Interesting question, but let me take a different tack.
I lived in Texas for a couple years, and now reside in NC: another state known for hot, humid summers. When I was young, my daddy told me I'd be cooler with a t-shirt or a-shirt on under my outershirt as the undershirt absorbs and evaporates sweat. I've tried both options and now agree with dad.
Personally, I perfer an undershirt on a hot day.