After experiencing our first 90-degree day of the season on Friday, many people are wondering what we might expect this summer.

It turns out that the number or 90-degree days each summer is extremely variable here in Madison. From 1971 to 2015, the average number of days at or above 90 in Madison was 10.96. This average, however, struggles to 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 two-thirds of the years will fall.

In this case the standard deviation is nine. Thus, we might expect that two-thirds of the years would range from having 20 to two days at or above 90. As it turns out, 33 of the last 45 summers have been in that range.

It is interesting that six summers have had 20 or more hot days (1975, 1976, 1983, 1988, 1995 and 2012) – the record being held by 2012 with 39 days.

Over the last four and a half decades, there has 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, ’80s, ’90s and ’00s, respectively. The half-completed 2010s, by virtue of the incredibly warm 2012, appear to be bucking this trend as, thus far, we have averaged 12 such days each summer in this decade, though only 2012 had as many as 12.

It remains to be seen what the rest of this summer and the decade will bring, but 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.