Roger Long wrote:

Another common over estimation is wind speed. I was quite surprised
when I
started carrying a pocket wind gauge with me while sailing. . .

Part of your surprise may be due to the velocity gradient. The wind
blows harder the higher you are above sea level, with the gradient
being stronger or weaker depending on how stable condtions are. It is
caused by friction with the sea; at the sea surface windspeed is
effectively zero, rapidly increasing in the first few feet, but is
still several knots different from deck level to mast top, where
sailboat wind sensors are.

I've seen conditions (building wind from a calm) where it's blowing 10
knots 55 feet up, and only 5 knots on deck, measured by acccurate
sensors. It's one of the reasons the big boys with the ninety foot
sticks seem to be able to move in drifting conditions while the punters
are becalmed.

On 24 Dec 2004 22:16:50 -0800, "Mark" wrote:

Roger Long wrote:

Another common over estimation is wind speed. I was quite surprised
when I
started carrying a pocket wind gauge with me while sailing. . .

Part of your surprise may be due to the velocity gradient. The wind
blows harder the higher you are above sea level, with the gradient
being stronger or weaker depending on how stable condtions are. It is
caused by friction with the sea; at the sea surface windspeed is
effectively zero, rapidly increasing in the first few feet, but is
still several knots different from deck level to mast top, where
sailboat wind sensors are.

I've seen conditions (building wind from a calm) where it's blowing 10
knots 55 feet up, and only 5 knots on deck, measured by acccurate
sensors. It's one of the reasons the big boys with the ninety foot
sticks seem to be able to move in drifting conditions while the punters
are becalmed.

That would probably explain why so many more trees were blown down in
my neighborhood in Pensacola when hurricane Ivan came ashore. We lost
8 of 10 trees in my front yard. That was normal in our neighborhood
on the NE side of Pensacola where we are at 115' above sea level. The
neighborhoods at only 10' above sea level did not lose nearly as many
trees.

mike
--

Mike Hendrix
Pensacola
http://travellogs.us/