What is a 20-foot wind?

Dear Tom,
In the recent episodes of red flag warnings issued by the National Weather Service there was a term: “20 ft. winds.” What does that mean?
— Laura M., Plainfield
Dear Laura,
We checked with Mark Ratzer, lead forecaster at the National Weather Service forecast office in Romeoville, and he shed some light on the subject. Ratzer informed us that 20-foot winds are the sustained winds averaged over a 10-minute period at a height of 20 feet above the surface. Since most of the wind observations are taken at the international standard height of 10 meters (33 feet), the weather service approximates the 20-foot wind speed — dividing the 10-meter wind speed by 1.2. In some heavily forested areas, wind speed is measured at 20 feet above the tree canopy, but that is not done in the Chicago area.
A 20-foot wind is the sustained wind speed averaged over a 10-minute period at a height of 20 feet above the surface.
Ask Tom, Tom Skilling, Chicago weather, WGN weather, wind speed, 20-foot wind