Wednesday, December 14, 2016

Record Setting Wave

"It's like an ocean out there!", I've heard many say on Lake of the Ozarks on a busy weekend. Hell, I've even said it, but it is obviously an exaggeration. When it comes to big waves, the Lake of the Ozarks doesn't come close to what awaits a sailor out on the open ocean.

There are lots of headlines today over an announcement by the World Meteorological Organization of a record wave height measured by a sensor buoy.  It was 19 meters (62.3ft) high, and not just some "rogue wave".  More interestingly, everyone is reporting this as if it just happened, when in fact the data point was collected on April 2, 2013.

Buoy Location 59°07.3'N, 11°42.5'W
The record measurement is called: "Highest Significant Wave Height as measured by a Buoy", but what does "highest significant wave height" actually refer too?
In this instance, "Significant wave height recorded is four times the RMS value of the water level above the average level of the water surface measured over a 17½ minute period. The factor of 4 applied to the RMS value is because the waves are trochoidal in nature. (Waves at sea, especially those growing under the influence of the wind, tend to be short-crested, i.e. the wave crests project further above the mean level than the troughs are below it.) The ‘average’ wave period, again over a 17½ minute sample, is the average of the periods over 7 successive 2½ minute samples (each determined from the number of wave cycles in the sample)." - WMO

Okay, that's a lot to unpack, but basically it means we're not talking about a single wave calculation. For all the non-math nerds, what this essentially says is that the number represents a sampling of waves over a period of time, and the "root mean squared" (RMS is a calculation typically used to find peak magnitudes for a set of numbers) value of those wave heights was multiplied by a factor of 4 to account for the shape of the waves (wind driven in this case) and the buoy's heave (up and down) sensor sampling rate.  So what we're talking about is not a 19 meter rogue wave, but a brief series of waves that statistically calculate to be of that average height.
Those are monster waves. I've been in 40ft seas in the Pacific on a 180ft Buoy Tender and can say it was an amazing, if somewhat frightening experience. Not one I'd like to repeat.