What was done
The authors analyzed hail-day and thunder-day occurrences over the 100-year period 1896-1995 in terms of 20-year averages obtained from records of 66 first-order weather stations distributed across the United States.

What was learned
Frequency of thunder-days peaked in the second of the five 20-year intervals, while hail-day frequency peaked in the third or middle interval.  Thereafter, both parameters declined to their lowest values of the century in the final 20-year period.  Hail-day occurrence, in fact, decreased to only 65% of what it was at mid-century.  There was also a drop in national hail insurance losses over the same period.

What it means
As the world has experienced its most dramatic warming of the century over the past two decades, so has the United States experienced its fewest hail-day and thunder-day occurrences over the same time period.  Hence, and contrary to what is typically claimed by those who say that global warming will lead to more and stronger storms, just the opposite appears to be occurring with respect to hail and thunderstorms in the United States, where we have some of the world's best records of these phenomena over a rather significant area of land.