Looking at the number of 90-degree days as the sole criterion for gauging how hot a summer has been is a fairly standard measure, but it can be misleading. For instance, it doesn't take into account the fact that in some years 90-degree days are spread over considerably more weeks than other years. That is the subject of this analysis. For my purposes I'll be referring to the period between a year's first and last 90-degree day as the "window". New York's typical 90-degree window (based on 1980-2020) is a few days shy of 13 weeks, starting in late May and continuing thru late August. During this window, one in five days, on average, will see a high in the 90s or triple digits.
Looking at records dating back to 1872, the most 90-degree days in a year has been 39 - and it happened twice - in 1991 and 1993. However, while 1991's occurred over a lengthy span of 23 weeks, 1993's were more concentrated, occurring over five fewer weeks. 1991's hot days, in other words, were more "diluted". While 1991 experienced 90-degree temperatures during 24% of its "window", 1993's corresponding figure was 31%.
Yet, neither of these hot summers come close to 1999. Although that year had ten fewer 90-degree days, they were concentrated in a sixty-day window. That means that close to half of the days during its 90-degree window were in the 90s. And 1988 wasn't far behind, with 33 90-degree days over 77 days (43% concentration). Four other years with a considerable number of 90-degree days (24 or more) also had a higher concentration of 90-degree temperatures than either 1991 or 1993. So, you be the judge over which summer was hotter.
The chart below ranks years with more than two dozen 90-degree days based on their concentration of hot days that occurred between their first and last 90-degree day. (Although the summer of 2015 doesn't qualify for inclusion on the chart because it had twenty 90-degree days, its concentration was 25%.)
I'd like to to thank fellow weather fanatic, Eugene DeMarco, for helping me by supplying and organizing some of the weather statistics that provide the background material for this post.