Significant Tornadoes/Tornado Days By Month, 1875-2011

Significant tornadoes represent a greater threat to life and property compared to weak tornadoes, which means that they deserve more attention.  What defines a "significant" tornado?  Some define a significant tornado to be any tornado that causes fatalities or one of at least F2/EF2 strength.  There is no right or wrong answer but the latter criteria will be used for the purpose of this post.

When examining Indiana's tornado record, it is clear that there is a long history of significant tornadoes.  Some years/decades have been more active than others to be sure, which may be due to cyclical weather patterns or just plain luck (a tornado must hit something to even have a chance to be rated EF2, after all).
In most instances on this blog, I use Grazulis ratings for tornadoes prior to 1950 and NOAA ratings for tornadoes since 1950.  I have done that again here, but this time will also utilize the Grazulis database up to 1991 to demonstrate the differences.

Combining Grazulis data from 1875-1949 and NOAA data for 1950-2011, there were a total of 602 significant tornadoes and 332 significant tornado days.




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The March to June period contributes a majority of the significant tornadoes and tornado days, but other months certainly should not be overlooked.  November, for example, has produced a respectable number of significant tornadoes despite the low number of significant tornado days.

When using Grazulis data from 1875-1991 and NOAA data for 1992-2011, there were a total of 505 significant tornadoes and 282 significant tornado days.  This is a substantial drop compared to above (97 fewer tornadoes and 50 fewer tornado days), but this trend is not unique to Indiana.  The Fujita scale was not invented until the early 1970's, which means that tornadoes that occurred prior to that were rated retroactively, often by inexperienced individuals.  There is not a large difference in the degree of damage between a high end EF1 and a low end EF2 tornado, and ratings (NOAA) were particularly liberal in the 1950's-1970's as the Fujita scale was just in its infancy.




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Although the totals vary depending on the database, the March to June period is still the most active.

There are probably countless ways to analyze 137 years of data and this just a quick look.  I will have another post later.