Is there a demographic bias in the Kansas tornado record? Dr. John Heinrichs Samuel Lane Department of Geosciences Fort Hays State University

Overarching research goal Identify the climatological factors that influence tornado occurrence in Kansas Tornadoes cause many deaths and injuries each year, predictions can assist disaster managers Simulations of global climate change suggest increases of extreme weather Argonia, KS May 29, 2004 (

Background Previous research indicated a clear relationship between the El Niño/Southern Oscillation and tornado frequency (in particular, tornado activity increases in the western portion of the state after La Nina events) Higher frequency of tornadoes in counties containing major metropolitan areas was noted

Background (continued) During that research, observed that Kansas tornado report occurrence shows dramatic positive trend with time Raised question about whether the observed increase was due to more observers (i.e., tornadoes were just as common in the past, we just missed them) Average number of tornadoes per year: 56 Standard deviation: 30 Note “lull” in tornadoes in 1970s, steep increase afterwards

Previous research on the topic Divided opinions: –Aguirre et al., 1994: population and income not related to detectability of weak (F0 and F1) tornadoes across the continental US – excellent paper but needs to be confirmed and updated (also county level may be too fine) –Anderson et al., 2005: spatial variability of tornado reports may be modeled by a measure of human population density

Data Tornado data obtained from the Severe Storms database at the National Climatic Data Center (NOAA/NESDIS) for total tornado reports All data assigned counties (post 1993, data set has more precise locations) Population data for 1950, 1960, 1970, 1980, 1990, 2000 from US Census Bureau

Dramatic increase in F0 tornadoes since 1975 (note that in the 70s, more F1s and F2s then F0s, though!); F4s and F5s have been stable over time Perhaps the weakest tornadoes were missed previously?

Methodology Spatial unit of analysis: climate division Calculate tornado trends (all tornadoes and F0s only), population trends for each division over , Test null hypothesis that the tornado and population trends are correlated across the divisions

Results: All tornadoes Divisions 3,7,8 have same trend sign, rest are opposite Division 6 (largest increase in population) had a decrease in tornadoes!

Statistical results for all tornadoes, Pearson’s correlation coefficient (assumes normality) over all divisions: Spearman’s rank correlation (nonparametric): Critical value for 7 degrees of freedom: Can reject null hypothesis at 95% level of significance

Results for all tornadoes Larger tornado trends, all positive 5 divisions with same trend sign Division 6: greatest population growth but lowest tornado trend

All tornadoes Pearson’s correlation: Spearman’s rank correlation: Null hypothesis can be rejected

Results for F0 tornadoes Every division has positive tornado trend Signs of tornado and population trends opposite for 5 divisions

F0 tornadoes Pearson’s correlation: Spearman’s rank correlation: 0.02 Null hypothesis can be rejected

Results for F0 tornadoes Tornado trends greater then full time period 4 divisions with opposite sign

F0 tornadoes Pearson’s correlation: Spearman’s rank correlation: Null hypothesis can be rejected

Graphical representation by decade Left graphs show tornado totals by decade, right show population by decade

Another interesting result If multiple tornado reports on the same day in the same county are combined (as done for the ENSO study), trend is largely eliminated Difference between reported tornadoes and tornado events/days is greatest after 1990

Conclusions Can reject hypothesis that increases in tornado report frequency are related to population change Results consistent with Aguirre et al. (1994) In fact, correlations surprisingly suggest very weak inverse relationship Divisions containing KC/Topeka, Wichita (rapid growth areas) have very different trend patterns Much of apparent growth of weak tornadoes is in multiple events

Discussion Is the observed increase in tornadoes a real one? Not necessarily! –Changes in reporting process? –More attentiveness/awareness? –More spotters/chasers? Could it be that the observed increase is because we are better at seeing what goes on in large mesocyclones with tornado swarms? Future work needed to test these hypotheses

References Aguirre, B. E., Rogelio Saenz, John Edmiston, Nan Yang, Elsa Agramonte, Dietra L. Stuart (1994). "Population and the Detection of Weak Tornadoes." International Journal of Mass Emergencies and Disasters 12 (3): Anderson, C.J., Wikle, C.K., Zhou, Q., and J.A. Royle (2005). "Population Influences on Tornado Reports in the United States" In Review.