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Scales and Ratings
(2005-08-20) Rationalized Beaufort Wind Scale
In "force n" weather, the wind speed is proportional to n3/2
= nÖn
The widely-used Beaufort scale was devised in 1806,
by Sir Francis Beaufort (1774-1857), rear admiral, hydrographer to the Royal Navy.
It was adopted by the British Admiralty in 1838,
and has been in international use since 1874.
Originally, the Beaufort Wind Scale did not refer to specific wind speeds,
but to the effect of the wind on a full-rigged ship, and the amount
of sail which should be carried.
Since "force 12" meant a wind that 'no canvas can withstand',
the original scale did not extend beyond that point.
Each Beaufort number still corresponds to a variety of common observations
which can be made at sea or inland.
For example, in a "force 0" condition:
'Smoke rises vertically. Sea is like a mirror.'
Since 1946, the Beaufort scale has been defined in terms of the speed of the wind,
measured by an anemometer placed 10 meters above the ground.
"Force n" means a wind speed around V.n3/2,
where V is a speed of about 1.871 mph.
Any speed V, in mph, between 62Ö26/169 and
146Ö46/529 yields agreement with
the rounded "mph" scale below, and also with the "km/h" scale, which is
somewhat less restrictive. (Some tables give 18 mph instead of 17 mph as the
upper limit for a moderate breeze; this is inconsistent
with the rest of the table, for any value of V.)
(Consistent) Beaufort Scale
Force
(n) |
Denomination of the wind |
Wind speed (V nÖn) |
| English | French | (mph) | (km/h) |
| 0 | Calm | Calme | 0 to 0.6 | 0 to 1 |
| 1 | Light air | Très légère
brise | 0.7 to 3 | 2 to 5 |
| 2 | Light breeze | Légère
brise | 4 to 7 | 6 to 11 |
| 3 | Gentle breeze | Petite brise | 8 to 12 | 12 to 19 |
| 4 | Moderate breeze | Jolie brise | 13 to 17 | 20 to 28 |
| 5 | Fresh breeze | Bonne brise | 18 to 24 | 29 to 38 |
| 6 | Strong breeze | Vent frais | 25 to 31 | 39 to 49 |
| 7 | Near gale, moderate gale | Grand
frais | 32 to 38 | 50 to 61 |
| 8 | Gale, fresh gale | Coup de vent | 39 to 46 | 62 to 74 |
| 9 | Strong gale | Fort coup de vent | 47 to 54 | 75 to 88 |
| 10 | Storm, whole gale | Tempête | 55 to 63 | 89 to 102 |
| 11 | (Violent) storm | Violente
tempête | 64 to 72 | 103 to 117 |
| 12 | Hurricane | Ouragan | over 73 | over 118 |
To find the Beaufort number corresponding to a
given speed, one divides that speed by V, and finds the whole number closest
to the cubic root of the square of that ratio.
As a result of this modern definition,
the Beaufort scale can be extended beyond the traditional limit
of "force 12" for extremely violent winds.
We have not traced the existence of a "standard" value of V; we shall simply
note that a value V = 0.8365 m/s (or any value between 0.83626 m/s and
0.8368 m/s) will agree with the above tables in mph or km/h, but
that --unexplicably-- tables published in knots imply a value of V falling
in the range 0.8401 m/s to 0.8433 m/s (once the inconsistent value of 16
knots published for the upper limit of a moderate breeze is lowered to 15 knots).
(2005-08-20) Saffir / Simpson scale for hurricanes
The customary scales for hurricanes (Beaufort force 12 and "above").
In August 1969, Hurricane "Camille" hit the Mississipi-Alabama coast
with what would be "force 23" winds in an extended Beaufort scale:
200 mph to 213 mph.
However, the Beaufort scale is rarely extended
(if ever) beyond force 12.
Instead, the strength of hurricanes is described with the following scale:
The
Saffir / Simpson Hurricane Scale
| Cat. | Pressure at center | Wind (km/h) | Surge (m) | Damage |
| 1 | above 980 hPa | 120 to 153 | 1.2m to 1.5m | Minimal |
| 2 | 965 hPa to 980 hPa | 154 to 177 | 1.8m to 2.4m | Moderate |
| 3 | 945 hPa to 965 hPa | 178 to 209 | 2.7m to 3.7m | Extensive |
| 4 | 920 hPa to 945 hPa | 210 to 249 | 4.0m to 5.5m | Extreme |
| 5 | below 920 hPa | 250 + | 5.5m + | Catastrophic |
In the Atlantic, the record-breaking hurricane season of 2005 included three
category-5 hurricanes, named Katrina, Rita and Wilma (in chronological order).
At this writing (Oct. 2005) Wilma is the most intense hurricane ever
observed in the Atlantic basin, featuring the lowest sea-level atmospheric pressure
ever recorded in the Western Hemisphere outside of
tornadoes (882 hPa).
In the Northwest Pacific Ocean, only 9 typhoons
have surpassed the intensity of Wilma.
(The terms typhoon and hurricane describe the
same phenomenon, but are used in different parts of the Globe.)
The costliest hurricane ever was hurricane Katrina
(August 23 to 31, 2005) which caused an estimated $200 billion in damages and at
least 1281 fatalities (official count at this writing).
After hitting land as a mere category-1 hurricane north of Miami on August 25,
the eye of Katrina made landfall again in Lousiana
at 6:10am (CDT) on Monday, August 29, 2005.
as a category-4 hurricane...
By 11 am, the storm surge had breached the levee
system protecting New Orleans from Lake Pontchartrain.
Most of the city was subsequently flooded.
(2005-08-20) Fujita scale for tornadoes
Local twisters are primarily measured against a 6-rung scale (F0 to F5).
Within tornadoes, the wind can reach speeds
in excess of 280 mph (450 km/h).
If the Beaufort scale was applicable, this would mean force 28 or 29.
Instead, all tornadoes are ranked using the following scale, from weakest to strongest:
The Fujita Tornado Scale
| Fn | Effects | Wind speed (km/h) |
| F0 | Twisted antennas, broken branches | 60 to 110 |
| F1 | Uprooted trees, vehicles turned over | 120 to 170 |
| F2 | Lifted rooves, small projectiles | 180 to 250 |
| F3 | Walls tipped over, large projectiles | 260 to 330 |
| F4 | Houses destroyed, some trees lifted | 340 to 410 |
| F5 | Large structures lifted, incredible damages | 420 to 510 |
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