For riding in this area, a summer base layer keeps me cooler by helping the body stay moist. When it's 110 degrees(with 8% humidity) like it was yesterday, the 18mph wind passing over you drys the jersey so quickly that you never get any cooling effect. A baselayer helps to keep the body moist, and cooler than when just wearing a tight cycling jersey. Most people doubt it, until they try it.

I've been cycling in Phoenix/Las Vegas summers since 1995, and have figured out what works.

FTR, I doubted sweat caps worked until last summer, but now I won't ride without one.