Here's a look at the hourly forecast for today, July 22, 2011 provided by Weather.com:
As a young lad in Boise, Idaho I once experienced the mercury rise to 111 degrees, tying the all-time Boise record set in 1960. But that was a typical Great Basin dry heat, with humidity in the 10-15% range. It was toasty, for sure, but bearable and also relieved nightly by the crisp interlude of the high desert night.
Philly's humidity today will range from 55 - 69% and temperatures will peak around 101-102 around the 4 o'clock hour for an insane heat index of 115. There will be no relief with darkness either. I plan to be outside for some or all of this phenomenon to get the most savage taste of suffocating heat seen this side of Libya.