Hot air is one thing when you're training...an obstacle you just have to deal with and move along. Add humid conditions and you begin to need gills. If you're fortunate your training runs will have a mix of shade and sun, variations in terrain and - most importantly - many places to hydrate.
Okay, porta-potties are also important, but that's for another time.
Lately, our heat index has been in the triple digits; yes, that's above and beyond 90 days of 90 degrees and 90 percent humidity. Think higher, my friends. There's not enough cooling you can do to yourself, internally or externally, over the course of a training run. Why is it folks don't have their lawn sprinklers on at the time of day when you want to run through them most?
My Sunday morning training run is nearly optimal; a hilly, moderately-shaded loop around a large body of water. The down side is that there are only two places (three if you count the park where I begin and end) to take in water. Both of these are on the same side of the bayou. If you're in a situation where your training run goes through a residential neighborhood, make friends so you can borrow their garden hose or leave a container of fluid there. I've heard horror stories of friends leaving gallon jugs of water in front yards, only to find some local nosy-nut has taken them away.
You CAN carry water with you, though. I don't like the idea of doing it, though, because there's not a good way to carry fluid without affecting your run. Carrying a bottle in the hand is all right for some, but I feel like Quasimodo as the run approaches the first mile. Okay, so the weight is decreased after the first ten minutes or so, but in the meantime... Some gear makers have built straps that make it easier for you to grip the bottle, but the issue is still the same. If I wanted hand weights, I'd have brought two of them.
I have a Camelbak waist-mounted hydration system, which is a 36 ounce bladder in an overgrown fanny pack with elastic straps to keep everything snug at the waist. This, I think, was developed for cross-country skiers and not runners. They don't have a lot of up-and-down motion as part of their sport; two extra pounds of weight strapped to my waist is not good for the knees...as I learned a couple of years ago after my last marathon.
What I'm going to start looking at, however, is a belt-mounted system that will allow me to either carry four 6-ounce flasks and gels, or holster a bottle. The flasks are starting to sound like a very good idea, since I can carry water, electrolyte or carbohydrate in the amounts I feel are necessary. The only other options are to hope for the goodness of my fellow man (being a glass half-empty AND glass needs cleaning kind of guy, I don't hold much hope) to not borrow/steal/discard fluid bottles I cache.
Of course, if I can't find my bottles I can always "borrow" their bushes.