GUIDE WAVE ANALYSIS AND FORECASTING - WMO

The descriptions of wind effects on the sea surface
which are used by mariners for observing the various
intensities of the wind have not been chosen arbitrarily.
The criteria are the result of long experience and represent
individually distinguishable steps on a specific
scale. Originally, these steps were characterized by
descriptive terms, such as light breeze and moderate
breeze, or gale and strong gale. When, in the last
century, weather reporting became an operational practice,
the steps were given numbers, and the wind scale
was named after Admiral Beaufort who introduced this
coding system in 1814 in the British navy. The
Beaufort scale was recommended for international use
in 1874.
Since it is possible for an experienced observer to
distinguish one step from another, a wind observation
should be accurate to within a half of one scale interval
— a fact which has indeed been verified empirically.
However, wind effects on the sea surface are sometimes
modified by other phenomena, of which the observer
may be unaware. The plankton content of the sea water
has an influence on foam forming, air stability affects
the steepness of waves to some extent, while strong
currents may also change the form of waves and hence
the general appearance of the sea state. As a result, the
standard deviation of a wind observation is, on the average,
somewhat greater than a half of one scale interval: it
amounts to 0.58 I (I being the width of the scale interval)
for each of the steps 1–10 of the Beaufort scale. This
means that the standard deviation of an individual
wind-speed observation varies from 0.76 m/s at step 1
(wind speed 2.0 m/s) to 1.34 m/s at step 5 (wind speed
10.2 m/s) and 2.6 m/s at step 10 (wind speed 24.2 m/s)
(Verploegh, 1967).
It should be stressed that wave height is not a criterion
used in wind observations at sea. Any mariner is
all too well aware that wave height depends on other
factors as well — such as the duration and fetch of the
wind. The influence of fetch, for instance, is clearly
noticeable when the ship is in the lee of land, or when
there are abrupt wind changes in open seas.
Many studies have been made to determine wind
speeds equivalent to the steps of the Beaufort scale
(WMO, 1970) and such studies are continuing at present
(Lindau, 1994). A scale recommended for scientific use
is given in Table 2.1. It shows the equivalent wind speed
for each Beaufort number and equivalent scale intervals
in metres/second (m/s) and in knots.
Since this scientific scale has not been introduced
for the operational reporting of wind speeds from ships,
ships’ observers use an older conversion table which was
introduced internationally in 1948. This table (using
knots) is given in Table 2.2 (WMO, 1990). For the
purpose of wave forecasting, therefore, a wind report
(expressed in knots) should be converted back into the
original Beaufort scale number and then converted into
the correct wind speed of the scientific scale. This
slightly complicates the work, but many years of
OCEAN SURFACE WINDS 19
TABLE 2.1
Conversion scales for Beaufort wind force
Beaufort Descriptive
Scale recommended for use
in wave forecasting
number term Equivalent
wind speed
Intervals
(m/s) m/s kn
0 Calm 0.8 1 or 0 0–2
1 Light air 2.0 2 3–5
2 Light breeze 3.6 3–4 6–8
3 Gentle breeze 5.6 5–6 9–12
4 Moderate breeze 7.8 7–9 13–16
5 Fresh breeze 10.2 9–11 17–21
6 Strong breeze 12.6 12–14 22–26
7 Near gale 15.1 14–16 27–31
8 Gale 17.8 17–19 32–37
9 Strong gale 20.8 19–22 38–43
10 Storm 24.2 23–26 44–50
11 Violent storm 28.0 26–30 51–57
12 Hurricane - 31 and 58 and
above above
wave-forecasting practices have shown that important
systematic errors are avoided in this way.
The wind direction is much easier to determine
from the orientation of the crests of wind waves. The
standard deviation of an individual observation of wind
direction amounts to 10° (Verploegh, 1967) and appears
to be independent of wind speed. The direction of
constant trade or monsoon winds can be determined with
greater accuracy.
The mix of wind observations between visual
estimates and anemometers varies considerably from
one ocean area to another. Anemometer winds are
prevalent in the Pacific whereas most observations in the
Atlantic are visual estimates. Surprisingly, a study by
TABLE 2.2
Scales in use in international reports
Beaufort Descriptive Range of values reported
number term by observers (kn)
0 Calm