The US is in the middle of tornado season, which has been particularly destructive this year, with 21 people killed in violent storms across the country and the threat of tornadoes remaining high in some areas well into July.
Tornadoes, like hurricanes, have a rating system that meteorologists use to determine the strength of a storm.
How is the strength of a tornado measured?
Tornadoes, like hurricanes, have a rating system that meteorologists use to determine the strength of a storm. The Improved Fujita Scale (abbreviated EF scale) has been used by the National Weather Service since 2007 to issue storm ratings based on winds and associated damage. The EF scale is different from the scale that rates hurricanes, and is determined after the storm due to the chaotic nature of tornadoes. Tornado strength, behavior, and damage potential are difficult to predict, and when a tornado warning or watch is issued, the danger is imminent and people should follow the advice.
The EF system replaces previous versions that only considered wind gusts.
Gusts of wind and the improved Fujita scale
EF Rating | 3-second wind gust (mph)
- Rating 0: 65～85
- Rating 1: 86 to 110
- Rating 2: 111 to 135
- Rating 3: 136 to 165
- Rating 4: 166 to 200
- Rating 5: 200+
Source: National Weather Service
The expanded version also takes damage into account by looking at physical structures such as schools, shopping centres, homes and trees and their impact on nature.
The most powerful tornadoes recorded in the United States
An F5 storm is the highest ranking in the original and enhanced Fujutia rating systems. The NWS has data on tornado intensity going back over 100 years, providing a long list that can be consulted to determine the most “destructive” storms. Tornadoes in the 19th century tended to cause more damage and deaths because the infrastructure and warning systems of the time were simply not up to the task of dealing with the threat. In terms of deaths, the five most destructive tornadoes occurred in the following areas:
- March 18, 1925 | 695 people died in Missouri, Illinois, and Indiana
- May 6, 1840 | Natchez, Mississippi: 317 dead
- May 27, 1896 | St. Louis, Missouri: 255 dead
- April 5, 1936 | Tupelo, Mississippi: 216 dead
- April 6, 1936 | Gainesville, Georgia: 203 dead
Source: National Weather Service
But when we look at the physical damage caused by storms, the five most destructive storms are:
- May 22, 2011 | Joplin, Missouri: $2.8 billion
- April 27, 2011 | Tuscaloosa, Alabama: $2.45 billion
- May 20, 2013 | Moore, Oklahoma: $2 billion
- June 8, 1966 | Topeka, Kansas: $250 million ($2.3 billion in 2023 dollars)
- May 11, 1970 | Lubbock, Texas: $250 million ($1.9 billion in 2023 value)
Three types of tornado warnings issued by the National Weather Service
When a tornado is expected, the NWS issues three warnings: a tornado watch, a tornado warning, and a tornado emergency. During a tornado watch, the NWS urges people to “be prepared!” and encourages residents to “check and talk about tornadoes.” […] “Develop an emergency response plan, take stock of supplies, and make sure you have a safe room.” The next level is a tornado warning, which means a tornado has reached the area and residents should “move to an indoor room on the lowest floor of a sturdy building” and “avoid windows.” During the most severe tornado emergency warning, the NWS advises residents to “immediately seek shelter and
