale
Windspeed
in MPH Description - Visible Condition
0 Calm smoke rises vertically
1 - 4 Light air direction of wind shown by smoke but not by wind vanes
4 - 7 Light breeze wind felt on face; leaves rustle; ordinary wind vane moved by wind
8 - 12 Gentle breeze leaves and small twigs in constant motion; wind extends light flag
13 - 18 Moderate breeze raises dust and loose paper; small branches are moved
19 - 24 Fresh breeze small trees in leaf begin to sway; crested wavelets form on inland water
25 - 31 Strong breeze large branches in motion; telephone wires whistle; umbrellas used with difficulty
32 - 38 Moderate gale whole trees in motion; inconvenience in walking against wind
39 - 46 Fresh gale breaks twigs off trees; generally impedes progress
47 - 54 Strong gale slight structural damage occurs; chimney pots and slates removed
55 - 63 Whole gale trees uprooted; considerable structural damage occurs
64 - 72 Storm very rarely experienced; accompanied by widespread damage
73+ Hurricane devastation occurs
Conversion table for knots to miles per hour
KTS to MPH
5 Knots = 5.8 MPH
10 Knots = 11.5 MPH
15 Knots = 17.3 MPH
20 Knots = 23.0 MPH
25 Knots = 28.8 MPH
30 Knots = 34.6 MPH
35 Knots = 40.3 MPH
40 Knots = 46.1 MPH
45 Knots = 51.8 MPH
50 Knots = 57.6 MPH
55 Knots = 63.4 MPH
60 Knots = 69.1 MPH
65 Knots = 74.9 MPH
70 Knots = 80.6 MPH
Whats the Beaufort Scale ?
The Beaufort wind force scale is one of those simple things that seems to have always been around. We think we know what it is meant to tell us -- a simple numerical relationship to wind speed based on an observation of the effects of the wind.
Rear-Admiral, Sir Francis Beaufort, Knight Commander of the Bath, was born in Ireland in 1774. He entered the Royal Navy at the age of 13 and was a midshipman aboard the Aquilon. Beaufort is said to have had an illustrious career on the seas and by 1800 had risen to the rank of Commander. In the summer of 1805 Commander Beaufort was appointed to the command of the Woolwich, a 44 gun man-of-war. It was at this time that he devised his wind force scale. An early surviving form the scale is replicated below. By 1838 the Beaufort wind force scale was made mandatory for log entries in all ships of the Royal Navy. Beaufort last served as Hydrographer to the Admiralty. He died in 1857 two years after his retirement.
In examining Beaufort's scale, it catches one's attention that the scale is is a force scale. There is no mention of wind speed! Given the current applications of the scale and the fact that meteorologist are generally unfamiliar with sailing ships underway, it is easy to see that Beaufort's intentions in creating the scale may be mistaken. Beaufort's specification is essentially an association of a set of integers (0 to 12) with a description of the state and behavior of a "well-conditioned man-of-war." While the choice of numbers is quite arbitrary, as a sailor Beaufort apparently felt there were 13 levels of behavior that he could recognize in a man-of-war. Although he describes them in terms that may be vague to a modern sailor, his descriptions would certainly convey the full meaning of the force of the wind to men who shared years of sailing in ships with characteristics similar to the Woolwich.
The effect of the wind on an 18th-century fighting ship is at the heart of Beaufort's scale. Note that Beaufort intends that you look at the ship not at the wind! The scale was devised for a group of men who shared the same experience -- years of unremitting blockade of Europe in sailing ships which were all quite similar in characteristics. His descriptions are couched in terms of the ship's characteristics under sail.The descriptions for Beaufort numbers 0 through 4 describe the wind in terms of the speed that it may propel the ship; those for 5 through 9 in terms of her mission and her sail carrying ability; and those for 10 through 12 in terms of her survival. So how then did Beaufort's wind force scale ever make the jump to a wind speed scale?
Special wind scales had been routinely suggested through the years but their lives were usually as short as mayflies'. What happened after 1838, when the Royal Navy made Beaufort's scale mandatory, helps to explain its incredible longevity. In one sense the story is a tale of the triumph of technology over rational thought. It begins with a couple of gadgets -- in 1837 Samuel Morse demonstrated the first practical telegraph and in 1846 T. R. Robinson invented the cup anemometer. Neither of these inventions would have saved Beaufort's scale, however, if it weren't for a catastrophe
Wind Chill
