One reasonable way to gauge how hot a summer season was is to consider how many days that year reached 90F or above. It turns out that this number is extremely variable here in Madison.
From 1971 to 2016, the average number of days at or above 90F in Madison is 10.9. As is often the case with statistics, however, the average does not convey a sense of the variability. A better way to express that variability is by calculating the standard deviation, which, when added to or subtracted from the average, sets a range in which approximately 2/3 of the years will fall.
In this case the standard deviation is 9.0. Thus, we might expect that 2/3 of the years would range from having 19.9 to 1.9 days at or above 90F. As it turns out, 34 of the last 46 summers have been in that range!
It is interesting to note that six summers have had 20 or more hot days (1975, 1976, 1983, 1988, 1995 and 2012) — the recent scorching summer of 2012 had 39 days (one short of the record 40 of 1955)!
Notably cold summers (by this measure) include 1979, 1996, 1998, 2000, 2004, 2008 and 2014 with 2004 being the only summer in the last 46 years in which the temperature never reached 90F.
Broken down into decades, there had been a trend toward fewer hot days each summer with the averages being 15.8, 11.7, 8.2 and 7.3 days for the 1970s, 1980s, 1990s and 2000s, respectively. The summer of 2012 singlehandedly accounts for a departure from this trend as this decade has thus far averaged 11.42 days (only 6.83 without 2012).
These data remind us how complicated the interplay between weather and climate can be since the global average temperature has been trending the other way in these same decades.