National Preparedness Month is recognized each September to raise awareness about the importance of preparing for disasters and emergencies that could happen at any time. This year, the campaign will focus on severe weather events. Last week, the Division of Public Safety at Case Western Reserve University shared information about what to do after a tornado.
This week, the series will wrap up with an explanation of how tornadoes are measured.
The Enhanced Fujita (EF) Scale is used to assign a tornado a “rating” based on estimated wind speeds and related damage. When tornado-related damage is surveyed, it is compared to a list of damage indicators (DIs) and degrees of damage (DoD), which help better estimate the range of wind speeds the tornado likely produced. From that, a rating (from EF0 to EF5) is assigned.
The EF Rating scale is as follows estimated in units of miles per hour:
- EF 0: 65-85
- EF 1: 86-110
- EF 2: 111-135
- EF 3: 136-165
- EF 4: 166-200
- EF 5: Over 200
The EF scale is a set of wind estimates (not measurements) based on damage. It uses three-second gusts estimated at the point of damage based on a judgment of levels of damage to indicators. These estimates vary with height and exposure.
The National Weather Service (NWS) is the only federal agency with authority to provide ‘official’ tornado EF Scale ratings. The goal is to assign an EF Scale category based on the highest wind speed that occurred within the damage path. First, trained NWS personnel will identify the appropriate damage indicator (DI). The tornado evaluator will then make a judgment within the range of upper and lower bound wind speeds, as to whether the wind speed to cause the damage is higher or lower than the expected value for the particular DoD. This is done for several structures—not just one—before a final EF rating is determined.