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Wind Shear
Wind is slowed down at the surface by trees and obstructions
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Wind moving across the Earth's surface is slowed by obstructions like buildings, trees and similar and the wind velocity increases with altitude.
The wind shear can be expressed as
v / vo = (h / ho)α (1)
where
v = the velocity at height h (m/s)
vo = the velocity at height ho (m/s)
α = the wind shear exponent
The wind shear exponent varies with the terrain.
| Terrain |
Wind Shear Exponent - α - |
| Open water | 0.1 |
| Smooth, level, grass-covered | 0.15 |
| Row crops | 0.2 |
| Low bushes with a few trees | 0.2 |
| Heavy trees | 0.25 |
| Several buildings | 0.25 |
| Hilly, mountainous terrain | 0.25 |
Example - Wind Velocity at a Terrain with Several Buildings
With Wind Shear Exponent 0.25 and wind velocity at level 10 m measured to 5 m/s - wind velocity at level 40 m can be estimated by transforming (1) to
v = (h / ho)α vo (1b)
Substituting with values
v = ((40 m) / (10 m))0.25 (5 m/s)
= 7.1 m/s
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