One thing not mentioned (apparently) is the effect of wind (wind chill). Hypothermia is easily achieved at temps of 50 F with enough wind, while high winds simply moderate the effects of high temperatures. Level of activity is a potent factor as well.

For that matter, relative humidity is very important. Dry heat is more bearable than wetter conditions.

There are good reasons for the practice of a mid afternoon siesta in warm areas. What is it that they say about mad dogs and Englishmen? (No offence intended for you Brits...)