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Wave height, how is it measured?

Old 08-22-2007, 12:23 PM
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Default Wave height, how is it measured?

I wonder, is it from the bottom of the trough to the top of the wave?, or is it 1/2 of that?

I always think that if I could stand on water, at the bottom of a trough, and the water would be to come up to my knee/ 1 1/2', then that would be a 1-2 foot sea as many grossly overestimate wave height thus the term, "THT-seas" What is the right way to measure wave height?
Old 08-22-2007, 12:34 PM
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Default Re: Wave height, how is it measured?

Wave height is usually measured by drunken rednecks on boats that are overpowered and travelling way too fast with too many fish onboard.
Old 08-22-2007, 12:36 PM
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Default Re: Wave height, how is it measured?

I use trough to crest....but almost everyone overstates it once you get above 2. I think it's because once reach a height where it's higher than the freeboard it just seems bigger.
Old 08-22-2007, 12:37 PM
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Default Re: Wave height, how is it measured?

Pretty sure its measured as the vertical distance from the trough to the top.

I use to think it was half that too. But that was because of surfing. if the seas were "3ft" it was normally a head high wave "6ft".
But I guess thats because they get jacked up on the sandbar.
Old 08-22-2007, 12:39 PM
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Default RE: Wave height, how is it measured?

Significant Wave Height

Is the average height (trough to crest) of the one-third highest waves valid for the indicated 12 hour period.


Significant wave height, Hs, is approximately equal to the average of the highest one-third of the waves. Hs is calculated using:


Hs = 4.0 * sqrt(m0)

where m0 is the variance of the wave displacement time series acquired during the wave acquisition period. However, since wave displacement time series are not returned from NDBC's wave measurement systems, variance is calculated using the nondirectional wave spectrum according to the following relationship:


m0 = sum(S(f)*df)

where the summation of spectral density, S(f), is over all frequency bands, f, of the nondirectional wave spectrum and df is the bandwidth of each band. NDBC wave analysis systems typically sum over the range from 0.03 to 0.40 Hz with frequency bandwidths of 0.01 Hz. Some newer systems sum up to 0.485 Hz with bandwidths that vary from 0.005 Hz at low frequencies to 0.02 Hz at high frequencies.

Dominant, or peak, wave period, Tp, is the period corresponding to the frequency band with the maximum value of spectral density in the nondirectional wave spectrum. It is the reciprocal of the peak frequency, fp:


Tp = 1/fp

Dominant period is representative of the higher waves encountered during the wave sampling period. Greater detail on the processing of NDBC wave data can be found in the 1996 technical document, Nondirectional and Directional Wave Data Analysis Procedures (a pdf file).

NDBC also provides estimates of the height and period of wind-seas and swell on each station page. Values for these quantities are calculated by applying the above process to the respective wind-sea and swell portions of the wave spectrum.

The algorithm used to estimate wave steepness is taken from work done by William Buckley, discussed in a paper that appeared in the Journal of Naval Engineers, September, 1988, titled " Extreme and Climatic Wave Spectra for Use in the Structural Design of Ships". The algorithm involves the relationship between significant wave height (Hs) and dominant wave period, or more precisely, its inverse, peak wave frequency (fp).

The algorithm follows:

val = exp(-3.3 * ln(fp))
if Hs > (1/250 * val)
steepness = 'very steep';
elseif Hs > (1/500 * val)
steepness = 'steep';
elseif Hs > (1/1000 * val)
steepness = 'average';
else
if Hswell >= Hwindwv
steepness = 'swell';
else
steepness = 'average';

where Hswell and Hwindwv are the respective significant heights of the swell and wind waves which are quantities that are also computed.


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Old 08-22-2007, 12:52 PM
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Default RE: Wave height, how is it measured?

I keep it simple..

If it's higher than my bow it's a 3.5'.

It if comes over the bow its a 4'

If I can't see the top its a 12'

Old 08-22-2007, 01:52 PM
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Default Re: Wave height, how is it measured?

I've always used the 1/2 of the distance from the trough to the crest. My thinking, as was my physics professor's, is that on calm seas the water level is stable. When you introduce wave mechanics into the situation the water level will drop from the stable level during the trough and rise during the crest in equal amounts. When I say 3 foot seas I mean 6 feet from trough to crest. Does anyone know how NOAA measures it? I guess I should look that up.
Old 08-22-2007, 01:55 PM
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Default Re: Wave height, how is it measured?

Whatever NOAA reports add 2 feet!
Old 08-22-2007, 01:57 PM
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Default Re: Wave height, how is it measured?

Found my answer on NOAA's website (they measure from from the bottom of the trough to the crest):

SWH Swell height is the vertical distance (meters) between any swell crest and the succeeding swell wave trough.

WWH Wind Wave Height is the vertical distance (meters) between any wind wave crest and the succeeding wind wave trough (independent of swell waves).
Old 08-22-2007, 02:01 PM
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Default Re: Wave height, how is it measured?

Here is NOAA's definition of "significant wave height"

http://www.srh.noaa.gov/graphical/de...aveHeight.html
Old 08-22-2007, 02:02 PM
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Default Re: Wave height, how is it measured?

Gringo - 8/22/2007 11:34 AM

Wave height is usually measured by drunken rednecks on boats that are overpowered and travelling way too fast with too many fish onboard.
Correct!
Old 08-22-2007, 02:03 PM
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Default Re: Wave height, how is it measured?

G8RDave - 8/22/2007 11:57 AM

Found my answer on NOAA's website (they measure from from the bottom of the trough to the crest):

SWH Swell height is the vertical distance (meters) between any swell crest and the succeeding swell wave trough.

WWH Wind Wave Height is the vertical distance (meters) between any wind wave crest and the succeeding wind wave trough (independent of swell waves).
Correct, and if you add those two numbers together, then you get a true picture of the sea conditions.

HH
Old 08-22-2007, 02:08 PM
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Default RE: Wave height, how is it measured?

Wave height, schmave height.

It's either rough (for the boat yer in), or it isn't.


Big Al
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Old 08-22-2007, 02:10 PM
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Default RE: Wave height, how is it measured?

hlights2 - 8/22/2007 11:39 AM

Significant Wave Height

Is the average height (trough to crest) of the one-third highest waves valid for the indicated 12 hour period.


Significant wave height, Hs, is approximately equal to the average of the highest one-third of the waves. Hs is calculated using:


Hs = 4.0 * sqrt(m0)

where m0 is the variance of the wave displacement time series acquired during the wave acquisition period. However, since wave displacement time series are not returned from NDBC's wave measurement systems, variance is calculated using the nondirectional wave spectrum according to the following relationship:


m0 = sum(S(f)*df)

where the summation of spectral density, S(f), is over all frequency bands, f, of the nondirectional wave spectrum and df is the bandwidth of each band. NDBC wave analysis systems typically sum over the range from 0.03 to 0.40 Hz with frequency bandwidths of 0.01 Hz. Some newer systems sum up to 0.485 Hz with bandwidths that vary from 0.005 Hz at low frequencies to 0.02 Hz at high frequencies.

Dominant, or peak, wave period, Tp, is the period corresponding to the frequency band with the maximum value of spectral density in the nondirectional wave spectrum. It is the reciprocal of the peak frequency, fp:


Tp = 1/fp

Dominant period is representative of the higher waves encountered during the wave sampling period. Greater detail on the processing of NDBC wave data can be found in the 1996 technical document, Nondirectional and Directional Wave Data Analysis Procedures (a pdf file).

NDBC also provides estimates of the height and period of wind-seas and swell on each station page. Values for these quantities are calculated by applying the above process to the respective wind-sea and swell portions of the wave spectrum.

The algorithm used to estimate wave steepness is taken from work done by William Buckley, discussed in a paper that appeared in the Journal of Naval Engineers, September, 1988, titled " Extreme and Climatic Wave Spectra for Use in the Structural Design of Ships". The algorithm involves the relationship between significant wave height (Hs) and dominant wave period, or more precisely, its inverse, peak wave frequency (fp).

The algorithm follows:

val = exp(-3.3 * ln(fp))
if Hs > (1/250 * val)
steepness = 'very steep';
elseif Hs > (1/500 * val)
steepness = 'steep';
elseif Hs > (1/1000 * val)
steepness = 'average';
else
if Hswell >= Hwindwv
steepness = 'swell';
else
steepness = 'average';

where Hswell and Hwindwv are the respective significant heights of the swell and wind waves which are quantities that are also computed.


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mirage2521 - 8/22/2007 1:02 PM

Gringo - 8/22/2007 11:34 AM

Wave height is usually measured by drunken rednecks on boats that are overpowered and travelling way too fast with too many fish onboard.
Correct!

Wrong...Here's how I do it: (And I never have more than thirty kingfish on board, thank you.)




Old 08-22-2007, 02:36 PM
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Default Re: Wave height, how is it measured?

Many posters on this board when describing their last trio take the most accurate estimate of the wave size and add 3 feet to it.
Old 08-22-2007, 02:37 PM
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Default Re: Wave height, how is it measured?

Wave height calculation is variable. It has alot to do with the distance from the bow of the boat you are standing on to the stern multiplied by the distance you are from shore & divided by the amount of experience the skipper has in rough water then multiplied again by the time the skipper has to get to the nearest bar to tell the story.

The result is at least triple what NOAA says.
Old 08-22-2007, 02:40 PM
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Default Re: Wave height, how is it measured?

SeaJay - 8/22/2007 12:36 PM

Many posters on this board when describing their last trio take the most accurate estimate of the wave size and add 3 feet to it.
My last trio involved a couple of sets of 32DD's, but nothing like 3 Feet!


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Old 08-22-2007, 03:02 PM
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Default RE: Wave height, how is it measured?

Take NOAA's forecast and multiple it by 3........
Old 08-22-2007, 03:53 PM
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Default RE: Wave height, how is it measured?

solarfry - 8/22/2007 10:52 AM

I keep it simple..

If it's higher than my bow it's a 3.5'.

It if comes over the bow its a 4'

If I can't see the top its a 12'

Sounds like you need a new prescription from your optometrist...

jky
Old 08-22-2007, 04:48 PM
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Default Re: Wave height, how is it measured?

Bottom tier boats, 2' seas feel like 6' seas
Mid tier boats, 4' seas feel like 6' seas
Top tier boats, 6' seas =jack

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