Weather Radio Marine Forecast Question

[Saul Trabal]

There's something I've been very curious about for years.

Whenever I listen to the marine forecast on my weather radio (station KWO35-162.55 mhz), they give buoy reports off the ocean.

My question is about wave measurements.

I'll hear a report that says:

"At Ambros Light Tower, the wind is at 15 knots. Wave period 3 feet every 3 seconds."

Why do they measure the height of the waves and the time that they rise to a given height? I never understood the need for knowing how long it takes for a wave to reach a given height.

:?

 

[Morgan Palmer]

I'm not a mariner, but a Texas Rangers fan. :lol:

Anyway, my thinking is that the wave period is important especially if you have very tall wave amplitude. For instance (and this is just made up) a certain captain may not want to sail his craft over 20 foot seas if the period is too short.

In other words, a long period will produce a gentler slope over those 20 foot crests than a short period.

That's my thinking. Maybe someone who isn't landlocked has an example.
mp

 

[Mike Sharp]

From my experience in the marine industry, I service marine safety equipment, for my daily crust.
Wave height can be high but with close gaps and smaller craft might sit on the top with bow and stern out of the water leading to the vessel breaking her back.
However it is all peak/strough related think of the classic film Perfect Storm there it made no difference what size the vessel was.

 

[Glenn Rivers]

Wave Mesurements

Sorry I didn't notice your post earlier. The wave height is determined by an accelerometer on the buoy that measures the displacement of the buoy. During a measurement period (typically 20 minutes) the wave height is given by either:

1. The average of the highest 1/3 waves that cross the still water level upwards (zero up-crossing waves)

2. 4 times the standard deviation of the water surface elevation in the record.

Both definitions give about the same result in most wave spectra. This definition of wave height is arbitrary and is largely a result of convention.

Wave period by definition is the inverse of the frequency. Whatever frequency band has the highest energy is used to determine the wave period. Typically, the wave energy is divided into frequency bins of .01 Hz width, ranging from .03-.30 HZ. For example, if the band centered on .20 Hz has the most energy, the wave period will be given as 5 seconds.

See the NDBC web site for definitions of the data http://www.ndbc.noaa.gov/rmd.shtml

 

[Saul Trabal]

Thanks for all the responses!

Happy New Year.

8)

 

[Cstok]

I have been a lifelong surfer here in Southern California. Before the internet, I used to listen to the bouy reports over my portable weather radio, and the wave height and period were VERY important (at least to surfers).

The height is self explanatory... bigger waves, generally better! However the period was important as well. It would generally tell you a few things. First if the period was short, 5-10 seconds or so, you knew that it was choppy, and that the waves were being generated by a local source. Here in California much of the time there is a short period swell which is generated from the local on shore breeze that is apparent 90% of the time, so that short period meant choppy crummy wind swell. Not much fun for surfing, but it also let you know how close the source was. Second if the period was very long 15+ seconds or so that indicated that the waves were being generated by a distant source (storm off of New Zealand in the Summertime, or off the Alleutians (sp?) during the winter). That generally meant big, good surf. The good report that would have us calling in sick to work would be "Wave height 10 feet at 15 seconds" !!! Also you knew when the swell was peaking because there was a difference in the period over time. Generally the periods were a little closer as the swell approached, it would peak, and then the period would lengthen.

There is an interesting site on the internet that I use to keep tabs on the Pacific.... It kind of reminds me of a hodo. Take a look at the "directional spectrum" in the lower lefthand corner.

http://cdip.ucsd.edu/?nav=recent&sub=nowca...xitem=socal_now

It is a good tool to see what is going on in the water, but also with the local wind. If there is alot of action on the out side rings, you know it is just local windy, choppy conditions. If you see a more tightly focused "blob or coloration" near the center rings that indicates long period.... or swells from a distant source. Finally this tool tells you the direction of the swells... again a very important data point for surfers here in S. Cal because of our off shore islands.. You can see from the map that the swell can very greatly depending on island and land "shadowing". The period has alot to do with this shadowing as well... Longer periods (in my experience) seem to "get into" or wrap around obsticles a little better, so that can help with knowing where the best place to surf will be.