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Global Guide to Tropical Cyclone Forecasting:
Chapter 9: Ready Reckoner
9.4 BEAUFORT SCALE
Several versions of the Beaufort wind scale exist. They mostly vary
by small degrees. The following is adapted from Pielke (1991) and Dunn and Miller (1964)
following work by Black and Adams (personal communication), and using information provided
by R. Simpson (personal communication). See also Table 4.1.
| Beaufort Force |
Estimated 10-min averaged wind (kt) at 19m. |
Description |
| 0 |
Calm |
Sea glassy. Appearance of being covered by oil. |
| 1 |
1-3 |
Slight ripple. |
| 2 |
4-6 |
Slight ripple. Isolated brief whitecaps. Unable to
determine direction. |
| 3 |
7-10 |
Gentle Breeze: Surface like wrinkled paper. Small. well
defined whitecaps of uniform size but few in number. White crests disappear quickly. First
able to tell direction but with difficulty. |
| 4 |
11-16 |
Moderate Breeze: Small foam patches. Number of breaking
crests increase slightly and are a little larger. First able to tell direction with
confidence. Wrinkle texture of surface is very evident. |
| 5 |
17-21 |
Fresh Breeze: Size and number of whitecaps and foam
patches increase significantly. Whitecaps on most wave crest. Very short streaks may
appear in foam patches. |
| 6 |
22-27 |
Strong Breeze: Well defined short streaks in foam
patches. Small whitecaps on most wave crests. Occasional medium-size foam patch or
breaker. Isolated green patches of short duration. Foam patches, short streaks, and
whitecaps (white water) cover 5-7% of sea surface. |
| 7 |
28-33 |
Moderate Gale: Medium-size breaking crest. Dense foam
patches and accompanying short streaks are numerous. Average length of streaks equal to
diameter of average foam patch. Small green patches occasionally visible. |
| 8 |
34-40 |
Fresh Gale - Tropical Cyclone: Streaks more numerous
and occasionally longer. Some streaks may appear unassociated with breaking waves or foam
patches. Area covered by whitecaps stabilises at 7-10%. Occasional large foam patch. Small
green patches continually visible with occasional moderate-size green patch. |
| 9 |
41-47 |
Strong Gale: Streaks readily apparent between foam
patches. Streak length varies from patch size to occasional regions of long, nearly
continual streaking. Streaks, patches, and breaking waves cover 15-20% of sea. 50% of foam
patches are green. |
| 10 |
48-55 |
Storm Force: Wind streaks become the most obvious
surface feature and are continuously or nearly so. Well-defined, thinly breaking waves
form on long crestlines, often preceded by short breaking wavelets giving a step-like
appearance. Occasional large foam patches are quickly fragmented and elongated into
streaks. Sea covered 2-25% by white water. |
| 11 |
56-63 |
Streaks are well-defined, parallel, thin, close
together, and continuous with very short capillary wavelets cutting across and
perpendicular to streaks, giving sea surface a 'shattered glass' effect in certain areas.
Some large breaking crests may take on 'rolling' or 'tumbling' appearance. Sea covered
30-40% by white water. |
| 12 |
64-69 |
Hurricane Force: Sea may occasionally be obscured by
spray and take on a murky appearance. Large, curved, breaking crests have undulating
effect on steaks, giving churning appearance. Streaks appear to thicken and become milky
or pale green. |
| 13 |
70-75 |
Surface features generally become murky. Streaks and
foam patches begin to sole their sharp definition and appear to smudge, thicken, or merge
together. Frequent, extremely large, almost semicircular crests outlined by thinly
breaking waves with occasional groups of large foam patches after entire crest breaks. |
| 14 |
76-80 |
Quantity of spray increases. Streaks thicken and appear
to have more depth. Previous crisp, shattered glass appearance now appears blurred. Most
features appear to be a submerged rather than a surface phenomenon, owing to obscuration.
Very short capillary wavelets which cut across streaks give surface a stressful appearance
as though undergoing compression. Sea surface 50% white. |
| 15 |
81-85 |
Sea appears flatter and entire surface takes on a
whitish/greyish cast. Streaks organise somewhat into broader, diffuse bands. All features
lose some definition and appear submerged. Surface 50-55% white. |
| 16 |
86-90 |
Many thin streaks are partially obscured and those
which can be seen may appear as bands spaced farther apart. Occasional cloud below
aircraft blots out or obscures surface. Sea appears almost flat. Whitish cast covers
60-65% of surface. |
| 17 |
91-95 |
Breaking waves and foam patches appear as diffuse,
white, puffy areas. Streaks become fuzzy bands. Surface 70-80% white. |
| 18 |
96-100 |
Cloud, spray, and foam patches merge into large, white,
indefinable areas historically referred to as 'white sheets'. Surface features have only
rough boundary definition. |
| 19 |
101-105 |
Isolated large, white puffs. Only strongest features of
previously seen thick streaks remain to be observed and result gives impression of only a
very few widely scattered and non-parallel streaks or wide bands. Whitish and greenish
cast covers 100% of surface. |
| 20 |
106-110 |
Foam patches, bands, and whitecaps merge into large
indefinable areas or white sheets. Variations in brightness are less distinct but still
result in churning appearance. |
| 21 |
>110 |
Sea 100% white and green. Only slight variation in
whiteness is apparent. |
The standard height for ship
observations in 19m, rather than the 10m used for land.
Contents Chapter9.5
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