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A wind-estimation system developed in 1806 by Admiral Sir Francis Beaufort of the British Navy. Admiral Beaufort related wind speed and sea surface conditions and designed a 0-12 (calm to hurricane) wind scale with descriptions sea conditions at each level. Beaufort's scale was adopted by the U.S. Navy in 1838. Presently, an adaption of the Beaufort Scale, called the Universal Sea State Code, is used internationally and ranges from 0 to 9.

Sea State Code & Description

Sea like a mirror; wind less than one knot. Average wave height is 0.

A smooth sea; ripples; no foam; very light winds, 1-3 knots, not felt on face. Average wave height is from 0-0.3 m (0-1 ft).

A slight sea; small wavelets; winds light to gentle, 4-6 knots, felt on face; light flags wave. Average wave height is 0.3-0.6 m (1-2 ft).

A moderate sea; large wavelets, crests begin to break; winds gentle to moderate, 7-10 knots; light flags fully extend. Average wave height is 0.6-1.2 m (2-4 ft).

A rough sea; moderate waves, many crests break, whitecaps, some wind-blown spray; winds moderate to strong breeze, 11-27 knots; wind whistles in the rigging. Average wave height is 1.2-2.4 m (4-8 ft).

A very rough sea; waves heap up, forming foam streaks and spindrift; winds moderate to fresh gale, 28-40 knots; wind affects walking. Average wave height is 2.4-4.0 m (8-13 ft).

A high sea; sea begins to roll, forming very definite foam streaks and considerable spray; winds a strong gale, 41-47 knots; loose gear and light canvas may be blown about or ripped. Average wave height is 4.0-6.1 m (13-20 ft).

A very high sea; very high, steep waves with wind driven overhanging crests; sea surface whitens due to dense coverage with foam; visibility reduced due to wind blown spray; winds at whole gale force, 48-55 knots. Average wave height is 6.1-9.1 m (20-30 ft).

Mountainous seas; very high-rolling breaking waves; sea surface foam covered; very poor visibility; winds at storm level, 56-63 knots. Average wave height 9.1-13.7 m (30-45 ft).

Air filled with foam; sea surface white with spray; winds 64 knots and above. Average wave height is 13.7 m and above (45 ft and above).
Source: An Introduction to the World's Oceans. Duxbury, Duxbury & Sverdrup. McGraw Hill, 2000.

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