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The Enhanced Fujita Scale: How tornadoes are measured

Here's how the National Weather Service uses the Enhanced Fujita Scale to determine tornado strength.

MOLINE, Ill. — On Monday, the National Weather Service issued measurements for the tornado that went through Winterset, Iowa, as an EF-4, and the two tornadoes in the Quad Cities as an EF-0 and EF-1. So what is the Enhanced Fujita Scale and how does it work?

The Enhanced Fujita Scale, previously known as the Fujita Scale, was created by Dr. Tetsuya Theodore Fujita in 1971 to estimate tornado wind speeds based on the damage it leaves behind.

However, there were problems with the original scale's measurements. Issues that arose included a potential lack of damage indicators, no account for construction quality and variability, and no definitive correlation between damage and wind speed. This often meant that tornadoes were being rated inconsistently and that wind speeds were often overestimated.

On Feb. 1, 2007, the Enhanced Fujita Scale (EF) was created and replaced the original. It takes more variables into account than its predecessor, such as damage indicators (DIs) and eight degrees of damage (DODs). Damage Indicators include building types, structures and trees; while DODs rank from visible damage, minor damage, major damage and complete destruction.

There are 28 damage indicators, starting at No. 1, small barns and farm outbuilding to No. 11 a large shopping mall, to the final indicator, No. 28, soft wood trees. Each indicator has eight degrees of damage; and a few may have one or two more degrees added to form a more complete understanding of the damage.

An example using damage indicator No. 1 — NWS surveyors would go to the scene of the tornado and evaluate the damage to small barns and farm buildings based on the following criteria:

Credit: NOAA

The first DOD would be a threshold of visible damage, while the eighth DOD would be total destruction of the indicator. Each correlates with what wind speed must have been present to cause this damage. EXP is the expected damage that is done, LB means lower bound, or less damage than expected, and UB is the upper mound, or more damage than expected. SBO is the abbreviation of for damage indicator No. 1.

The creation of the new scale reorganized the way in which wind speeds are classified, consisting of EF0 (gale tornado), EF-1 (Moderate), F-2 (Significant), EF-3 (Severe), EF-4 (Devastating) and EF-5 (Incredible).

An EF-0 has winds between 65 – 85 mph. Light Damage includes damage to chimneys, branches broken off of trees, shallow trees uprooted and sign boards damaged.

An EF-1 has winds speeds of 86 – 110 mph. These cause moderate damage that includes roof surfaces peeling off, mobile homes pushed off their foundations or overturned, loss of exterior windows other glass broken.

An EF-2 has winds speeds of 111 – 135 mph. These cause considerable damage, tearing roofs off of houses, demolishing mobile homes, pushing cars over or lifting them off the ground, snapping or uprooting larger trees and turning small objects into dangerous projectiles.

An EF-3 has winds of 136 – 165 mph. These bring to mind stories of destroyed houses, heavy damage to large buildings, overturned trains, large cars being thrown from the ground and mobile homes blown away.

An EF-4 has winds speeds of 166 – 200 mph. These cause devastating damage like leveling a home's entire frame, throwing cars into the air and creating missiles out of debris.

An EF-5 has winds speeds over 200 mph and an F5 had wind speeds of 261 – 318 mph. They do the most damage possible, sweeping away the strongest houses, carrying cars over a hundred yards away, heavy structural damage to high rise buildings and debarking trees.

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