Saturday, December 29, 2012

Wind shadows

During my last session in Wellfleet Harbor, I noticed a pretty wide variation in the wind which seemed to be linked to where exactly on the bay I was. So I went back to my GPS tracks and compared them with a topographical map of the area:

The image on the left shows the GPS track from one of my two crossings (the other one was similar). At two areas, I had problems to stay planing, as indicated by red and blue colors in the tracks: about in the middle of the bay, and about 4/5th of the way to the lower left. In both cases, I was directly behind two land masses, relative to the wind direction: Great Island in the middle, and Great and Little Beach Hill at the bottom left. We can use the GPS data and the topo map to get an idea how far downwind the wind shadow extends.
  • I was about 1150 m (3,800 ft) downwind of Great Island. The map shows a maximum elevation of about 80 feet. The island has a dense tree cover which adds probably 40-70 ft to the height, giving us a total height of 120-150 ft, or about 40-50 m. At a distance that was 25 x the height of the island, the wind shadow was pronounced; I'd guess that the wind dropped from 25 mph to maybe 15-18 mph.
  • The distance to Great Beach Hill was only about 520 m. The maximum elevation is 60 ft, or about 100-120 ft with trees (30-40 m). At a distance of about 12-15 x the height of the "obstacle", the wind shadow was so strong that I was not able to stay on a plane. My guess is that the wind dropped to somewhere near 12-15 mph.
I am a bit surprised to see such large effects at a pretty considerably distance - about 25 times the height of the obstacle. From a bit of internet research I had done, I would have expected a smaller effect. From experiences with local speed slicks like Duxbury Bay, however, the larger numbers make sense. One more thing to keep in mind when searching for new speed slicks...

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