Just going by number of 90 degree days, 1983 had our record prior to 2010. JFK did this amazing thing in 1983, 100 degrees in both July and August and then we hit 99 on 9/11/83 with 7 90 degree days just in September, 37 for the season.
1983 was hot June, July, August and September.
I don't go by average temperatures, NWS ranks summer heat by number of 90 degree days. We can extend that to very hot summers by ranking them by 95 degree days or even 100 degree days.