Join Date: Apr 2010
Location: Thunder Bay, Ontario
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Re: Understanding wave information
Originally Posted by Minnesail
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.
Why go fast, when you can go slow.