How does the tornado scale work?

Answer 1

The tornado rating scale currently used in the United States is the Enhanced Fujita Scale. It uses the severity of the damage caused by a tornado to assign a rating, ranging from EF0 for the weakest tornadoes to EF5 for the strongest.

In more detail:

After a tornado strikes, meteorologists and engineers go out to survey the damage done to man made structures and trees. They look at the degree of damage done by the tornado and, based on the guidelines of the scale, assign a wind speed estimate to the damage. The quality and type of construction are taken into account in assigning wind speed estimates. The wind speed estimates is then used to assign a rating to the damage. Different structures and areas in a tornado's path may receive different ratings, as some areas may be more severely impacted. The highest rated damage along the path of the tornado will be the tornado's rating. In other words, a tornado can cause mostly EF0 and EF1 damage, but will be rated EF2 as long as there is at least one instance of EF2 damage. This is useful as tornadoes often fluctuate in intensity, and damage severity can be erratic.

One major flaw of the scale involves tornadoes that occur in area where there are few or no damage indicators. These tornadoes are often rated below what their actual intensity is. A tornado that reaches EF5 strength, but does not hit any structures with EF5 winds cannot be rated as such. Additionally, some weaker structures cannot be used to assign high ratings. For example, barns, depending on quality of construction, will be completely obliterated by winds of EF1 or EF2 strength. As a result, damage to barns cannot be rated higher than EF2. Because of this, it is quite likely that the number of tornadoes that reach violent (EF4+) intensity is several times higher than actual record of ratings indicated.

Here are the ratings with their wind speed estimates and damage that might be expected. Note that the wind speed estimates have not been fully verified to correspond with these damage levels, and so should be taken with a grain of salt.

• EF0 (65-85 mph): Some shingles may be peeled from roofs. Vinyl siding, gutters, and siding may be peeled torn away. Weak outbuildings may be destroyed. Fences may be knocked down. Some windows may break: Some trees and tree limbs may be knocked down.
• EF1 (86-110 mph): Roofs of houses are severely damaged. Poorly secured roofs may be removed. Weak exterior walls in some structures may collapse. Barns, garages, and trailer homes will likely be severely damaged or destroyed. Trailers may be flipped.
• EF2 (111-135 mph): Roofs are torn from well-built houses. Exterior walls in some houses may collapse. Poorly anchored houses may shift off their foundations. Trailer homes are completely obliterated. Larger trees are snapped. Cars may be thrown short distances and freight trains derailed.
• EF3 (136-165 mph): Well-built houses lose exterior walls and some interior walls. Some houses may have only a few interior walls left standing. Weaker houses may be completely leveled. Most trees will be snapped or uprooted, and some debarking may occur.
• EF4 (166-200 mph): Well-built houses are completely leveled. Some houses may be blown away. Asphalt may be stripped from roads. Cars and similar objects become projectiles. Vehicles mangled beyond recognition.
• EF5 (over 200 mph): Well-built houses are completely swept away, leaving empty foundations. Reinforced masonry structures are leveled. Severe ground scouring may occur.