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  #1  
Old 11-25-2013
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Understanding wave information

I've been looking at the NOAA weather reports for Lake Superior and I'm kind of confused by the numbers for waves. (I'm always kind of confused on a Monday morning, but these numbers are confusing me more than usual.)

Significant Wave Height (WVHT): 11.2 ft
Swell Height (SwH): 6.6 ft
Swell Period (SwP): 10.0 sec
Swell Direction (SwD): WSW
Wind Wave Period (WWP): 8.3 sec
Wind Wave Direction (WWD): WSW
Wave Steepness (STEEPNESS): STEEP
Average Wave Period (APD): 6.7 sec

What's the difference between Wave Height and Swell Height?

And there are three different numbers for period. Swell Period, Wind Wave Period, and Average Wave Period. What do those three numbers mean?
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Old 11-25-2013
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Re: Understanding wave information

According to
What is the difference between waves and swell? | Weather Help

Quote:
What is the difference between waves and swell?
Not a lot!


Swell is the regular longer period waves generated by distant weather systems - think of a rock thrown in a pond, and you'll see swells.

Waves & Swell are often used to mean the same thing .. in common language many like to think of "swell" as the regularly spaced lumps that approach your local surf spot, and "waves" as when they break/something to stick your surfboard on.

Wave height (trough to crest) refers the average height of the highest one-third of the waves.
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Re: Understanding wave information

Off Pachena there can be a big difference. Ten meter swell from the NW and no wind so it's a gentle up and down in glassy conditions. Add a good local wind from the SE and now its 2 meter waves against the swell .
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Old 11-26-2013
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Re: Understanding wave information

My primitive, nicely over-simplified take:

I think of the significant wave height as the total of the base swell and the more recent wind waves, as a number to represent the slightly-higher-than-average wave.
Weekee: "In physical oceanography, the significant wave height is defined traditionally as the mean wave height of the highest third of the waves. ..."

The period gives you an idea of how close together the waves are. Shorter period = potentially steeper, more of a pain in the seat to sail.

I think of swells as more mellow, typically -- at least while they're still offshore.

So a lot of wind wave, moving fast, short wave period = better day for surfing than sailing.
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Old 11-26-2013
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Re: Understanding wave information

Quote:
Originally Posted by Minnesail View Post
I've been looking at the NOAA weather reports for Lake Superior and I'm kind of confused by the numbers for waves. (I'm always kind of confused on a Monday morning, but these numbers are confusing me more than usual.)

Significant Wave Height (WVHT): 11.2 ft
Swell Height (SwH): 6.6 ft
Swell Period (SwP): 10.0 sec
Swell Direction (SwD): WSW
Wind Wave Period (WWP): 8.3 sec
Wind Wave Direction (WWD): WSW
Wave Steepness (STEEPNESS): STEEP
Average Wave Period (APD): 6.7 sec

What's the difference between Wave Height and Swell Height?

And there are three different numbers for period. Swell Period, Wind Wave Period, and Average Wave Period. What do those three numbers mean?
Not an expert either (although have sailed Superior for a number of years).

First off, here's a nice explanation of waves and wave forecasts.

Wave hight and swell height are both pretty much the same. In terms of the forecast given, swell is old typically rounder waves, whereas wind-waves are newer, often sharper waves. Significant Wave Height (Hs) is an average, but it's important to realize this numbers is the average of the top 1/3rd of waves.

Further (according to the document):
  • The mean wave height (H) is approximately 2/3rds of Hs. H is very close to the most probable wave height.
  • One in 10 of the largest waves will be 27% larger than Hs.
  • One in 100 of the largest waves will be 67% larger than Hs.
  • The largest possible wave is twice Hs.

So your forecast is saying you should be able to handle a 22.4' steep wave if you go out.

What makes Superior so challenging is that that wave nights can get quite large, but the period remains relatively short (compared to oceans). Steep waves are quite common.
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Re: Understanding wave information

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Originally Posted by MikeOReilly View Post
So your forecast is saying you should be able to handle a 22.4' steep wave if you go out.


OK, I think I get it. There are "baseline" waves that are the swell waves. In this forecast they're 6.6' and 10 seconds apart. The wind waves are on top of those with a period of 8.3 seconds, which means that they'll sometimes be in the middle of the swell waves and other times on top of them, giving an average wave period of 6.7 seconds. When the wind waves are in between the swell waves they'll be smaller, and when they're on top they'll be bigger, in this case building the 6.6' swell wave up to 11.7' (for the average of the upper third).

Thanks for the link to that PDF, it was very helpful.

It says that the highest 1% of waves will be 1.67 times the reported significant wave height, so in this case that means you can expect a 20' wave with a period under 10 seconds about every ten minutes or so.

Yikes.
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Re: Understanding wave information

Nearly this is a great understanding of this set of details with regards to waves.

Thanks for this Information!

~Austin
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