What About Waves?

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wc-SWANwaves A wave in nature can be defined as any pattern with some roughly identifiable periodicity in time or space.  Patterns are found everywhere in nature and so are waves.  Waves exist in the ocean, in the atmosphere and in interstellar space.  In biology you may think of waves of migration, growth and decay.  Earthquakes produce shock waves.  Electromagnetic forces yield radio waves, micro-waves and waves of light, electricity and radiation.  We even see waves in stock market prices, boat sales and social media activity.  Often, the closer you look the more waves you will find.  In the atmosphere we have long waves, short waves, gravity waves and density waves.  In the ocean waves can range in length between tsunami (several hundred miles) to a capillary wave or ripple (less than half an inch). Water waves help shape the sky and seascapes of the Florida Keys on a daily basis.  They can be more or less out of mind on a calm, summer morning or they can serve as constant companions on a day with a moderate breeze.  Occasionally, they can quite literally make one sick to the stomach and rarely, they may build rapidly, surge violently and carry an unforgettable destructive force. Wind waves grow and decay according to the speed, duration and fetch of the wind.  An individual wave height is the vertical distance between the crest and trough.  Wave period is the time elapsed between successive traveling wave crests (or troughs).  Swell is a word for waves which have moved out of the region from which they were born.  When waves get together they form a “sea”.  In fact, the term “seas” appears routinely in the coastal waters forecasts prepared by NOAA/National Weather Service marine forecasters.  Seas represent a statistical wave measure called “significant wave height” and they generally include both wind waves and swell.  The idea of a significant wave height or “seas” grew out of a project designed to improve wave forecasting for marine military operations in World War II.  The significant wave height tends to be the wave height most readily observed by the human eye.  Statistically, it is the average of the highest one third of the waves in a wave field.  And therein lays the rub:  the average of the highest one third of the waves. What does this mean?  It means that in any sea, you will always encounter individual waves both higher and lower than the significant wave height, a fact of which every seasoned skipper is well aware.  Long records of wave behavior collected from sea buoys in the open ocean show that about one out of every 10 waves will be about 27% higher than the significant wave height.  In addition, one out of every 100 waves will be about 67% higher, and about one out of every 1000 waves will be close to twice the significant wave height!  Think about the implications.  Seas of 2 feet yield a maximum expected wave height of 4 feet.   However, let us imagine that the seas are 4 feet, with a dominant wave period of 6 seconds.  In this situation, you should encounter a wave of 5 feet about once a minute, and a wave approaching 7 feet about once every 10 minutes.  In fact, if you are in seas of 4 feet for an hour and a half, you may very well encounter a wave approaching 8 feet!  This kind of wave behavior applies to a “fully developed sea” and mainly to the open ocean where waves do not shoal, reflect, refract or diffract too much.  When seas are in a rapidly building or subsiding phase, the statistical theory breaks down. As the water depths decrease below 20 feet wave motion becomes increasingly modulated by “boundary effects” which refer to the sea floor, islands, shoals and other barriers.  Wave activity in these regions seems best expressed through words rather than by numbers (for example, “smooth”, “light chop”, “choppy”, “rough”).  Wave behavior is more complicated closer to shore, especially around the Keys where water depths are shallow with numerous barriers like the reef tract, islands and shoals.  Numerous deeper channels are interspersed among the flats, each providing a conduit for waters moving both seaward and shoreward as well as nutrients and the marine wildlife they attract.  As such, the nearshore environment teems with life both under and over the membrane which separates sea from sky. The sea state in the waters adjoining the archipelago which is the Florida Keys is highly dependent upon wind direction because of its great influence on resulting wind fetch.  A directional wind change of perhaps only 20 degrees on the compass can mean the difference between leeward and windward in the Florida Keys.  Such a change could result in seas of 1 foot versus 4 feet, or “smooth” versus “rough”, or “go” versus “no go”.  Navigating a vessel through such complex waters in a high wind usually requires a healthy stock of local knowledge possessed by an experienced skipper. Forecasting waves in such an environment has in the past been more of an art than a science.  However, advances in both science and technology offer some hope for improvement in the future.  Increases in computer power during the last decade have allowed Florida Keys marine weather forecasters to apply “shallow water wave physics” to the problem of local wave prediction.  A wave model named “SWAN” has been adapted for this purpose (the acronym stands for “simulating waves nearshore”).  Of course, a good wave forecast always depends on a sound wind forecast and the wind forecast is not perfect all the time, everywhere.  In addition, wave data are scarce in the Florida Keys and come mainly in the form of subjective reports from attentive captains (which are greatly appreciated!).  Without systematic wave observations it is difficult to verify or validate a wave model.  Nevertheless, marine weather forecasters can now offer a visual to complement the words.  It is not perfect, but it is a step in the right direction. Remember to be weather-ready and stay safe!


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