Born and raised in WI, now live in VA, and Florida is way more humid than PA ever is. It was in the 80's a few days while we were there. You are surrounded by concrete, so I think the parks are hotter than the forecast actually calls for. AK almost killed us!
That said, I wear jeans always - my legs are not made for shorts - but they were lightweight. If we were going to be going through the evening hours, I brought a light jacket that I could tie around my waist until it was needed (not often, since the running around kept us warm - I used it on BTMRR at night and waiting for a parade). Except for that very warm day at AK, it did not bother me as much the other days. And it cooled off toward the end of the week, but still warm enough to get drenched on SM and dry off in a few hours.