20foot wind speedn. Wind speed 20 feet (six meters) above the surrounding vegetation.
The standard height above the vegetation for measuring open wind speed in the United States is 20 feet; the rest of the world measures open wind speed ten meters (33 feet) above the surrounding vegetation (tenmeter wind speed). Divide 10m wind speed by 1.15 to estimate 20foot wind speed; multiply 20foot wind speed by 1.15 to estimate 10m wind speed (Turner and Lawson 1978).
In forest stands, 20foot wind speed is measured 20 feet above the top of the tree canopy, in shrublands, it is measured 20 feet above the top of the shrub vegetation.
20foot wind speed is used in combination with a wind adjustment factor to estimate midflame wind speed.
