The Beaufort Scale
The scale was created in 1805 by Sir Francis Beaufort, a British
naval officer and hydrographer. At that time naval officers made
regular weather observations, but there was no scale and so they
could be very subjective - one man's "stiff breeze" might be
another's "calm conditions". The initial scale from zero to 12
did not reference wind speed numbers, but related qualitative
wind conditions to effects on the sails of a man of war, then
the main ship of the Royal Navy, from "just sufficient to give
steerage" to "that which no canvas [sails] could withstand."
The
scale was made a standard for ship's log entries on Royal Navy
vessels in the late 1830s.
The scale was adapted to non-naval
use from the 1850s, with scale numbers corresponding to cup
anemometer rotations. In 1906, with the advent of steam power,
the descriptions were changed to how the sea, not the sails,
behaved and extended to land observations. Rotations to scale
numbers were standardised only in 1923. George Simpson, Director
of the UK Meteorological Office, was responsible for this and
for the addition of the land-based descriptors. The measure was
slightly altered some decades later to improve its utility for
meteorologists. Wind speed on the Beaufort scale can be
expressed by the formula:
- vv = 0.837 B3/2
m/s
where v is wind speed and B is Beaufort scale
number.
|
No. |
Knots |
Mph |
Description |
Effects at sea |
Effects on land |
|
0 |
0 |
0 |
Calm |
Sea like a mirror |
Smoke rises vertically |
|
1 |
1-3 |
1-3 |
Light air |
Ripples, but no foam crests |
Smoke drifts in the wind |
|
2 |
4-6 |
4-7 |
Light breeze |
Small wavelets |
Leaves rustle. Wind felt on face |
|
3 |
7-10 |
8-12 |
Gentle breeze |
Large wavelets crests, not breaking |
Small twigs in constant motion. Light flags
extended |
|
4 |
11-16 |
13-18 |
Moderate wind |
Numerous whitecaps |
Dust, leaves and loose paper raised. Small
branches move. |
|
5 |
17-21 |
19-24 |
Fresh wind |
Many whitecaps, some spray |
Small trees sway |
|
6 |
22-27 |
25-31 |
Strong wind |
Larger waves form. Whitecaps everywhere. More
spray |
Large branches move. Whistling in phone wires.
Difficult to use umbrellas |
|
7 |
28-33 |
32-38 |
Very strong wind |
White foam from breaking waves begins to be
blown in streaks |
Whole trees in motion |
|
8 |
34-40 |
39-46 |
Gale |
Edges of wave crests begin to break into
spindrift |
Twigs break off trees. Difficult to walk |
|
9 |
41-47 |
47-54 |
Severe gale |
High waves. Sea begins to roll. Spray may reduce
visibility |
Chimney pots and slates removed |
|
10 |
48-55 |
55-63 |
Storm |
Very high waves with overhanging crests. Blowing
foam gives sea a white appearance |
Trees uprooted. Structural damage |
|
11 |
56-63 |
64-72 |
Severe storm |
Exceptionally high waves |
Widespread damage |
|
12 |
63 |
73 |
Hurricane force |
Air filled with foam. Sea completely white.
Visibility greatly reduced |
Widespread damage. Very rarely experienced on
land |
|
|
