Creds: I am a card-carrying volunteer Outdoor Emergency Care anon. I live in Phoenix, and have lived in worse places for suffering heat exhaustion.
Download this chart, which shows you the same thing as wind chill, but for heat and humidity. They combine and gang up on you
Heat Index Charts
Or this one:
NWS Pueblo, CO
The humid areas will have more of a problem than Phoenix: When it's 105° and the relative humidity is 5% it's easy to evaporate sweat enough to stay cool ... as the humidity rises, you evaporate less sweat and you don't get the cooling effect. I would rather be in Phoenix in May at 105° than Atlanta in August at 95°
What to wear:
HATS!!!! A lightweight straw cowboy hat, anything with a full brim.
Lightweight, longsleeved, loose cotton shirts.
What to Drink:
Anything alcoholic = EPIC FAIL because it increases urine output and you dehydrate.
Ordinary soft drinks = EPIC FAIL because their sodium content is higher than the sodium content of your body fluid, and they are loaded with caffeine and you dehydrate. Not so epic if you have them heavily iced and drink the dilute stuff and the melted ice too.
"Energy drinks" = FAIL! Not so epic, but if they have caffeine in them you can OD on the caffeine trying to stay hydrated. Caffeine comes labelled as "guarana" and a host of other names to disguise it.
Electrolyte replacement drinks = Win as long as they don't have caffeine.
WATER = WIN!!!! +5 internets to you.
Water + Electrolyte drink + ice = EPIC WIN!!!! and +7.43 internets.
How much to drink:
Don't wait until you feel thirsty. By then you are a quart low. If you plan to be there for 4 hours, bring 4 quart/liter bottles and drink one an hour.
Here's an example of how much water you will need: In Phoenix in late May, if I am working in the yard before noon, staying mostly in the shade of the trees, I will drink about a quart of water an hour.
A roofer working in the sun will drink 4 times that much - and pour more onto his shirt.