U.S. Tornado Statistics and Data Set Analysis

Jun 17, 2007
SIlver Spring MD
I recently asked SPC for the U.S. annual list of tornado count, fatalities, and killer tornadoes.
They sent me the list. Official records begin in 1950.

This provides a good opportunity to examine an official data set and realize they can sometimes
be misleading, especially when trying to determine trends over an extended period of time.

1) There is a significant spike in annual tornado count starting in 1953. This was the year the Severe
Local Storms (SELS) unit, forerunner of SPC, was established for around the clock severe storm
monitoring. Tornado reporting was encouraged by the U.S. Weather Bureau for documentation
and watch/warning verification purposes, hence the increase in count.

2) Starting in 1990, you see another significant spike to above 1000 in tornado count. This year
marked the first installation of NEXRADs and around this time, storm chasing was beginning to ramp
up in popularity. Both of these resulted in more tornadoes being reported, verified, and documented.

3) After 1990, the NWS Modernization occurred over the next several years, and investigation of
tornado reports and post damage storm surveys at each new WFO county warning area ramped up
considerably, leading to a more representative count of tornadoes per year, staying above 1000 for
all years afterwards except 2002.

4) Storm chasing really took off after the release of the movie 'Twister' in 1996 and has continued
to grow ever since. This continued to increase/sustain higher annual tornado counts.

5) By the middle part of the first decade of the new millennium, digital cameras were rapidly replacing
traditional slide and print film. This eliminated the long processing times of film, allowing for not only
more tornadoes to be verified and documented, but also in a more timely manner.

6) By the end of the first decade of the new millennium, social media such as Facebook and Twitter
allowed instant reporting of tornadoes in real time. Wireless Internet had been firmly established for
the now ubiquitous smartphone/mobile device, combined with built-in digital still picture and video
cameras, made tornado reporting and verification even quicker and more efficient. Also, tornadoes
took front and center in the mainstream media, both in the daily news and reality shows.

So all the above shows that a data set can be misleading over an extended period of time if you
do not take into account changes in things like technology, population, and public awareness, as
three examples. Despite all this, we are still too low in the annual tornado count. By how much on
average?, difficult to say, but likely by a few hundred. We document nearly all the the significant
tornadoes, but smaller, short lived ones are missed, due to darkness, poor visibility, and/or remoteness
of location. We are finding tornadoes in the Rockies and Great Basin are more common than once
thought. Given the low population density and large area, it is likely these areas of the country are
where we miss the most tornadoes.

Some other notes...you can see the number of tornado fatalities can highly vary from year to year going
back to to 1950. Overall though, there has been downward trend in fatalities long term. However, the
benefits of better warnings and awareness/preparation are constantly being tempered by the growing

Year U.S. Population
1950 151,325,798
1960 179,323,175
1970 203,211,926
1980 226,545,805
1990 248,709,873
2000 281,421,906
2010 308,745,538

We have doubled in population in 60 years. Put simply, there are just more people in the way of
tornadoes every year. In addition, population in and around the larger cities has become denser, so
the potential of very high death tolls for when a large, violent tornado plows through any of the larger
cities is becoming more and more likely as time goes on.

I think this shows reasonable evidence that tornadoes are becoming "worse" not necessarily due to
meteorological or climatological reasons, but more due to technological and sociological reasons.

I agree. Thanks for providing the statistics that makes it that much clearer. I think the evidence to this is last year in April in AL, MI, MO, and LA. Tornadoes tore through fairly dense areas of population. If one looks at the statistics of 2008 - which had nearly matched last year for the number of tornadoes in a year - one sees 1/4 of the number of deaths. Due to the fact that although more tornadoes touched down, they occurred in rural areas for the most part.