I've lived in the desert for most of my life. I've always ridden throughout the year, during summer I'd stay hydrated and wear a jacket with good ventilation as well as protection. Then we found another solution a few years ago called a
Phase Change Cooling Vest. These work well, they can be charged fairly quickly and maintain the charge for a couple of hours, long enough to go through a whole tank of gas while driving. They actually stay cold, they do not rely on evaporation. That's a great advantage because evaporation doesn't happen when the humidity is high.