posted 05-21-2004 03:10 PM ET (US)        

For the record, wave height is not measured from the trough to the peak. Rather, it is measured from the midrange of the wave to the peak. The midrange is the middle of the wave from the trough to the peak. The peak of the wave is the same distance from the midrange as the trough.

So, when you are in the trough of 10 foot seas, you are looking at wall of water that is 20 feet high; in 20 foot seas, the wall is 40 feet. Fyi.

We should leave hyperbole and exaggeration to the landlubbers and women.

Kind regards.

posted 05-21-2004 03:28 PM ET (US)         

JMHXYO

posted 05-21-2004 03:56 PM ET (US)            

NOAA
http://www.erh.noaa.gov/okx/marine_def.html

US Navy training materials
http://www.tpub.com/content/aerographer/14269/css/14269_58.htm

"In oceanography, wave height (fig. l-31) is the vertical distance, usually measured in feet, from the crest of a wave (the highest portion of a wave) to the trough of the wave (the lowest portion of the wave). This differs from the "wave height" or "amplitude" normally used in physics, in which the distance is measured from the "at rest" or midline position to the crests and troughs."

This one is the most descriptive.
Rutgers University Institute of Marine and Coastal Sciences
http://marine.rutgers.edu/dmcs/ms200/waves.html

"Height (H) - vertical distance between a crest and trough.
Amplitude (A) - vertical distance from equilibrium surface to crest or trough (= H/2)."

posted 05-21-2004 04:48 PM ET (US)            

I saw a wave THIS (___________________________) big.

posted 05-22-2004 01:19 AM ET (US)            

I agree wave height is more often than not overstated by those who claim that the waves were such and such a height. But having a reputation for veracity, I am shy about exaggerating, and may on occasion understate things.

I believe such is the case with my part of Cetacea page 40:

http://continuouswave.com/whaler/cetacea/cetaceaPage40.html

The photo doesn't show it, but it was really rough that day. I explain how I estimated the wave height to be 8 or 10 feet. The guys I was with claimed the waves were much larger. I attributed that to a lack of experience on their part in judging wave height.

But a couple years ago the guy who took that photo, sent me another photo from that same roll of film. I had never seen the photo before. It made me admit that I had made an error in my estimate. I believe the waves WERE much larger that I originally claimed.

Here is that photo:

posted 05-22-2004 02:06 AM ET (US)            

One lazy man's way to measure wave height is to check the NOAA weather buoy readings...that's what I base my statements about wave height north of San Francisco upon (check the Bodega Bay weather buoy every now and then to get a feeling for the seas out here--NDBC/Station 46013). And I assume you understand that a reading taken at a weather buoy fourteen miles from shore in 420 feet of water just gives you the starting point to estimate the size of swells as they approach land and build in shoaling water, interacting with tidal currents and other preceding waves which have been reflected back in all directions from the ragged shoreline.

I board surfed from the age of ten, before the Giget movies back when a crowded day at Malibu during a good break (anything bigger than four feet!) meant maybe twenty guys. I've board- and body-surfed moderate sized waves in California, Mexico, Costa Rica, Hawaii, Port Sudan, Djoubouti, and the Gulf of Aden. Serious surfers pride themselves in UNDERESTIMATING wave height. It's a reverse snobbery kind of thing. FYI.

Tony

posted 05-22-2004 08:25 AM ET (US)            

--the strength of the wind
--the distance of open water (fetch)
--the duration of the wind

After a few hundred miles and about 24 hours, the wave height will grow into a fully developed sea.

The sea surface is very non-uniform and random, so wave height prediction become more of a statistical description than anything else.

In his excellent book, Oceanography and Seamanship, William G. Van Dorn provides this example:

If a steady wind of 30-knots blows for 24 hours over a fetch of 340 miles, a fully developed sea will result with the following wave heights:

Most Frequent Wave Height = 8.5 feet

Average Wave Height = 11 feet

Significant Wave Height = 17 feet

Average Wave Height
of the highest ten
percent of all waves = 22 feet

Ten percent of all waves will be higher than 18 feet.

Ten percent of all waves will be lower than 3.6 feet.

Ninety percent of all waves will be between 3.6 and 18 feet high.

There will be a five percent chance of encountering a single wave higher than 35 feet among every 200 waves that pass in about 30 minutes.

In five hours approximately 2,600 waves will pass, among which there will be a five percent chance of encountering a single wave higher than 40 feet(!).

Van Dorn cites this as an example of how there is a chance of encountering a really big wave even in moderate weather.

These heights are for the open sea, and will increase near shore due to shoreline effects.

To get back to our fundamental topic, how to measure wave height, Van Dorn's book measures wave height vertically from trough to crest.

posted 05-22-2004 12:22 PM ET (US)            

I love Tom's photo every time I see it. Whalers in the open ocean...doesn't get any better.

BTW, anyway you look at it, a 20ft wave is really big.
I have been in some in my 65' and it scared the #$#% out of me! Would never want to see one in my whaler.

Andy
>>))))">

posted 05-22-2004 12:28 PM ET (US)            

That chop on Lake Ray Hubbard must have been at least 3 ft.

posted 05-22-2004 02:00 PM ET (US)            

In the Navy we refer to it as Sea State.

It varies. For example a Sea State of 1 would mean Sea of 2-3 Feet. Sea State of 2 would be 2-5 Feet. Sea State 3 would be 3-6 feet etc....

Allpoints what is the big deal if someone says 3 foot waves or 8 foot waves? It is their story let them tell it. As for the differances? Well 2 foot waves on a Aircraft Carrier mean nothing yet two foot waves to a Kayak means green water...

posted 05-22-2004 02:04 PM ET (US)            

A famous example of this occurred in World War II in the Indian Ocean southeast of Capetown, South Africa. A large warship was steaming westward toward the Cape of Good Hope in some heavy weather. It took the crest of an ordinary wave, then plunged down into a very deep trough. The ship sustained significant damage and had to have major repairs.

posted 05-23-2004 12:09 AM ET (US)            

Thanks for sharing...I have it set as my wallpaper. Those waves are really towering over your boat.

It's amazing the difference between ocean waves and great lakes waves.

Dad and I were out two weeks ago in his 22' Guardian off Michigan City, Indiana for a fishing tournament...steady 6 footers with some 7 and 8 footers thrown in for good measure. Those looked huge, but nothing like the ones in your photo...much shorter spacing between waves.

Thanks again.

Very cool.

posted 05-23-2004 11:00 PM ET (US)            

If: Ten percent of all waves will be higher than 18 feet and
ten percent of all waves will be lower than 3.6 feet.

Then

EIGHTY (not ninety) percent of all waves will be between 3.6 and 18 feet high.

posted 05-24-2004 12:46 AM ET (US)            

The waves were swells but they were big swells that were rolling in off the Pacific Ocean into the mouth of the Straight of Juan de Fuca against a strong ebb tide where the bottom shoals from over 1000 feet of depth to less than 200 feet.

posted 05-24-2004 10:38 AM ET (US)            

That's an amazing picture! How far were you offshore? I don't think I would have the stomach to go out in conditions like that.

Erik

posted 05-26-2004 10:42 PM ET (US)            

I've got a poster of that cover on my bathroom wall...it's been hangin' there thirteen years in my current house and five years before that in my last one. Says it all about many New Yorkers' views of the rest of the nation--I know, I worked back there for half-a-dozen years.