Riding the TransAm I met a fellow who swore by Cabela's or Brass Pro "fishing shirts" made for being on the water and in the sun all day. My observation was they had great airflow while riding.
Agreed, 110%! I have a bunch of jerseys, mostly from ACA and other clubs I am part of, but when I am riding any real distance, I wear "fishing" or "expedition" shirts. They have an SPF of 40-50 (depending on the brand), are loose fitting and breath very well, and they have pockets! You can get them in long sleeves, which can be rolled up, and the better shirts have straps to hold the sleeves up. If I am riding on the road, I wear yellow (It has become something of a trademark.) and on trails, light green or tan. I also wear baggy shorts, so if I decide to eat at a restaurant, I look like a guy in shorts and a fishing shirt, rather than an over-stuffed sausage in spandex.
Willard Munger/Hinckley Fire State Trail, Moose Lake, MN