Well, anecdotally - since I've been here I've always thought we have a longer "spring/ fall" climate than a really hot, humid summer. I would assume the elevation of the city and the Cumberland Plateau have some effect.

It definitely gets hot in the mid-summer months, but not nearly as severely as other cities like Nashville, Memphis, Atlanta, even DC.

Also, I think calling Chattanooga "deep south" would be a severe misnomer.