Pictures from around western Colorado and eastern Utah
The Fujita Scale
Tornadoes are classified according to the damage they cause. Through observational studies, T. Theodore Fujita created the following scale in the late 1960's to classify tornadoes. The scale correlates wind speeds with damage: (F-0 is the weakest and F-5 the strongest).

ScaleWind Estimate (MPH) Typical Damage
F0 < 73 Light damage . Some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged.
F1 73 - 112 Moderate damage . Peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos blown off roads.
F2 113 - 157 Considerable damage . Roofs torn off frame houses; mobile homes demolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.
F3 158 - 206 Severe damage . Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown.
F4 207 - 260 Devastating damage . Well-constructed houses leveled; structures with weak foundations blown away some distance; cars thrown and large missiles generated.
F5 261 - 318 Incredible damage . Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 meters (109 yds); trees debarked; is the U.S. government's official web portal to all federal, state and local government web resources and services.