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Also if you feel like heat is going to be a bother and about to pass out put a wet cloth, a cold bottle of water etc. under your arm . oit will bring your body temp down a little faster than applying a cold cloth to your forehead. I sometimes also wear a wet cloth around my neck to help with the heat.
Bing Bing Bing!
It is very good advice to put a wet cloth on your neck. A neck exposed to the sun can heat you up FAST. A wet cloth with evaporate and make you cool off fast or keep you cool in the first place.
A wide brimmed white cloth hat with vent holes can do wonders in the cooling department. Just be careful of a hat that would collect heat because of color or construction.
A cloth hat is nice because you can soak it in water which evaporates and keeps your head cooler. And a wide brimmed hat is even better since it keeps the sun off your face and neck. Keeping your head and neck covered does wonders to keeping cooler.
Drinking water is good but if you are really sweating you are loosing minerals that have to be replaced. If you are very hot you won't likely be hungry. But eat something. A banana is high in potassium which is a mineral you loose and need.
More than a few years ago I was working an event in a polyester uniform that included long pants, a ballistic vest, and 20 pounds of gear. I was standing on black top in weather that was not too hot but hot enough. I was drinking lots of water and eating bagels. But after 6 hours or so I was feeling bad. I took a break and found a banana to eat. Eating that banana was like flicking a switch. I went from about going down to being perfectly fine over a few minutes.
This weekend I was working in our garden. Very nice weather it might have gotten into the low 70s and we had low humidity. I found my self having to take frequent breaks in the shade because I was over heating. I was NOT wearing my hat.

Put on said hat and was able to work long before needing a break. Bending over to garden exposes the neck to the sun which heats you up ASAP even when it is a nice day out. If it had been hotter I would have gotten the hat wet and wore a wet towel to cover my neck.
Wear a wide brimmed hat if you can. Wet it if you get hot. Wet a cloth and put it on your neck if needed.
Drink water but don't forget to eat and/or drink a sports drink if you can handle the sugar.
Take breaks in the shade or better yet the AC. Hall of President or COP come to mind.

At EPCOT there the building behind Club Cool is very COOL and great place to sit out of the heat.
Later,
Dan