Height of Wind Measurement
Due to surface drag, even over seemingly smooth surfaces laminar wind speed decreases to near Zero at the surface of the earth.3 In fact wind speed on a flat grassy surface under normal non turbulent wind conditions the wind speed can be seen to increase logarithmically with height above the surface. In flat grassy areas (with low grass height) at standard temperature this change in wind speed as a function of height can be approximated using the formula: V= Vref(H/Href)0.142 and shown in the below chart 4
Figure 2.5 Wind Speed as a Function of Height above Ground
4 Note: According to one Canadian Study by the CNRC the exponent used above “0.142” should be changed to 0.333 over rough terrain and wooded areas or outskirts of towns and to 0.5 over cities.
As can be seen on this chart wind speed of 12 knots measured at 5 meters above the ground will be 13.3 knots 10 meters above the ground and 8.2 knots 1 meter above the ground due to surface friction creating a boundary layer of air near the surface of the earth.
The international standard height for wind measuring instruments is 10 meters (32.8 feet) above ground level. Wind instruments sited at lower heights can be corrected for surface friction to a 10 meter equivalent measurement (providing there is no surface induced turbulence) using the equations similar to the above calculation or sited higher than 10 meters and corrected to the lower speed that would be measured at 10 meters. This effect is considered negligible above 300 meters in most terrain. Keep in mind that differences in surface roughness and temperature will require modification of this calculation to more precisely correct wind speed for height above the earth’s surface.
3 Now you know why it seems warmer when you lay down close to the ground on a cold windy winter day.
Point to Ponder: Why are airport wind socks at 16 feet above the ground and airport anemometers at 33 feet above ground level?
