20-foot wind speed
n. 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 (ten-meter wind speed). Divide 10-m wind speed by 1.15 to estimate 20-foot wind speed; multiply 20-foot wind speed by 1.15 to estimate 10-m wind speed (Turner and Lawson 1978).
In forest stands, 20-foot 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.