We just got back yesterday. While riding on the bus with a long-time Disney CM, we had a lengthy discussion about Disney and such. This man was very knowledgeable.
We were telling him how hot it was at AK. He advised us that that park is at least 10 degrees hotter than the rest because there are not a lot of trees and the animals need the heat. We took the train to Rafiki's area (petting zoo and such), and it was even worse. No shade--just sand and heat! I love the heat, but I could hardly stand it. So beware--save that park for a cooler day!