What was done
For the period 1896-1995, the author analyzed thunderstorm data obtained at Cracow, Poland, which is "one of the few continuous records in Europe with an intact single place of observation and duration of over 100 years."

What was learned
Over the entire length of record, there were 2470 days with thunderstorms, or an average of about 25 days per year.  The highest annual number of thunderstorm days was 37, recorded in 1968 and again in 1975.  The lowest annual number was 9 in 1904.  Close analysis of the data, as reproduced in the figure below, reveals a slight but non-significant linear increase of 1.6 storms from the beginning to the end of the record (dashed black line).  From 1930 onward, however, the trend is negative, revealing a linear decrease of 1.1 storms per year from 1930 to 1996 (solid black line).  The author also reports there has been a decrease in the annual number of thunderstorms with hail over the period of record, and there has been a decrease in the frequency of storms producing precipitation greater than 20 mm.

What it means
Once again, the climate alarmist claim of CO2-induced increases in the number and severity of storms fails to pass the test of reality provided by actual data.  Over the past 70 years, the frequency of thunderstorms, thunderstorms with hail, and thunderstorms with extreme amounts of precipitation in Cracow, Poland has actually decreased, in stark contrast to climate alarmist predictions.