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.
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! |
