I play outdoors in North Texas in early and late summer. It's HOT... REAL Hot !! but a few words of wisdom (I hope) for you outdoors folks.
Always bring water (preferably chilled) and plenty of it. Do not drink sodas or liqor - it dehydrates you and you WILL pay for it !!
Bring a box or similiar type of fan(s). If you don't need it then point it at the PA amps or your monitor amp. You'll be glad you did.
Try top play early evening - the humidity is still around, but the direct heat of the sun has dissipated some. And set up in any shade - allow for the fact that the sun DOES move... !!
IMPORTANT - if you store your gear in A/C or a cooler place, put it ins safe place to acclimatize for a couple of hours before playing.. This will prevent any impact of condesation or moisture buildup that occurs due to the temperature differences... You have far fewer repairs and/or potential damage if you do.
Did I say bring/drink plenty of water ??
Regards,
Steve