The wind speeds assigned to the F-scale by Fujita were mostly abritrary, and were never calibrated. More recently, research by the Texas Tech wind engineering folks, and a committee of other damage experts (including Tim Marshall) are finding that the actual wind speeds that cause the described damage in the Fujita descriptions might be as much as 30-40% lower than what Fujita guessed. Also add to the fact that Fujita's descriptions were for well-built single-family residential homes only. Unfortunately, as we are discovering on almost every damage survey, very few homes hit by tornadoes are "well-built".
Regarding the comment about direct measurement of tornadoes by the Doppler On Wheels (DOW). This is not possible, because even at very close ranges (1-2 miles), there are still some horizon/clutter effects blocking any beam aimed near the ground. I think the closest to the ground that the measurements have been estimated is still 50-100 feet above ground level, which is still above the roofs of most single-family residential structures. There are rapid changes in vortex wind speed and acceleration in that lowest "skin" layer within 100 feet of the ground.
And yes - those like me who do damage surveys as part of our job are trained to know that the F-scale is a *damage intensity* scale, and that the distribution of wind speeds capabile of producing said damage actually have distributions that ovelap each scale category. There are many other factors to consider, such as exposure, angle to the wind, building construction, secondary damage caused by debris impacts, etc., such that a wind speed of a constant xxx mph could cause a range of damage to similar structures.
greg
