There are apparently several, not just one way, to measure waves. As explained in David Burch's book, <underline>Kayak Navigation</underline>, p. 298, the Canadians publish wave data in a measurement called "significant wave heights." Swell is illustrated on at least some of the canadian charts showing the direction and height of the various components (i.e. a vector which illustrates the swell from the cardinal points about the compass). I could not find an example of these vector diagrams, but I have seen them, I think in the Canadian Sailing Directions (my copy got left in a motel room somewhere). Definition of "SIGNIFICANT WAVE HEIGHT" = The average height of the highest 1/3 of all waves. *********************************************************************** Bascom's _Waves and Beaches_ has tables that include average wave height (H), significant wave height (H3), and H10 wave height, which is the average of the highest 10% of waves. H10 wave height is twice average wave height. In other words, in a typical sea, one out of every ten waves is twice as high as the average wave height. For all of these, wave height is measured from trough to crest. Linda and I were in Canada last week, and the Canadian marine weather reports and forecasts all used significant wave height rather than average wave height, which I think is reasonable -- after all, it is usually the highest waves that cause the most trouble, if only because they are least predictable. NOAA weather buoys also report significant wave heights in addition to average wave heights. Wave height and period are mathematically derived from accelerometer data. FMI, see http://seaboard.ndbc.noaa.gov/wave.shtml. Chuck Holst *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Chuck wrote; -- > >There are apparently several, not just one way, to measure waves. As >explained in David Burch's book, <underline>Kayak >Navigation</underline>, p. 298, the Canadians publish wave data in a >measurement called "significant wave heights." Swell is illustrated on >at least some of the canadian charts showing the direction and height >of the various components (i.e. a vector which illustrates the swell >from the cardinal points about the compass). I could not find an >example of these vector diagrams, but I have seen them, I think in the >Canadian Sailing Directions (my copy got left in a motel room >somewhere). One should not confuse the way wave conditions are reported versus the way individual waves are measured. The measurement of wave height is the distance from trough to crest or twice the amplitude. The confusion may come from assuming wave height and amplitude are the same thing. The reporting of wave conditions - average wave height, significant wave heights, etc. - are based upon many waves not individual waves. If I recall correctly the original question had to do with observations of individual wave heights not observations of wave systems. A minor point perhaps but, as Chuck points out, there is a difference between individual waves and wave heights as reported by a sea condition report. One should not assume that because wave heights are reported as being 1 meter that they will in fact be 1 meter. They will cover a range some being much higher and some much lower. Height is but one factor. Perhaps of greater interest to paddlers will be ratio of length to height. It is the steep breaking waves that cause capsizes not the long gentle old rollers. Generally for waves of the same height new waves are steeper than old waves and waves in shallow water are steeper than waves in deep water. One can put this information to good use. You can avoid steep waves by staying offshore where waves are unaffected by shoaling water and can pay heed to changing weather conditions that will create new wind driven waves. Cheers, John Winters Redwing Designs Specialists in Human Powered Watercraft http://home.ican.net/~735769/ *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
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