So far this August, 78% of the US has been below normal temperature, with the central part of the country 6-8 degrees below normal.

MonthTDeptUS.png (688×531)
During August 1936, almost 100% of the US was above normal temperature, with the central part of the country 10 degrees above normal.

docs.lib.noaa.gov/rescue/mwr/064/mwr-064-08-c1.pdf
Heidi Cullen and other climate experts* tells us that global warming is raising summer temperatures, and they will be 12 degrees warmer in the US by 2100.
Research Report by Climate Central
If it feels hot to you now in the dog days of this summer, imagine a time when summertime Boston starts feeling like Miami and even Montana sizzles.
Thanks to climate change, that day is coming by the end of the century, making it harder to avoid simmering temperatures.
By the end of the century, assuming the current emissions trends, Boston’s average summer high temperatures will be more than 10°F hotter than they are now, making it feel as balmy as North Miami Beach is today. Summers in Helena, Mont., will warm by nearly 12°F, making it feel like Riverside, Calif.
Blistering Future Summers for 1,001 U.S. Cities | Climate Central
* In order to understand what a climate expert is, think in terms of a cross between Bernie Madoff and an Enron accountant.