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Tornado Intensity Index

August 23, 2014

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.)





[ 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.  




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


EF Number mph Squared
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.  




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

  1. August 24, 2014 2:04 am

    Reblogged this on cosmoscon and commented:
    Remember when teh AGW cult told us that increasing CO2 concentrations in the atmosphere would cause stronger tornadoes in the US? Add this to the large heap of failed Global Warming predictions….

    • David permalink
      August 24, 2014 7:45 am


      “Remember when teh AGW cult told us that increasing CO2 concentrations in the atmosphere would cause stronger tornadoes in the US?”

      That might be the case among certain dubious ‘green’ propagandists but I can’t find anywhere in either IPCC AR4 or AR5 that says increased CO2 concentrations (or climate change) are predicted to influence tornado trends in the US in any way.

      AR4 summary of projections for North America is here:

      The AR5 North America Regional Aspects report is available from Volume 2 here:

      References to cyclonic activity in both reports are specific to coastal communities and appear to refer only to large scale ocean-based cyclonic activity (tropical and extra tropical storms, etc). Even then, the risk is defined as ‘very low’ to ‘low’ for both the present and near-term (to 2040).

    • August 24, 2014 9:38 am

      Kevin Trenberth is quite clear that global warming should be leading to stronger tornadoes.

      He also said

      “it is irresponsible not to mention climate change” in the context of these extreme tornadoes.

      Still I suppose he is really just a dubious green propagandist, as David says!

      • David permalink
        August 24, 2014 3:54 pm

        Thanks for the links Paul. Well researched article as usual, by the way.

        I wouldn’t rule Trenberth out as being a green propagandist in his spare time! But the question is, has he placed these ideas into the scientific literature? Does the actual science of climate change say that tornadoes should become more common/intense in the US?

        That’s not a cop out. If Kevin or others haven’t done so, then there’s probably a good reason for that, i.e. there’s *no* strong evidence supporting such views. That’s why they didn’t make the IPCC reports.

        To my mind at least, it’s better to focus on what the scientific literature actually says, rather than on the quoted views of scientists in the media, etc. Certainly you are right to draw attention to unsupported views expressed by individual climate scientists. They should be more careful what they say to the media and stick to the facts.

  2. tom0mason permalink
    August 24, 2014 8:48 am

    Nice figures Paul but I see what you are doing.
    By using the approximation of raw figures your trend will be inaccurate. You’ve neglected to infill, homogenize, and adjust to get the consistantly rising values year on year which you know you require.
    Well that is NASA’s prefered method. Isn’t it?

  3. August 24, 2014 10:04 pm

    My biggest gripe comes from the fact that wind speeds must be estimated from damage. Unlike hurricanes, for which these large scale systems can have good measurements made of actual wind speeds, you can’t do that in nearly as formally and rigorously for tornadoes. If you are going to make a scale like that, I think you’d be better off using Doppler estimates and track time.

    As an aside, I suspect that tornadoes follow a Poisson distribution and a power dissipation based on integrated wind speed and track time would make the final distribution almost uniform for a given tornado season. I might be wrong, though.

  4. August 25, 2014 3:52 pm

    It’s whack-a-mole!

    “Bottom line: New data published in the journal Climate Dynamics suggest that the risk of big tornado days in the U.S. is on the rise. The increase in tornado outbreaks is thought to be partially driven by increases in convective energy in the atmosphere due to climate change.”

    The paper –

    analyzing the density of tornadoes within the clusters. There is a consistent decrease in the number of days with at least one tornado at the same time as an increase in the number of days with many tornadoes. These changes are interpreted as an increasing proportion of tornadoes occurring on days with many tornadoes. Coincident with these temporal changes are increases in tornado density as defined by the number of tornadoes per area. Trends are insensitive to the begin year of the analysis. The bottom line is that the risk of big tornado days featuring densely concentrated tornado outbreaks is on the rise. The results are broadly consistent with numerical modeling studies that project increases in convective energy within the tornado environment.

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