I remember being in Clearwater Beach at a miniature golf course at 10am and it was so hot that my makeup was "melting" off my face! That was the first time that I vowed not to go to Florida in the summer.
Touring the parks is really hard when its' hot. After a while you just want to go back to the hotel and take a nap or jump in the pool!
Now I don't go to the parks until the late afternoon when the sun has gone down a little. I really like to tour the parks after dinner and stay until it closes.
Another way to "beat the heat," is to stay by the pool in the day. When you're swimming and enjoying the pool, you don't really notice the heat so much. If your energy gets zapped, it doesn't matter since you can lay around and get a tan while sipping a nice cool drink!
In the winter, it's not bad at all. The weather is perfect for "park hopping." But, the evening can get COLD! Yes, cold. Especially if you're watching "Illuminations Reflections of Earth," which is a fireworks show at Epcot. The show takes place over the central lake called the "World Showcase Lagoon," which is in the middle of the World Showcase. It's so cold, you can usually see your breath!
Staying at a Disney resort can really help "beat the heat" in the summer. You can take advantage of the Extra Magic Hours! In the summer, that means being able to go to the parks when it's not too hot. Either you can take advance of the morning EMH and enter the park one hour early. Most parks open at 9am, so that means you'd be able to enter around 8am.
The Magic Kingdom usually stays open as late as 3am for the EMHs with a regular park closing at 11pm or 12am.
Epcot sometimes will stay open until midnight, with a regular closing time of 9pm or whenever "Illuminations Reflections of Earth" is over.
Disney's Hollywood Studios stays open at times as late as 12am. While the Animal Kingdom stays open only as late as 10pm or 11pm.
One other tip, if you are going to the Animal Kingdom, you may want to go in the evening as well. The Animal Kingdom seems a lot hotter, since most of the attractions are outside! Plus the animal tend to be out in the morning and once the sun starts to go down.