This has nothing to do with board design, but can any of you veteran surfers give me some info on buoy reports? I live in Hawaii, I know that 17 seconds and above indicate ground swell and 14 sec and below is wind swell. I would just like some more info on the subject so that way i can learn more about the ocean’s ways better, Aloha & Mahalo.
Pick up the book Waves and Beaches, by Willard Bascom. Everything you would ever what to know about waves. In fact more then you may want to know. This book is kind of hard to find but it is worth look for.
As I remember from Bascom’s book: Multiply an open ocean swell’s period in seconds by 1.65 to get the wave set’s speed in miles per hour. A buoy 500 miles away measures a 20 second swell period headed in your direction. Speed = 20 x 1.65 = 33 mph. To calculate arrival time: 500 miles / 33 mph = 3 hours 38 minutes travel time. It’s 6:00 am, so the measured set will arrive at 9:38 am. Bascom died a few years ago. Thanks for the legacy, Willard.
You consider 17 and up groundswell? Damn, there is never groundswell in the North atlantic then. regards, Håvard
In the western North Atlantic swell period above 10 seconds is a “ground swell” and enough to suit up in booties, gloves, and hoods no matter the weather. Rob Olliges
Noodle, You made me go back and read. I think you are off a little but I’m sorry to say your math sucks. To get the speed of a wave in open ocean you multiply the period in sec. by 3.5. This gives you the speed in MPH. But since the front wave is over taken by the second wave and so on and so on. The speed of the set of waves can be rough estimated by V=(T X 3.5).5. or in other words multiply the period in sec. X 3.5, then multiply this X .5. So if you have a buoy that is 500 miles away and you have a swell moving at 20 sec. period. You would have 20 X 3.5= 70 X.5 = 35. 500/35 = 14.28 hours. Please if I’m wrong on this I will say up front to Noodle I’m sorry. But I think I’m right. Can someone correct me or tell me I’m right. TK
there’s a bunch of info online, try reading up at the link below: http://www.stormsurf.com/page2/tutorials/menu.html
I think the Stormsurf tutorial is a little more driven towards surfing, but there some good info here too… http://www.co-ops.nos.noaa.gov/restles1.html
I thought it was more like X 2.3 or 2.5 , I might be wrong. Pat Caldwell used to explain it on his page. I think Mike Perry does as well - www.surfalert.tm Here is some info. from Stormsurf : Chop tends to have a period ranging from 3-8 seconds. That is, there is anywhere from 3-8 seconds between each wave crest. Wind waves range from 9-12 seconds. Ground swells range from 13-15 seconds, and strong ground swells have a period anywhere from 16-25 or more seconds. The next time you’re in the water, try measuring the amount of time between wave crests. Though shallow water tends to distort the times as compared to what you’ll see reported at buoys, the comparative effect is still the same. What you should find is that on days where the surf seems weak, the period will be short. And on days where waves of equivalent height seem strong, the period will be longer. You should also find that on long period swells, it seems like a lot more water is moving around as compared to shorter period swells of the same height. That’s because it is! Longer period swells have a lot more energy and are moving a lot faster than their short period counterparts, and so there’s a lot more water movement. A 3 foot swell with an 8 second period moves at about 12.48 nautical miles per hour. But the same 3 foot swell with a 22 second period moves at about 34.32 nautical miles per hour. That’s almost 3 times as fast! And moreso, the longer the period, the larger the wave will be when it breaks (all other things being equal). That’s because a longer period swell affects water much deeper in the ocean than short period swells. Long period waves move faster and deeper. In short, swell period is more important than height. Also, swell speed is directly proportional to its period. It’s a linear relationship. As period increases so does swell speed. And all swells of the same period travel at the same speed, regardless of size. That’s right. A 2 ft swell with a 20 sec period moves the same speed as a 25 ft swell with a 20 sec period. If you can predict the period of a forecasted swell, and know how far away the storm is from your location (in nautical miles), you can accurately determine the arrival time of the swell, regardless of it’s size.
In the western North Atlantic swell period above 10 seconds is a “ground >swell” and enough to suit up in booties, gloves, and hoods no matter the >weather. Yup. That’s the way it is on the eastern North Atlantic too. Albeit I do think 12 sec is the limit for really good surf. bdw. The local buoy peaked at 21.79 at 14 of feburary this winter. I saved that html page for the record(and broke my leash). regards, Håvard
here in hawaii anything above 13sec can yield good surf depending on the swell height and other factors. remember the period is just one part of the equasion.
Your calculations yield a 1.56 period-to-speed constant… not far off from my 1.65 constant. Using this method is far from exact. It’s an approximation, so the error bars for either of our constants would probaby be outside the other constant. As I remember the bigger errors come from geodetics (earth’s curvature), from water currents, and from bottom features. A wave’s energy reaches 1/2 its wavelength down into the water. There are 3600 seconds in an hour. The 20 second period wave traveling at 33 mph has 3600/20=180 waves crammed into 33 miles. There are 5280 feet in a mile, and 171,600 ft in 33 miles. 171,600 ft / 180 waves = 953 ft wavelength. Half of the wavelength is 477 ft. The 20 second period wave’s energy reaches 477 ft deep in the ocean. Any bottom feature which reaches shallower than 477 ft will shorten the wave’s period and slow its speed. 477 ft of water is deep. Most shores have much shallower features well offshore. Such a swell would slow considerably approaching most shores.
Bagman, Yes, you’re right, but that doesn’t mean Im wrong. As you stated, a wave set travels at twice the speed of the waves inside it. If a wave set travels (in mph) at 3.5 times its period (in seconds), then 3.5/2=1.75, essentially my constant. You merely applied the set-to-wave factor (1/2) at another place in the equation. My calculations probably suck, but I lucked out on this one. …but you’re still ugly, and your mother dresses you funny.
“a wave set travels at twice the speed of the waves inside it” Should read: a wave set travels at HALF the speed of the waves inside it …sorry
ummm… I think someone needs to draw a line before stuff like the Froude number and Chaotic theory start to arise. All those formulae are approximations, and lets face it… who cares if a wave is coming in at 24 or 27 mph? I know when I see a 18 sec. T swell rolling in that: A) it will break larger on shore than the measured open water swell height B) it will wrap around land masses and island blockage better than a small T C) That we will have more powerful waves D) that it’s time to get ready to go surf…
I didn’t luck out. I did the Elapsed Time calculation on a spreadsheet. I programmed it to read time. It read clock time from midnight. I should have added 12 hours to 3:38, making the elapsed time 15 hours 38 minutes.
Noodle, that is what I ment, when I said your math sucked. I think you should also look at your other cal. If you use your factors 1.65 in place of 3.5 you are off on the depth of the water efficting the wave. But all in all Rook is right. I don’t care about waves at my brake until it get to 17 sec. and that it is coming in from between 290 deg. and 190 deg. and that is really all I need to know. That is as long as I’m in town.
I did a cursory logic check for the 20 sec wave and confirmed 477 ft energy depth, or there abouts. Since first seeing the depth of open ocean ground swells, it’s amazed me. Some pacific islands are so amazing because they have no continental shelves, no bottom features to conflict with ground swell energy until… Bam! some shallow rock shoal unleashes all 477 ft of energy… earthquake proportions… which brings me to question the accepted model which makes California bedrock planes vibrate enough to allow trillions of tons of rock to release its stored energy. There’s gotta be wave-caused chaos in that equation somewhere. I’m probably anal about this stuff, but I’d like to get it right. Where’s my error.?
As I remember from Bascom’s book: Multiply an open ocean swell’s period in seconds by 1.65 to get the wave set’s speed in miles per hour. A buoy 500 miles away measures a 20 second swell period headed in your direction. Speed = 20 x 1.65 = 33 mph. To calculate arrival time: 500 miles / 33 mph = 15 hours 38 minutes travel time. Say it’s 6:00 pm, so the measured set will arrive at 9:38 am tomorrow morning. Bascom died a few years ago. Thanks for the legacy, Willard.
Actually very little of the wave energy hits bottom. The water moves in circles or rather spirals, ever decreasing towards the the bottom. Thus there are very little energy hitting the bottom when the depth is a little less then half that of the wave period. Thus the wave doesn’t loose all that much energy to the bottom untill it hits really shallow water. Energy is also lost to air friction and I guess to some extent the water molecules friction against each other. From http://www.vorticity.org/alt.surfing/physics.txt "In shallow water [h