Tornadoes are one of the most dangerous forms of weather. However, since we cannot normally intercept tornadoes, it is difficult to measure their exact wind speed. So, how do we know their strength?
Well, unlike hurricanes which are categorized by wind speed, tornadoes are categorized by the amount of damage they do, and in turn, translated to wind speed based on previous studies. The table used to determine this is called the EF scale, or "Enhanced Fujita" scale. This is a newer version of the F scale, or "Fujita" scale which was replaced in February of 2007. Changes were made to the F scale because it was inconsistent in determining wind speeds based on damage. This scale didn't take in the structural integrity of buildings, trees, powerlines / poles and many other objects if they were torn down. A building could have been 100 years old, rotting, and leaning over on a farm; if it got knocked over, there is the potential that a rating could have been skewed since the building was weak anyway. However, with the EF scale, it incorporates 28 different damage indicators "DIs" including structural integrity, types of trees, soil moisture etc.
The EF scale also adjusted the estimated wind speeds compared to the damage done. For example, using the Fujita scale, an F-3 tornado was thought to have produced wind speeds around 158 - 206 mph, though with extensive research and post storm examination, wind speeds were weakened with tighter gradients in the new EF scale. On the Enhanced Fujita scale, 158 - 206 mph winds range from EF-3 to EF-5 strength. One of the details that were ironed out over time was the extent of damage changed significantly with wind speeds between 150 and 200 mph, so the range needed to be shortened. Before the EF scale came out, there were numerous overestimations of tornadoes because of the large ranges. So, this needed to be changed.
Now, this process to change the tornado scale wasn't a guess and check. Wind speeds were tested using wind tunnels, different materials to resemble structures, computer programming (although the technology wasn't great, there were some very helpful tools) and other instruments. This was designed by several of the most prestigious meteorologists as well as wind engineers, so it was a thorough process.
Enhanced Fujita Scale. Found at: https://www.weather.gov/oun/efscale
We still use the EF scale today and there are 6 different strengths of tornadoes ranging from EF-0 to EF-5. Again, tornadoes are rated after they occur and they are based extensively on the amount of damage they do. If there is a tornado over farmland, it becomes a bit more difficult determining the strength, but there are ways to come to a conclusion. In these cases, the meteorologists will go around looking for any type of debris, path, or "tornado scar" to help determine the width. A tornado scar, in literal terms, is a marking on the surface caused by a tornado. It displays the path, width even multiple vortices helping determine how long it was on the ground, how wide it was and how strong it could have been when comparing it to other tornadoes with similar attributes. This can give clues to how strong a tornado was, but it will not describe the whole story. There have been times in the past when tornadoes tracked over farmland and only knocked over powerlines / poles since those were the only standing objects in its way. It is likely that some of these tornadoes were underestimated since there is minimal evidence to base it off.
Nonetheless, the transition from the F scale to the EF scale has been proven to be more accurate and precise when it comes to rating tornadoes. Maybe down the line we will have a way to send drones or other instruments into tornadoes safely without them being lost in the debris. For now, we will stick with the Enhanced Fujita scale.
