Geoscience Reference
In-Depth Information
Table 7.2 The modern Beaufort scale
Beaufort
number
Description
Wind
speed (m/s)
Wave
height (m)
Sea conditions
0
Calm
<0.3
0
Flat
1
Light air
0.3
1.5
0
0.2
Ripples without crests
-
-
2
Light breeze
1.6
3.4
0.2
0.5
Small wavelets
-
-
3
Gentle breeze
3.5
-
5.4
0.5
-
1
Large wavelets
4
Moderate breeze
5.5
7.9
1
2
Small waves with breaking
crests
-
-
5
Fresh breeze
8.0
10.7
2
3
Moderate waves of some
length
-
-
6
Strong breeze
10.8
-
13.8
3
-
4
Long waves begin to form
7
High wind,
moderate gale,
near gale
13.9
-
17.1
4
-
5.5
Sea heaps up
8
Gale, fresh gale
17.2
20.7
5.5
7.5
Moderately high waves with
breaking crests forming
spindrift
-
-
9
Strong gale
20.8
24.4
7.5
9
High waves whose crests
sometimes roll over
-
-
10
Storm, whole
gale
24.5
-
28.4
9
-
11.5
Very high waves with over-
hanging crests
11
Violent storm
28.5
-
32.6
11.5
-
14
Exceptionally high waves
12
Hurricane-force
≥
32.7
≥
14
Huge waves
intended to apply only to special cases, such as tropical cyclones. Nowadays, the
extended scale is only used in Taiwan and mainland China, which are often affected
by typhoons. Wind speed on the 1946 Beaufort scale is based on the empirical
formula:
837B
3
=
2
m
v
¼
0
:
=
s
where v is the equivalent wind speed at 10 m above the sea surface and B is
Beaufort scale number. For example, B = 9.5 is related to 24.5 m/s which is equal to
the lower limit of
“
10 Beaufort
”
. Using this formula the highest winds in hurricanes
would be 23 in the scale.
Today, hurricane force winds are sometimes described as Beaufort scale 12
through 16, very roughly related to the respective category speeds of the Saf
r-
Simpson Hurricane Scale, by which actual hurricanes are measured, where Cate-
gory 1 is equivalent to Beaufort 12. However, the extended Beaufort numbers
above 13 do not match the Saf
r-Simpson Scale. Category 1 tornadoes on the Fujita
and TORRO (TORnado and storm Research Organization) scales also begin
roughly at the end of level 12 of the Beaufort scale but are indeed independent
scales. The TORRO scale (or T-Scale) is a scale measuring tornado intensity
between T0 and T11. It was developed by Meaden (1985) of the TORRO a
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