WX0507 · Waves & Weather
18 questions · 18 multiple choice · 0 written · ASA 109 Marine Weather — Waves & Weather
See Beaufort scale in glossary or actual in G231.
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The Beaufort scale is designed to correlate wind speed with observable sea state conditions. It is, however, often used as in this question to refer to wind speed alone.
65 knots falls in the Beaufort force 12 range (64+ knots, hurricane force).
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The dramatic effect of the relationship between sea state and current flow is one of the key issues when applying practical knowledge of marine weather.
Wind blowing against the current creates steeper, shorter waves with greater height — a dangerous sea state.
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The final sea state we observe is usually made up of many kinds of waves.
Swells are waves that have moved beyond the area where the wind generated them.
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G238 shows a “picture” of the sea state for any wind speed. The question asks effectively about some underlying assumptions behind that diagram.
Fetch is the unobstructed distance over water that the wind blows. Waves need adequate fetch and duration to reach full potential height.
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Is a full day long enough for a 20-knot wind to produce “fully developed seas”? Also, there will be many wave speeds so we are asking about an average of some distribution, not necessarily the peak value.
Average speed about 18 kn. 24h is plenty for fully developed seas at 20 kn. The range of wave speeds is about 9 to 33 kts, with the mean around 18 kts.
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Maximum height means height of fully developed seas, which is still a statistical description.
Rule of thumb: wind must blow a number of hours equal to the wind speed in knots. 25 kn ≈ 25 hours ≈ about 1 day.
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The desired right answer here has been modified from the theoretical value to bring it more in line with the behavior of real waves.
Theoretically 1/7 but in practice closer to 1/5.
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We assume that all these winds are possible at equal strengths.
West wind blows along the longest dimension (~60 miles), providing maximum fetch.
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When waves run up onto a beach, they are slowed down as they enter the shallow water. Waves are also forced to slow down when meeting a contrary current.
When waves slow they shorten in wavelength and steepen. Energy is conserved so height increases.
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There may be other causes, this one is based on educated speculation. Such waves are very rare occurrences.
Volcanoes and earthquakes make tsunamis (very low waves at sea). Downbursts at squall lines are a leading candidate for rogue waves.
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The “floating” bridge does just that — it is a couple miles long, floating right on the surface.
The bridge interrupts the fetch. Waves on the windward side have built up over the full fetch, while the leeward side starts with essentially zero fetch.
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The idea of putting oil on water to calm the waves began in the days of whalers spilling oil overboard.
There is little scientific evidence that this works practically. The amount of oil needed, difficulty dispersing it, and environmental concerns make it impractical.
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A fundamental part of seamen’s weather terminology.
Force 5 encompasses a specific wind speed range (17–21 kn), a descriptive name (“fresh breeze”), and a sea state description. All three are part of its definition.
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What is it about the sea state implied here that could affect the fog in the atmosphere?
Waves bring cold deep water to the surface. The cooling of surface water can trigger advection fog (sea fog), the most important fog formation mechanism on the coast.
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Consider all the factors that contribute to final sea state.
It takes time to develop. Wave height won’t have changed much in 20 minutes, but the surface will be choppier with more whitecaps.
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The question applies to the oceans adjacent to the US. North of the equator.
The USCG broadcasts remain the primary and most reliable source of high seas weather voice broadcasts for waters adjacent to the US.
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Draw a picture of a Low with circulating winds and remember that waves form from and move in the direction of their driving winds. Northern Hemisphere assumed.
In the right front quadrant, wind direction and storm motion are aligned, so wave speed is effectively storm speed plus wave speed. These waves are most likely to outrun the storm.