The "air-conditioning" technology behind Real X Gear's Cooling Caps and Visors is as basic as this: Wet it, wear it, and let evaporation do the cooling.
The trick is in extending the cooling evaporation time as long as possible. The headband area of Real X Gear's head wear is lined with a strip of absorbent and water-retaining fabric called Poly-Vinyl Alcohol.
Embedded alcohol beads chemically react with water to lower the temperature of the material and slow the drying process.
This may sound way too low-tech to be any more effective than dunking any old hat, but a test run on a recent hot summer day showed Real X Gear's caps and visors do indeed stay cooler substantially longer than standard head wear.
Keep an extra "wet" cap or visor in a cooler (perhaps along with picnic food) to switch off when your head wear runs out of cool.
Info: The caps and visors come in blue, black, white, camouflage, khaki, red or pink. Caps or cooling visors from Real X Gear cost $20.