By Paul Homewood
As we, fortunately, head towards the end of the third quiet tornado year in a row, let’s take a closer look at the intensity of tornadoes in the US.
We often hear claims that tornadoes are growing stronger as a result of global warming. But what do the facts tell us?
NOAA’s Storm Prediction Center maintains a database of tornadoes back to 1950. However, it is generally accepted by tornado experts that data is unreliable from the 1950’s and 60’s, which were known as the “tornado growth period”, as observation practices began to develop.
Another problem during these early decades was that, according to the SPC’s Greg Carbin, there were too many higher-rated tornadoes because of post rating.
Therefore, any analysis can only be reliably started from 1970.
It is also well accepted that many more small EF-0 tornadoes are spotted nowadays, that would have gone unnoticed previously, so these too need to be excluded from any analysis.
[Quick note – The EF-Scale was introduced in 2007, to replace the F-Scale. All above references to EF-Scale refer to both.]
From the SPC data, we can therefore plot the annual number of EF-1+ tornadoes.
As we see, the number of tornadoes has been on a declining trend.
But within this total, are stronger tornadoes on the increase? We can plot the percentage totals of each category.
The results are perfectly clear – the weakest EF-1 tornadoes have been increasing as a proportion, while all the other categories have been declining. (I have not shown EF-5’s separately, as these are such a small number, about one a year, as to make any trends meaningless).
Can we rely on tornado classifications being consistent over time? There is no easy answer to this, but when the Enhanced Fujita system was introduced in 2007, it was stressed that the new system was designed to ensure compatibility with the original Fujita scale.
In any event, NOAA themselves still publish official analyses using these classifications, stating:
To better understand the variability and trend in tornado frequency in the United States, the total number of EF-1 and stronger, as well as strong to violent tornadoes (EF-3 to EF-5 category on the Enhanced Fujita scale) can be analyzed. These tornadoes would have likely been reported even during the decades before Doppler radar use became widespread and practices resulted in increasing tornado reports. The bar charts below indicate there has been little trend in the frequency of the stronger tornadoes over the past 55 years.
Analysis of long term climatic trends can often be fraught with these sort of data issues, whether tornadoes, hurricane or temperatures. Nevertheless, the official data is the best we have got. And its message is clear.
Tornadoes since 1970 have been declining both in number and intensity.
All data from the SPC