Doesn't everybody already know?
The extreme heat and humidity is present up and down the entire EAST COAST in the summer, extending up into Virginia and beyond, even Jersey beaches......and of course that goes for Florida beaches too. When it's hot at Hilton Head, it's also hot for hundreds of miles in both directions simultaneously.
Ever been to Williamsburg, VA in July????? Absolutely prohibitive temps because it's stagnant---no breezes!
Ever been to NYC in July???? Ever been to Philadelphia in July????
Chicago even has numerous deaths annually from the high temps...it's everywhere. I think Canada had a stifling heat wave recently.
There is no escaping summertime heat. At least you can run and hit some water really quickly at Hilton Head, and the wonderful breezes on the beach definitely help. (They don't happen inland.) In fact, the beach breezes make it quite pleasantly tolerable.
No place we'd rather be.....we spend 4-5 consecutive weeks at Hilton Head in June/July. Love it.
You just have to learn what to do and not do in the middle of the day.