Tornadoes since 1970 have been declining both in number and intensity.
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).