**Anamometer heights and wind speeds**

For Joel Wright.....Is there any way you could post pictures of what your exposure looks like in different directions?

When the wind is relatively strong, or there is not much heat flux from the ground or a thermal inversion, the wind profile will tend to be a logarithmic function with height, provided that the aerodynamic roughness is essentially uniform for some distance upwind.

There is a famous equation that relates wind speed with height for the above conditions:

U(Z) = Ln[(Z/Zo)+1]*(U*/K)

U = mean wind speed at height (Z)

Ln is the "natural logarithm function" having a base of 2.71828 instead of 10. (To convert to Log10 just multiply the Ln function by 0.4343)

Zo = roughness length. This is the height at which a logarithmic profile extrapolated downward goes to zero. Estimating its value takes some experience, but it is usually not more then than 10% of the height of the physical roughness elements..i.e. trees buildings, grass, tarmac, ripples and waves on water etc. acting on the wind. Zo can be less then 10% of the height of the roughness elements if they are too widely spaced, or too closely packed.

U* refers to "friction velocity" and is equal to the square root of the wind stress divided by the air density.

An added complication is that the apparent height at witch the winds go to zero (based on measurements above the roughness elements) does not equal the ground surface, but rather an apparent "zero-plane" displaced a good fraction of the height of the roughness elements above the ground. A classic value for the zero plane displacement is at a height of 0.6 times the height of the roughness elements, but this fraction varies a good deal with factors such as the packing density of the roughness elements.

K = the "Von Karman" constant, considered to be near 0.4

Typical values of Zo:

Mature Hardwood forest...1 m (3.3 feet)

One story bungalows with sparse trees.....0.3 m (1 foot)

4-5' high corn....0.1 m (0.33 feet)

4-6" high grass and weeds....0.01 m (0.033 feet)

Plowed or furrowed field.......0.01 m (0.033 feet)

Typical tarmac or concrete....0.0001 m (0.00033 feet)

Ratios of (U33/U24):

For Zo = 0.3m and Zero plane 8' above the ground....1.24

For Zo = 0.1m.....1.07

For Zo = 0.01m...1.05

These ratios refer to wind speeds averaged over periods of 1 minute or greater. The ratios for gust speeds will be smaller.

I hope this will be helpful.

Best regards...Glenn Rivers

P.S. if you would like, I could try to find a site with this stuff.