It's really the humidity more than the heat. With the humidity, your sweat does not evaporate and your body is unable to cool itself. You have to move that sweat away in order to cool, sweat stuck to you is just a layer of hot water which further inhibits cooling. I'll take 95 and 0% humidity over 85 and 70% humidity all day.
Last Summer was very mild but high 90's and 70+% percent humidity are the norm. With long pants on, the sweat doesn't evaporate and I end up soaking wet and miserable. If you sweat heavily, you do anything you can to cool down and dry off. Some guys around here do prefer wearing long pants, some even long shirts, I ain't one of 'em. Some of my guys barely sweat, while others, me included, sweat enough for three people. When the air is so wet you can almost feel it weighing down on you, all you want to do is crawl back to the truck and turn the air on high. When spraying you're stuck with long pants and rubber boots, miserable to me.
I've done work in the Phoenix Arizona area and the whole keeping covered and keeping the sun off is the way to go there, but it's a whole different ballgame when you factor in humidity.
Heat stroke is not just from not drinking water. In fact, my trouble was I was drinking too much water and not eating. You have to replace salts, and the more water you drink, the more you have to replace. I had probably had close to 2 gallons of water that day, by 2pm when this happened, but I had the habit of not eating breakfast or lunch in the Summer. I stopped at an Exxon to buy Gatorade, walked up to the counter and the next thing I knew I was sitting down with a wonderfully sweet lady holding my hands and saying, "Don't worry Honey, the ambulance is on the way". I thought to myself, ambulance, for who? Then the world started to go black and I distinctly remember thinking, "Oh, that ambulance is for me".
Almost killed me, I got lucky. Had I been out on a property and fell out, probably wouldn't have had anybody to help me so quickly. I was in my early 20's when this happened and it was months before I fully recovered.
Nowadays I wear a wide-brimmed straw hat, wicking shirts and lightweight shorts. I buy my guys straw hats if they want them and wicking shirts. They can wear whatever khaki pants/shorts they like. I also buy the spf40 or 50 Coppertone Sport for the guys, I think it's about $5 a can at Costco when you get it in the 6 pack. At least as far as high humidity goes, it's a matter of moving heat out of your body, not shielding it from the heat.
I also make certain to eat. Gatorade and such is ok and helps out, but nothing replaces eating. Plenty of fresh fruits, good carbs and protein. I like peanuts and jerky, bananas, apples, peaches, biscuits, crackers and McDonalds french fries in the Summer when it gets real hot. Not eating when it's real hot will drop you quick. I also buy gallons of water at Walmart or Costco for the trucks. Was using coolers on the trailers but somehow they keep falling off or walking away.
I actually hate the heat and have been trying to get the Wife to move, with me preferably, to Idaho or Montana for years.