# Variation of Wind Velocity with Height In this article named “Variation of Wind Velocity with Height”, how wind speed change as a function of structure height is discussed.

The viscosity of air reduces its velocity adjacent to the earth’s surface to almost zero, as shown in Figure 1. A retarding effect occurs in the wind layers near the ground, and these inner layers in turn successively slow the outer layers. The slowing down is reduced at each layer as the height increases, and eventually becomes negligibly small.

The height at which velocity ceases to increase is called the gradient height, and the corresponding velocity, the gradient velocity. This characteristic of variation of wind velocity with height is a well-understood phenomenon, as evidenced by higher design pressures specified at higher elevations in most building codes.

At heights of approximately 1200 ft (366 m) above ground, the wind speed is virtually unaffected by surface friction, and its movement is solely dependent on prevailing seasonal and local wind effects. The height through which the wind speed is affected by topography is called the atmospheric boundary layer. The wind speed profile within this layer is given by Equation 1

where
V
z = mean wind speed at height Z aboveground
Vg= gradient wind speed assumed constant above the boundary layer
Z = height aboveground
Zg= nominal height of boundary layer, which depends on the exposure (Values for Zare given in Fig. 1.1.)
α = power law coefficient