We live in Tampa. Orlando is very hot in August. It averages between 90 and 93 degrees during the day. It really does not cool off much past 80 degrees at night either! What is really bad is the humidity. It rains almost every afternoon for about 30 minutes to an hour. This rain is usually in the form of thunderstorms. They come up fast and sometimes have lots of wind and lightning. When they are done you have about 30 minutes of cool temps (75-80) before the humidity kicks back in. There are not usually a lot of hurricanes in August.
Now, we live here so by August we are pretty used to it by August. We do not let it stop us from getting out and riding our bikes, going to the beach, or going to Busch Gardens and WDW. However, we see people who are not used to the weather and they are Blizzard Beach --- ie MELTING in the Fla. sun.
Here are some tips to keep cool. Don't wear heavy cotton. It is not as cool as people think and it retains water. We try and wear as much nylon/polyester as possible. I know this sounds funny, but what I am talking about is the bathing suit type of material like Nike Dri-fit or Umbros shorts. Also, invest in some sandals like Tevas or Nike walking sandals. Keeping your feet cool is important. We dress our daughter (2) in little dresses and skirts to help keep her cool. Also, drink lots of water. Nothing is better in the heat that good ole H2O. Oh, try and stay out of the heat between 1 and 3pm. Plan lunch, indoor attractions, or sims in the pool. Oh, if you are not used to the sun, remember your sunscreen. Banana boat makes a great spray sunscreen that is not oily and is sweat and water resistant.
Enjoy your trip!