Tornado Intensity Index

By Paul Homewood

 

 

Is the intensity of tornadoes increasing in the United States, (or, for that matter, falling)? It’s a perennial question.

NOAA gives us some clues, with their charts of EF-1+ and EF-3 to EF-5 tornadoes since 1954. (NOAA ignore EF-0’s, because many more of these weak tornadoes get to be reported nowadays than in the past because of Doppler radar, better reporting practices, increasing population etc – for the background on this, see here.)

 

http://www.ncdc.noaa.gov/climate-information/extreme-events/us-tornado-climatology/trends

 

[ The original Fujita grading system, using “F” numbers, was replaced in 2007 by the Enhanced Fujita scale, hence “EF” numbers. The new system was designed to ensure compatibility with the original Fujita scale-  see here. All references to either Fujita or Enhanced Fujita should be regarded as interchangeable]

 

But these graphs tell us little about the distribution within the totals. For instance, could there be more EF-4’s relative to EF-3’s?

For tropical storms and hurricanes, there is the measure of Accumulated Cyclone Energy, or ACE, which is calculated by summing the squares of wind speeds for each storm, over 6-hourly intervals.

There is a similar method, called the Power Dissipation Index, or PDI, which, instead of squaring wind speeds, cubes them.

It should therefore be possible to use similar methodology with tornadoes.

 

Let’s start by looking at the estimated wind speeds, assumed under the EF system.  

 

http://www.spc.noaa.gov/efscale/ef-scale.html

 

And if we take the mid range speeds, then square and cube them,we get:

 

EF Number	mph	Squared /10,000	Cubed /1,000,000
0	75	0.56	0.42
1	97	0.94	0.91
2	123	1.51	1.86
3	152	2.31	3.51
4	183	3.35	6.13
5	230	5.29	12.17

N.B There is no maximum speed for EF-5 tornadoes, it is unlimited. I have therefore made an assumption of a mid range of 230 mph for this exercise.

 

Although both methods of squaring and cubing are valid, I personally feel that the cubing method gives a better fit. Nevertheless, I show the results of both calculations below.

 

 

 

As mentioned above, EF-0 tornadoes should be excluded, as improved tornado observation practices can create a misleading appearance of an increasing trend in tornado frequency.

There is also a great deal of evidence that the same applies to EF-1 tornadoes. As Figure 1 illustrates, there was a marked increase in the percentage of EF-1’s to total numbers between 1953 and 1990, since when the proportion has levelled off.

This is clear evidence that many such tornadoes occurred, but were never reported in earlier decades.  

Therefore, the analysis that follows will ignore both EF-0’s and EF-1’s.

 

 

Figure 1

 

Using the data provided by NOAA’s Storm Prediction Center, I have taken the annual tornado numbers by EF category, and applied the wind speed factors, as shown in the Table above. The totals for each category are added together for each year, to give the result in the indices shown below.   

 

 

 

Whichever method is used, there is a clearly declining trend in intensity.  

 

 

Sources

The number of tornadoes by category for each year are available from the Storm Prediction Center.

http://www.spc.noaa.gov/wcm/annualtornadomaps/