Science Source

Tippett, Michael K.

Geophysical Research Letters

Published date October 6, 2014

Changing volatility of U.S. annual tornado reports

States that a simple measure of the US tornado climatology is the average number of tornadoes per year; however, even this statistic is elusive because of nonstationary behavior due in large part to changes in reporting practices
States that differencing of the annual report data results in a quantity without mean trends, the standard deviation of which is denoted as volatility, since it is an indication of the likely year-to-year variation in the number of tornadoes reported
Finds that while volatility changes detected prior to 2000 can be associated with known reporting practice changes, an increase in volatility in the 2000s across intensity levels cannot
Finds a volatility increase in a tornado environment index which measures the favorability of atmospheric conditions to tornado activity, providing evidence that the recent increase in tornado report volatility is related to the physical environment