Light air Ripples with appearance of scales; no foam crests
Light breeze Small wavelets;
crests of glassy appearance, not breaking Gentle breeze Large wavelets;
crests begin to break; scattered whitecaps
Moderate Small waves, breeze becoming longer;
numerous whitecaps Fresh breeze Moderate waves, taking longer form; many whitecaps; some spray Strong breeze Larger waves forming; whitecaps everywhere; more spray
Fishing smack just has steerage way
Wind fills the sails of smacks, which then travel at about 1-2 mph Smacks begin to careen and travel about 3^1 mph
Good working breeze, smacks carry all canvas with good list Smacks shorten sail
Smacks have doubled reef in mainsail; care required when fishing
Calm; smoke rises vertically Smoke drift indicates wind direction; vanes do not move Wind felt on face; leaves rustle; vanes begin to move Leaves, small twigs in constant motion; light flags extended Dust, leaves and loose paper raised up; small branches move Small trees in leaf begin to sway
Larger branches of trees in motion; whistling heard in wires
9 41-47 47-54 20.8-24.4 75-88 Strong gale
10 48-55 55-63 24.5-28.4 89-102 Storm
11 56-63 64-72 28.5-32.6 103-117 Violent storm
12 64 and 73 and 32.7 and 118 and Hurricane over over over over
Sea heaps up; white Smacks remain in foam from harbor and those breaking waves at sea lie-to begins to be blown in streaks
Moderately high All smacks make waves of greater for harbor, if near length; edges of crests begin to break into spindrift; foam is blown in well-marked streaks High waves; sea begins to roll; dense streaks of foam; spray may reduce visibility 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 Exceptionally high waves; sea covered with white foam patches; visibility still more reduced
Air filled with foam; sea completely white with driving spray; visibility greatly reduced
Whole trees in motion;
resistance felt in walking against wind
Twigs and small Very rough (4-6) 6 branches broken off trees; progress generally impeded
Slight structural damage occurs; slate blown from roofs
experienced on land; trees broken or uprooted; considerable structural damage occurs Very rarely Very high (9-14) 8
experienced on land; usually accompanied by widespread damage
This aspect is important since many underwater acoustic propagation models require an input of wind speed or wave height (and sometimes both) for specification of sea-surface roughness. This input is used to initialize an internal sub-model that generates surface losses. If the wrong statistical relationship is used for the wave heights, then errors can be introduced into the computed solution due to an improper characterization of surface roughness. It is therefore important to understand what type of statistical wave height is expected as an input by the propagation model being used.
Wind-wave generation in coastal regions may be limited by the geometry of the water body, which is often irregular. Therefore, it may be necessary to consider the effects of fetch shape (both distance and width) in order to estimate wave spectra in coastal environments, especially when sea conditions are not fully developed. The Pierson-Moskowitz spectrum, which was discussed earlier for open-ocean applications, assumed that sea conditions were fully developed.
Air bubbles are produced by the breaking of waves and are carried beneath the surface by turbulence. They are also generated in the wakes of ships where they can persist for long periods of time. Free air bubbles in the sea are quite small since the larger bubbles tend to rise quickly to the surface. Bubbles only form a very small volumetric percentage of the sea. However, because air has a markedly different density and compressibility from that of sea water and because of the resonant characteristics of bubbles (e.g. Leighton, 1994), the suspended air content of sea water has a profound effect upon underwater sound. Urick (1983: 249-54) summarized these effects, which include resonance and changes in the effective sound speed. Norton et al. (1998) developed a numerical procedure to parameterize bubble clouds in terms of an effective complex index of refraction for use in high-fidelity models of forward propagation.
Aside from reflection losses, there are other acoustic effects associated with interactions with the sea surface. A moving sea surface produces frequency-smearing and shifting effects on constant-frequency signals. Large and rapid fluctuations in amplitude or intensity are also produced by reflection at the sea surface. Furthermore, Lloyd mirror (or image-interference) effects produce a pattern of constructive and destructive interference between direct and surface-reflected signals. This effect is diminished when the sea surface is roughened by wind.
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