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Wind Shear

Wind slowed down at surface by trees and obstructions

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Wind velocity increases with altitude and wind moving close to Earth's surface is slowed down by obstructions like buildings, trees and similar. Wind slow down along the surface is "wind shear".

Wind shear can be expressed as

v / vo = (h / ho)α                                             (1)

where

v = the wind speed at height h (m/s)

vo = the wind speed at height ho (m/s)

α = the wind shear exponent

The wind shear exponent varies with the terrain.

TerrainWind 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 Speed in a Terrain with Several Buildings

With a Wind Shear Exponent 0.25 and wind velocity at level 10 m measured to 5 m/s -  wind sped at level 40 m can be estimated by transforming (1)

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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