A tornado that hit an Illinois city during a storm that killed at least six people there Wednesday has preliminarily been given a rating of EF4 – the second-most powerful on the Enhanced Fujita Scale, according to the National Weather Service.
A report indicated the tornado in Harrisburg, Illinois, had winds estimated at 170 mph. EF4 tornadoes have wind speeds between 166 mph and 200 mph.
The Enhanced Fujita Scale rates tornadoes by estimated wind speed, which is determined not by measurements, but by observations of the damage left behind, according to the National Weather Service’s Storm Prediction Center.
To get the wind speed, the weather service goes to “damage indicator” tables that describe the wind speeds that would be required to do certain types of damage to certain structures. For example, if the exterior walls of one- and two-family homes collapsed, but not the interior walls, the service would consult the one- and two-family home table and find that wind speeds of about 132 mph were needed to do that.
A tornado of 132 mph would have an EF2 rating, which encompasses tornadoes between 111 mph and 135 mph.
EF0 is the weakest Enhanced Fujita rating, and EF5 is the strongest. Below is a description of the categories, with the general kinds of damage that may be associated with them.
EF0: 65-85 mph. Light damage. Some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; signboards damaged.
EF1: 86-110 mph. Moderate damage. Peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos blown off roads.
EF2: 111-135 mph. 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.
EF3: 136-165 mph. 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.
EF4: 166-200 mph. Devastating damage. Well-constructed houses leveled; structures with weak foundations blown away some distance; cars thrown and large missiles generated.
EF5: 200 mph. 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; incredible phenomena will occur.