Tornado and Hailstorm Behavior for 2011 Consistent with Weekly Cycles Observed in Earlier Years

The paper by Rosenfeld and Bell (2011) contains a figure showing the day of the week favored by hailstorms and tornados for each summer for 1995–2009. There is a clear tendency for storms to avoid weekends — particularly for hailstorms, possibly because there are many more hailstorms than tornados and the statistics for hailstorms are clearer as a consequence. Data for 2010 and 2011 are now available, and an updated figure is shown above. The weekly cycles detected for these years are rather weak but seem consistent with the patterns seen in earlier years.

Figure 1: Each colored balloon shows the day of the week when storm activity is most likely, for a given summer (Jun–Aug) for the year whose last two digits are inscribed in each balloon. The balloon’s distance from the origin shows the “strength” of the cycle. For example, a balloon close to the origin means that the favored day of the week was difficult to determine because the weekly cycle was “noisy”. The gray circles around the origin are labeled by “p-values” and indicate how far from the origin a weekly cycle balloon might appear when there is in fact no “real” weekly cycle in the data, when all that is happening is the natural variability of weather. For example, there is only a 3% chance that a weekly cycle could appear in the data “by accident” beyond the outermost gray circle.

Rosenfeld, D., and T. L. Bell (2011): Why do tornados and hailstorms rest on weekends? Journal of Geophysical Research, 116, D20211, doi:10.1029/2011JD016214.

Data Sources:
Data were collected by NOAA’s Storm Prediction Center [SPC], in the form of lists of locations, times, and strengths of tornados or hailstorms, available from the web site