The maximum wind speeds of tornadoes are not expected to reach the F6 wind speeds. This scale relates the wind speed of a tornado to the amount of damage done. This scale is not a perfect system for linking damage to wind speed, but it has advantages over what had been used previously. An important thing to remember is that the size of a tornado is not necessarily an indication of its intensity.
Large tornadoes can be weak and small tornadoes can be strong.
The Fujita Scale is based on damage , not the appearance of the funnel. Storm spotters, storm chasers and other weather observers often try to estimate the intensity of a tornado when they are in the field, basing their judgment on the rotational speed and amount of debris being generated as well as the width. However, the official estimate is made after the tornado has passed. Personnel from the National Weather Service office that issued the warning survey the site to determine the F-Scale rating. Sometimes they call in experts from out of the area.