re: How many times has southern heat and humidity been a major factor

It doesn't matter as much as you think. Its hot everywhere in August. Maybe not as humid but it matters about as much as the other myth of home field advantage (unless you are Auburn) I remember Boston College and Flutie coming to B'ham on Labor Day 1984 and thought that would be a huge factor. Flutie had our D's tongues hanging out in the 4th.