|
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 (age 26) had risen to the rank of Commander. In the summer of
1805 (age 31) 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 (age 83) 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.
Figures to
Denote the Force of the Wind
|
| 1 |
Light
Air |
Or
just sufficient to give steerage way. |
|
| 2 |
Light
Breeze |
Or that in which a man-of-war with all sail set, and clean
full would go in smooth water from. |
1 to
2 knots |
| 3 |
Gentle Breeze |
3 to
4 knots |
| 4 |
Moderate Breeze |
5 to
6 knots |
|
| 5 |
Fresh
Breeze |
Or that to which a well-conditioned
man-of-war could .
just carry in chase, full and by. |
Royals, &c. |
| 6 |
Strong Breeze |
Single-reefed topsails and top-gal. sail |
| 7 |
Moderate Gale |
Double reefed topsails, jib, &c. |
| 8 |
Fresh
Gale |
Treble-reefed topsails &c. |
| 9 |
Strong Gale |
Close-reefed topsails and courses. |
|
| 10 |
Whole
Gale |
Or
that with which she could scarcely bear close-reefed main-topsail
and reefed fore-sail. |
|
| 11 |
Storm |
Or
that which would reduce her to storm staysails. |
|
| 12 |
Hurricane |
Or
that which no canvas could withstand. |
| |
In 1854 the English and French
were entrenched in fighting at Sevastopool. The fleets carrying almost all
their winter supplies was struck by an intense, early winter storm on the
morning of November 14. In 12 hours the English and French suffered losses
(no less than 21 supply ships by the British alone) that exceed the most
savage fleet action that had ever been fought. In response to the losses
and with the hope that there might be some way to forecast future storms,
the British Admiralty and the French Marine jointly sponsored a weather
network -- the ancestor of the World Meteorolgoical Organization -- to
provide storm warnings. And here then is when Sir Beaufort's scale begins
its protean growth.
Since the task of forecasting storms was
commissioned partly by the Royal Navy for use by mariners and they had
made the use of Beaufort numbers mandatory, it "naturally" developed that
Beaufort numbers would be used for a meteorolgoical purpose. At the same
time, meteorologists of the time were excited about the possibilities of
the new weather net and the deployment of anemometers everywhere. And how
better to code and telegraph this wealth of new wind information than
Beaufort numbers!
Ah, but here the trouble begins. In central Europe a peasant who
had never seen the ocean, let alone an 1805 man-of-war, observed 37
revolutions of his anemometer and, after looking up the equivalent in his
conversion table, sent a Beaufort 7; his cohort in Kansas, who had never
seen the ocean either, looked up the same 37 revolutions in his table and
sent it as a Beaufort 5. The confusion only increased with the
proliferation of more than 30 sets of wind speed equivalents by 1900 --
some disagreeing by more than 100 percent. It was no longer clear just
what the old force scale meant (and few men survived who were competent to
judge what the behavior of an 1805 man-of-war would
be!).
In
1912 the International Commission for Weather Telegraphy sought some
agreement on velocity equivalents for the Beaufort scale. A uniform set of
equivalents was accepted in 1926 and revised slightly in 1946. By 1955,
wind velocities in knots replaced Beaufort numbers on weather maps. But
there were still a need for eyeball estimates by seamen to fill the gaps
in the global observing network. Thus it became imperative to relate the
seaman's guess logged in Beaufort numbers to the wind speed in knots. And
so Beaufort's scale had transfomed itself from a tool of the mariner to a
means for the meteorologist!
Meteorologists set in motion
the search to define a set of wind velocity equivalents for the Beaufort
force numbers. That the numbers were ever used to transmit anemometer
readings may well be one of those minor stories of history that has a much
more signifcant affect than warranted. If 100 years ago there had been a
way to extend weather observerations across the oceans using only the
science of meteorology, perhaps Admiral Beaufort's scale and numbers might
have been buried long ago -- preferably at sea!
|