So far, through June 23, we've had 8 total hours with heat index readings at/above 100 degrees in Cleveland. Compare this year to other years shows that the recent extreme heat is well ahead of most years.
Even by the end of June historically rarely shows a big increase in instances with a heat index at/above 100.
Here are the yearly 100 degree heat index hourly totals going back to the early 1930s. The most ever was the summer of 1936 followed by 1995 and the drought summer of 1998. More recently, 2012 stands out.
Interestingly, the summer of 1936 featured two instances where the heat index reached and stayed 100+ for 9 straight hours. July 12 between 11am and 8pm and July 13 between 12pm and 8pm.
Finally the totals for each summer since 2011:
What are the longest periods with heat index readings at/above 100?
1936: 10 hours on July 12 between 11am and 8pm. Average was 105 (highest: 105)
9 hours on July 13 between 12pm and 8pm. Average was 104
1995: 12 hours on July 14 between 12pm and 11pm. Average was 112 (highest: 115)
9 hours on July 15 between 9am and 5pm. Average was 108
9 hours on August 13 between 12pm and 8pm. Average was 105
1988: 10 hours on July 16 between 11am to 8pm. Average was 104 (highest: 109)
9 hours on August 17 between 11am and 7pm. Average was 102
2012: 8 hours on July 7 between 11am and 6pm. Average was 107 (highest: 113)