Another little trick is:
Carry a white cotton tshirt with you. If it gets hot, hot, during the day and you need a cool down. Stop, hydrate yourself and soak the tshirt in cool water. Wring it out and wear it under your vented jacket with all the vents open. Hit the road and the wind will dry the shirt out, and suck the heat out of your body. It really does feel like air conditioning at 70mph.
It makes for a nice break in the heat of the day... I used to do it on 100 degrree days coming back from a weekend in Austin and hill country mid trip... Re-energizer.