======== Newsgroups: alt.surfing Subject: Re: Why are there sets of larger waves? From: tbmaddux@alumnae.caltech.edu (Timothy B. Maddux) Date: 5 May 1997 18:41:24 GMT In article <19970504014200.VAA25310@ladder01.news.aol.com>, Gleshna wrote: >Here's another question: Why are there sets of larger waves? I think this question already got asked and answered within the past three months; take a look at DejaNews or Ric's site to see if it's archived. >I have learned that some waves combine as they travel... They group without decreasing in number, and remain together in this group as they travel, this is something that all waves of relevance to surfers do. All waves come to shore in groups, but it is the largest and most rare that are the ones that we call 'sets'. >... [do] we have sets of bigger waves because gusts come in sets? Gusts of wind are merely turbulent fluctuations of wind speed above the mean wind velocity, and in the absence of some sort of obstruction they are essentially random. Turbulent wind speeds don't exhibit behavior that is anything like wave grouping; waves don't even require unsteady winds to form groups. -- .-``'. Timothy B. Maddux, Ocean Engineering Lab, UCSB .` .`~ http://www.engineering.ucsb.edu/~tbmaddux/ _.-' '._ "From the essence of pure stoke springs all creation." ======== Newsgroups: alt.surfing Subject: Re: Why are there sets of larger waves? From: gleshna@aol.com (Gleshna) Date: 6 May 1997 03:19:27 GMT Followup: I know this was discussed about a month or so ago, but I don't think that a concise and clear answer was achieved. Additional observations: * Waves don't travel as fast as the wind. Even in storms. * Small lakes and even rivers have sets. * Often the first and last waves in a set a smaller than the middle wave(s). I usually see sets of three, and the middle wave is always the biggest. * Wave height builds to a peak and then declines. The biggest and fastest waves are not the first to reach the shore. Bob ======== Newsgroups: alt.surfing Subject: Re: Why are there sets of larger waves? From: robt2@ix.netcom.com(Rob(t.) Brannan) Date: 6 May 1997 05:44:16 GMT In <19970506031700.XAA02969@ladder01.news.aol.com> gleshna@aol.com (Gleshna) writes: > >Followup: > >I know this was discussed about a month or so ago, but I don't think that >a concise and clear answer was achieved. > Contrary to popular belief all waves are not created equally! ======== Newsgroups: alt.surfing Subject: Re: Why are there sets of larger waves? From: adam_eberbach@qmgate.corp.apple.com (Adam Eberbach) Date: Tue, 06 May 1997 18:22:12 +1000 In article <5kmgfg$pqd@sjx-ixn8.ix.netcom.com>, robt2@ix.netcom.com(Rob(t.) Brannan) wrote: > Contrary to popular belief all waves are not created equally! It's nature's way of laughing at kooks trying to make it out the back :-) -- Adam Eberbach adam_eberbach@qmgate.corp.apple.com Newton Developer ceMis pty. ltd. 339 Coronation Dve, Milton QLD 4064 ======== Newsgroups: alt.surfing Subject: Re: Why are there sets of larger waves? From: tbmaddux@alumnae.caltech.edu (Timothy B. Maddux) Date: 8 May 1997 17:42:17 GMT In article <19970506031700.XAA02969@ladder01.news.aol.com>, Gleshna wrote: >Waves don't travel as fast as the wind. Even in storms. This sounds like something you observed visually. It's very difficult to observe a storm sea and get any idea of what sorts of wave frequencies are present since there are so many waves piling atop one another. It's even difficult to pick them out by looking at a time-series record of wave heights. A spectrum will clearly show waves present of a period large enough to move as fast as or even faster than the prevailing winds in a storm fetch. >Small lakes and even rivers have sets. ... and wave groups. >Often the first and last waves in a set a smaller than the middle >wave(s). I usually see sets of three, and the middle wave is always the >biggest. This is typical behavior for a group, as Tauras noted while windsurfing and remarked on here in the 'fetch for wave height' thread. >Wave height builds to a peak and then declines. >The biggest and fastest waves are not the first to reach the shore. Yes, the open ocean significant wave height does climb and then decrease. The wave period, however, jumps sharply with the longest-period (and hence fastest) waves always arriving first. You can see this most easily on the Santa Monica Basin buoy after a long SoCali flat spell followed by a landing S. hemi, where it has been my observation that wave height peaks within the first day of a swell and declines slowly over the course of up to four or five days. All this changes if your original swell source changes direction, wind velocity, or fetch, and frequently there is more than one source of waves in the water. -- .-``'. Timothy B. Maddux, Ocean Engineering Lab, UCSB .` .`~ http://www.engineering.ucsb.edu/~tbmaddux/ _.-' '._ "From the essence of pure stoke springs all creation." ======== Newsgroups: alt.surfing Subject: Re: Why are there sets of larger waves? From: tbmaddux@alumnae.caltech.edu (Timothy B. Maddux) Date: 8 May 1997 19:10:51 GMT In article <5kt39p$flo@gap.cco.caltech.edu>, I wrote: >... A spectrum will clearly show waves >present of a period large enough to move as fast >as or even faster than the prevailing winds in >a storm fetch. ... provided that the waves have had enough time to become 'fully developed', as described in the 'fetch for wave height' thread. If the wind blows for too short a time or over too short a distance, the waves produced will not be moving as fast as the wind. -- .-``'. Timothy B. Maddux, Ocean Engineering Lab, UCSB .` .`~ http://www.engineering.ucsb.edu/~tbmaddux/ _.-' '._ "From the essence of pure stoke springs all creation." ======== Newsgroups: alt.surfing Subject: Re: Why are there sets of larger waves? From: gleshna@aol.com (Gleshna) Date: 10 May 1997 04:06:26 GMT . provided that the waves have had enough time to become 'fully developed', as described in the 'fetch for wave height' thread. If the wind blows for too short a time or over too short a distance, the waves produced will not be moving as fast as the wind. This seems counterintuitive. Large waves can be produced by 40 mph winds, so the waves are traveling at 40 mph? What about hurricance force winds - the waves are traveling at over 75 mph? This doesn't seem right. Other than fetch, there could be some other factors limiting the maximum speed of waves? Bob ======== Newsgroups: alt.surfing Subject: Re: Why are there sets of larger waves? From: Tauras Date: Sat, 10 May 1997 12:56:21 -0700 CC: tbmaddux@alumnae.caltech.edu Gleshna wrote: > > Mad dog wrote:. > >provided that the waves have had enough > >time to become 'fully developed', as described > >in the 'fetch for wave height' thread. If the > >wind blows for too short a time or over too > >short a distance, the waves produced will not be > >moving as fast as the wind. > > This seems counterintuitive. Large waves can be produced by 40 mph winds, > so the waves are traveling at 40 mph? What about hurricance force winds - > the waves are traveling at over 75 mph? This doesn't seem right. Other > than fetch, there could be some other factors limiting the maximum speed > of waves? Friction would seem to be the limiting force. It takes fetch to get the energy out of the wind into the sinusoidal energy traveling in the water that becomes swells. Remember there is a tremendous amount of difference in density between the water and the air. I figure... for the less dense air to pump the seas up (so to speak) it takes fetch to manifest any significant wave size. (using seat of the pants 11th grade physics ;) The friction of the wave train is( usually? Tim?) dependant on winds to be favorable during its journey here as well as the normal decay of energy within the wave as it trades size for speed and it radiates from its source of origin. (Going out on a limb) It would seem to me that even though you have a nice fetch and a resulting powerful wavetrain from a distant S. Hemi pointing your way the friction from opposing winds along the way could have a significant negative effect in decreasing the significant wave height. But I could be totally off on this one ;) ~~~~Tauras Sulaitis~~~~ <<>> http://www.slonet.org/~tsulaiti/ My real email address is: tsulaiti@slonet.org ======== Newsgroups: alt.surfing Subject: Re: Why are there sets of larger waves? From: nos@nexus.cprinc.com (Nosferatu) Date: 12 May 97 01:54:03 GMT On the open sea, where wind may blow over a fetch of thousands of miles, waves travelling at different speeds are most often whipped together by a storm. Did you ever notice that larger sets have a wave period that is higher than smaller sets? I think this lends some credence to the idea of wave aggregation. Especially when coupled with the theory that sustained size can only be maintained when the height of the wave is less than one seventh the distance between the crests. Of course, I profess no authority on this subject, and expect (and maybe warrant) many flames. NoSURFatu --the surf nazi-- ======== Newsgroups: alt.surfing Subject: Re: Why are there sets of larger waves? From: tbmaddux@gear.ucsb.edu (Timothy B. Maddux) Date: 13 May 1997 09:35:08 -0700 In article <19970510040500.AAA03641@ladder01.news.aol.com>, Gleshna wrote: >This seems counterintuitive. Large waves can be produced >by 40 mph winds, so the waves are traveling at 40 mph? Sure. A linear gravity wave with a phase speed of 40 mph (C = gT/2pi, so T = 2piC/g) has a period of 11 seconds. A linear gravity wave with a phase speed of 75 mph has a period of 21.5 seconds. >What about hurricance force winds - the waves are >traveling at over 75 mph? This doesn't seem right. Feel free to check my math. We don't get many waves over 20-second periods from relatively closer storms, but we also don't get hurricane-force winds over the long fetches and durations required to produce fully developed seas. >Other than fetch, there could be some other factors >limiting the maximum speed of waves? Fetch/duration and wind speed, and that is all. The first two are grouped together as they're really expressions of the same thing: how long the wind acts over a wave as it travels. Waves care nothing for distinctions between reaching the end of a fetch or the wind dying off with time. -- .-``'. Timothy B. Maddux, Ocean Engineering Lab, UCSB .` .`~ http://www.engineering.ucsb.edu/~tbmaddux/ _.-' '._ "From the essence of pure stoke springs all creation." ======== Newsgroups: alt.surfing Subject: Re: Why are there sets of larger waves? From: Stephen Lear Date: Fri, 09 May 1997 09:03:26 -0700 Timothy B. Maddux wrote: > > In article <19970506031700.XAA02969@ladder01.news.aol.com>, > Gleshna wrote: > >Waves don't travel as fast as the wind. Even in storms. > > This sounds like something you observed visually. > It's very difficult to observe a storm sea and > get any idea of what sorts of wave frequencies > are present since there are so many waves piling > atop one another. It's even difficult to pick > them out by looking at a time-series record of > wave heights. A spectrum will clearly show waves > present of a period large enough to move as fast > as or even faster than the prevailing winds in > a storm fetch. I remember having a discussion with a TV newscaster many years ago about which hits land first - the storm (rain, wind,etc.) or the surf? I've always believed (and observed) the later. He opined that the storm arrived first and then the waves. Even when I explained that some storms never reach landfall, but the waves do, he remained unconvinced. What does the physics say? I made up a mental experiment that I'm sure isn't totally correct, but I think I'm on the right track. It goes like this: A person stands across a room from you. You take a deep breath and blow air in their direction. Time how long it takes for your breath to get there (or if it does at all). Now, hang a small diameter wooden dowell from the ceiling with a string attached to both ends, the dowell being nearly the breadth of the room. The person places his finger on one end of the dowell while I blow on the other end. His end of the dowell immeidately moves. I imagine that water is somewhere between air and a solid object like the dowell as a medium for transfering this type of energy. Therefore, the waves arrive before my breath :>) !! Or something like that! Anyway, I'd be interested in other, more qualified opinions. ~Jet Stephen P. Lear - The opinions expressed herein are mine alone nospam_slear@scruznet.com -- http://www.scruz.net/~slear ^^^^^^^ remove this portion when emailing me ======== Newsgroups: alt.surfing Subject: Re: Why are there sets of larger waves? From: John Allan Date: Sat, 10 May 1997 11:33:13 +0100 Stephen Lear wrote: > > Timothy B. Maddux wrote: > > > > In article <19970506031700.XAA02969@ladder01.news.aol.com>, > > Gleshna wrote: > > >Waves don't travel as fast as the wind. Even in storms. > > > > This sounds like something you observed visually. > > It's very difficult to observe a storm sea and > > get any idea of what sorts of wave frequencies > > are present since there are so many waves piling > > atop one another. It's even difficult to pick > > them out by looking at a time-series record of > > wave heights. A spectrum will clearly show waves > > present of a period large enough to move as fast > > as or even faster than the prevailing winds in > > a storm fetch. > > I remember having a discussion with a TV newscaster many years > ago about which hits land first - the storm (rain, wind,etc.) or the > surf? I've always believed (and observed) the later. He opined that > the storm arrived first and then the waves. Even when I explained that > some storms never reach landfall, but the waves do, he remained > unconvinced. What does the physics say? I'm not sure what Physics say - but The low pressure areas have winds that generally travel around the centr of the LOW, therefore although the wind must travel much faster than the subsequently disturbed water (waves) the wind then goes off around the ow pressure area. whilst the waves travel in straight lines, once formed. Consequently the waves have a chance of travelling direct to our favourite break, whilst the winds carry on going round and round. Sometimes though the winds may hit first, this depends on how fast the low pressure system is actualling moving. It is possible for the low pressure system to be very slow moving and continue to whip up the waves for quite sometime. On the other hand it may travel across the atlantic to towards the UK in a couple of days or so, in which case the winds will get there first and we get the swell after the winds have passed. ======== Newsgroups: alt.surfing Subject: Re: Why are there sets of larger waves? From: sidles@u.washington.edu (John Sidles) Date: 11 May 1997 05:05:24 GMT >John Allan wrote: >>Stephen Lear wrote: >>>Timothy B. Maddux wrote: >>>> Gleshna wrote: Time for Sidles to write! People first began to discover why waves appear in sets when they built a wave tank which launched an absolutely regular train of waves out of a little wave-making machine at one end of the tank. Trouble was, when this train of waves reached the other end, it was no longer regular. It had organized itself into tiny litle "sets". Hey! said the engineers. Our wave machine must not be working smoothly. So they put a lot of effort into making the tank and the machine absolutely precise and regular. But no go .. the wave trains *still* organized themselves into sets, no matter how carefully and uniformly they were launched. So the engineers sat down and studied the equations that govern wave motion. They found that certain (very tiny and usually neglected) nonlinear terms in the equations of motion of deepwater waves act to transfer energy (very slowly) from the leading and trailing edge of a set of waves, toward the center waves. Thus the central waves get bigger at the expense of leading and trailing waves. As a result, even if the wind is blowing absolutely uniformly, waves will still organize their energy into sets. This organization happens slowly, which is why local windswells are less-well-organized than swells that have propagated thousands of miles. (If my fading memory serves, this stuff is studied by scientists under the technical name of "Benjamin-Feir instability".) In any case, it is pretty cool that this obscure energy transfer mechanism, acting over thousands of miles, creates sets for us surfers! The best place to learn more about waves is the very cool and accessible paperback book "Waves and Beaches" by Willard R. Bascombe. Take a copy on your next surfari! The photos of Bascombe surfing a 20-ton WWII amphibious landing vehicle, one mile offshore on a maxed-out 20' PNW beach break swell, are more than worth the price! Cawabunga ... JAS ======== Newsgroups: alt.surfing Subject: Re: Why are there sets of larger waves? From: tbmaddux@alumnae.caltech.edu (Timothy B. Maddux) Date: 11 May 1997 01:04:38 GMT In article <33734ACE.E78@isd.com>, Stephen Lear wrote: >...which hits land first - the storm (rain, wind,etc.) or the >surf? I've always believed (and observed) the later. He opined that >the storm arrived first and then the waves. ... just goes to show you can't believe what you hear from people who are on TV, even while they're not on the TV. Unless it's a particularly fast-moving storm system, the waves produced will arrive first. Heck, we get waves all the time from the southern hemisphere storms that never cross the equator. >I made up a mental experiment... Nice. For a moving storm system, you'd need to blow continuously, moving across the room towards your target as you did so, to get a better idea of how it works. -- .-``'. Timothy B. Maddux, Ocean Engineering Lab, UCSB .` .`~ http://www.engineering.ucsb.edu/~tbmaddux/ _.-' '._ "From the essence of pure stoke springs all creation." ======== Newsgroups: alt.surfing Subject: Re: Why are there sets of larger waves? From: freddyv@geocities.com (Fred) Date: Tue, 13 May 1997 21:14:20 -0800 In article <33734ACE.E78@isd.com>, Stephen Lear wrote: >> Gleshna wrote: >> >Waves don't travel as fast as the wind. Even in storms. > >I remember having a discussion with a TV newscaster many years >ago about which hits land first - the storm (rain, wind,etc.) or the >surf? I've always believed (and observed) the later. He opined that >the storm arrived first and then the waves. Even when I explained that >some storms never reach landfall, but the waves do, he remained >unconvinced. What does the physics say? > >I made up a mental experiment that I'm sure isn't totally correct, Forget the mental experiments. Go to the library and read a book on the subject. I did, and found out that it is all fairly well understood and is often counter to what you might expect. The book I was looking at just the other day was written by a Navy such-and-such who obviously knew whereof he spoke. The only uncertainty at that time(1970) seemed to be wether or not the wave period increased as the swell traveled. Fred TubeTime! - http://www.geocities.com/TheTropics/Shores/2303/TubeTime.html ======== Newsgroups: alt.surfing Subject: Re: Why are there sets of larger waves? From: Stephen Lear Date: Wed, 14 May 1997 07:51:13 -0700 Fred wrote: > > In article <33734ACE.E78@isd.com>, > Stephen Lear wrote: > > >> Gleshna wrote: > >> >Waves don't travel as fast as the wind. Even in storms. > >I made up a mental experiment that I'm sure isn't totally correct, > > Forget the mental experiments. Go to the library and read a book on the > subject. I did, and found out that it is all fairly well understood and is > often counter to what you might expect. > Do tell -or- Title? Author? SLear (Jet) > > Fred > TubeTime! - > http://www.geocities.com/TheTropics/Shores/2303/TubeTime.html -- Stephen P. Lear - The opinions expressed herein are mine alone nospam_slear@scruznet.com -- http://www.scruz.net/~slear ^^^^^^^ remove this portion when emailing me ======== Newsgroups: alt.surfing Subject: Re: Why are there sets of larger waves? From: gadgetpjt@aol.com (Gadget PJT) Date: 6 May 1997 22:24:18 GMT gleshna@aol.com (Gleshna) writes: >Here's another question: Why are there sets of larger waves? Bloody obvious... so you can paddle out back without getting your hair wet. ;-) -- Gadget.