The Enhanced Fujita Scale for tornado damage was implemented
in the U.S. on Feb. 1, 2007.  Below is a chart that compares the new scale
to the old Fujita Scale.

IMPORTANT NOTE ABOUT ENHANCED F-SCALE WINDS: The Enhanced F-scale
still is a set of wind estimates (not measurements) based on damage. Its
uses three-second gusts estimated at the point of damage based on a judgment of
8 levels of damage to the 28 indicators listed below. These estimates vary with
height and exposure. Important: The 3 second gust is not the same wind as
in standard surface observations. Standard measurements are taken by weather
stations in open exposures, using a directly measured, "one minute
mile" speed.

Source: http://www.spc.noaa.gov/faq/tornado/ef-scale.html