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I've looked and looked for a good calculator or graph of the way wind velocity decreases with altitude. I've come up disappointingly short. I'll link one article below, but give you the common sense observation that the reason that a turbine as high above the ground as possible is that getting it away from the ground allows it to see higher power winds. It costs a lot to make a tower higher, though, and that limits how tall you make the tower.

There is an approximate power law:

Va / Vr ~= (Ha / Hr) ^ F

Va== velocity at altitude in question
Vr== velocity at reference height
Ha== height of altitude in question
Hr== reference height
F== power factor - think of as frictional effect of the surface on the air.

In general, over “average” ground, F is taken as 1/7
Over water or very smooth ground, F is closer to 1/9
Very rough areas, it could even be 1/2.

The reference height for most of the world is 10 meters. In the US, it traditionally has been 20feet, and that means in quite a few disciplines (fire control) they will suggest that you multiply the US wind velocity in an article by 1.15 when reading articles.

I could not find on the NOAA site what altitude they are now measuring wind speed - I hope it is 10meter, but it might be 20ft or even 6 ft.

The article I link here has a graph that shows velocity versus altitude for a couple terrains, though it has nothing to do with the subject at hand:

http://umsis.miami.edu/~lpittack/cae614proj.html