As we head from early to late spring during the month of May, there are a number of ways to measure this progress. One way is to consider how often we experience a temperature 90 degrees during May. The last time Madison reached 90 degrees in May was just two years ago — on May 24, 2010. This is a relatively rare occurrence, however, as Madison has reached 90 degrees in May only 10 times since 1971 (once each in 2006, 1991, and 1988; twice each in 1978 and 1977 and three times in 1975).
A 90-degree temperature in May is not a particularly good indicator of the expected intensity of the heat during the subsequent summer either as 1988 had 36 days over 90 degrees while 2006 had only eight.
Another less parochial way to measure the advance of spring into summer is to consider the inevitable shrinkage of the pool of frigid air that rings the North Pole. We have recently calculated the daily average area over the Northern Hemisphere covered by air (at one mile above the surface) with temperatures below various thresholds.
It turns out that air at that level that is colder than minus-13 degrees has, on average since 1960, disappeared from the Northern Hemisphere on May 25 and does not rear its chilly head again until (on average) Sept. 13. So, by this rather obscure but precise measure, the Northern Hemisphere warm season begins in earnest on May 25.