Sea and Swell 

SEA [in open sea]
Description Height (metres) Effect
Calm (glassy) 0 No waves breaking on beach
Calm (rippled) 0 - 0.1 No waves breaking on beach
Smooth 0.1 - 0.5 Slight waves breaking on beach
Slight 0.5 - 1.25 Waves rock buoys and small craft
Moderate 1.25 - 2.5 Sea becoming furrowed
Rough 2.5 - 4 Sea deeply furrowed
Very rough 4-6 Sea much disturbed with rollers having steep fronts
High 6-9 Sea much disturbed with rollers having steep fronts (damage to foreshore)
Very high 9-14 Towering seas
Phenomenal over 14 Precipitous seas (experienced only in cyclones)

Sea height is the height difference between the crest of a wave and the preceding trough.



SWELL
Description Wave Length Period Wave Height
Low swell of short or average length 0 - 200 m Less than 11 sec 0-2 m
Long, low swell over 200 m Greater than 11 sec 0-2 m
Short swell of moderate height 0-100 m Less than 8 sec 2-4 m
Average swell of moderate height 100-200 m Greater than 8 sec, less than 11 sec 2-4 m
Long swell of moderate height over 200 m Greater than 11 sec 2-4 m
Short heavy swell 0-100 m Less than 8 sec over 4 m
Average length heavy swell 100-200 m Greater than 8 sec, less than 11 sec over 4 m
Long heavy swell over 200 m Greater than 11 sec over 4 m


Swell is the average height of every third wave.



Beaufort Scale - Wind Speed & Observed Effects.

Force Knots Mph Kph Sea Term Sea Conditions On-land Appearance
0 <1 <1 <1 Calm sea like a mirror smoke rises vertically
1 1-3 1-3 1-6 Light air ripples with appearance of scales: no foam crests smoke drifts and leaves rustle
2 4-6 4-7 7-11 Light breeze small wavelets: crests of glassy appearance, not breaking wind felt on face
3 7-10 8-12 12-19 Gentle breeze large wavelets: crests begin to break: scattered whitecaps flags extended, leaves move
4 11-16 13-18 20-30 Moderate breeze small waves, becoming longer: numerous whitecaps dust and small branches move
5 17-21 19-24 31-39 Fresh breeze moderate waves, taking longer form: many whitecaps: some spray small trees begin to sway
6 22-27 25-31 40-50 Strong breeze larger waves forming: whitecaps everywhere: some spray large branches move, wires whistle
7 28-33 32-38 51-61 Near gale sea heaps: white foam from breaking waves begins to be blown in streaks trees in motion, resistance felt when walking
8 34-40 39-46 62-74 Gale moderately high waves of greater length: edges of crests begin to break into spindrift: foam is blown in well-marked streaks walking impeded
9 41-47 47-54 75-87 Strong gale high waves: sea begins to roll: dense streaks of foam: spray may reduce visibility structural damage may occur
10 48-55 55-63 88-102 Storm very high waves with overhanging crests: sea takes white appearance as foam is blown in very dense streaks: rolling is heavy and visibility is reduced trees uprooted, structural damage likely
11 56-63 64-72 103-117 Violent Storm Exceptionally high waves: sea covered with white foam patches: visibility still more reduced damage to structures wide spread
12 >63 >72 >117 Hurricane air filled with foam: sea completely white with driving spray: visibility greatly reduced severe structural damage to buildings, wide spread devastation, flooding



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