More tornados and hailstorms observed in the middle of the week

November 1, 2011 — Click on image to enlarge.

Each colored balloon with the last two digits of the year inscribed shows the day of the week favored by storms for a given summer (Jun–Aug).  The balloon’s distance from the origin shows the “strength” of the cycle: a balloon  close to the origin means that the cycle was very noisy and the favored day of the week difficult to determine.

Hailstorm and tornado activity increases in the middle of the work week (Tuesday-Thursday) compared to weekends, as shown in a recently published paper1 by Rosenfeld and Bell in the Journal of Geophysical Research.  Weekly cycles in weather behavior are a clear sign of human influence on our climate.

The weekly cycle is believed2 to be caused by the well-known weekly changes in pollution levels with the day of the week.  Aerosol pollution decreases the size of water droplets coalescing in clouds.  They are lighter and don’t fall out as rain, but instead rise to much higher altitudes where they freeze and release additional heat (“latent heat of fusion”).  This invigorates the storm and produces more ice aloft.  This might explain the increase in hailstorms as well as the increase in lightning that has also been observed.3  It is conjectured by Rosenfeld and Bell (2011), based on numerical model simulations, that storms, amped up by pollution, nevertheless produce weaker cold pools at their base.  Tornados develop less easily when a cold, rapidly moving pool forms beneath the storm.  By weakening cold pool formation, pollution may lead to storms with better chances of forming a tornado than is the case for storms formed in clean air.

The Figure shows statistics for the year 2010 added to the years 1995–2009 analyzed in the paper by Rosenfeld and Bell (2011).  The weekly cycle detected for the summer of 2010 was weak.  The day of the week with maximum activity for 2010, Friday, again falls outside the weekend sector.

Since each summer supplies only 13 weeks of data (i.e., 13 “experiments”), and hailstorm and tornado occurrences are quite sporadic, statistical results for a single summer are themselves bound to be noisy.  Although the balloons obviously tend to avoid the “weekends” (Sat-Mon), those that fall in the weekend sector also tend to cluster near the origin, indicating that the data are not very conclusive about the day of the week in those cases.

Since 10% fluctuations in aerosol pollution relative to background levels may be producing 10% fluctuations in severe storm activity (not shown here), this suggests that the “background” anthropogenic aerosol levels may be making storms more severe than what we would be experiencing in a cleaner atmosphere.

References:
1Rosenfeld, D., and T. L. Bell (2011): Why do tornados and hailstorms rest on weekends? Journal of Geophysical Research, 116, D20211, doi:.1029/2011JD016214.

2Bell, T. L., D. Rosenfeld, K.-M. Kim, J.-M. Yoo, M.-I. Lee, and M. Hahnenberger (2008): Midweek increase in U.S. summer rain and storm heights suggests air pollution invigorates rainstorms, Journal of Geophysical Research, 113, D02209, doi:10.1029/2007JD008623.

3Bell, T. L., D. Rosenfeld, and K.-M. Kim (2009): Weekly cycle of lightning: Evidence of storm invigoration by pollution. Geophysical Research Letters, 36, L23805, doi:10.1029/2009GL040915.