Sorry to revive an old thread, but I have an update.

When this thread was first posted, I actually bought this jersey (Pactimo Summit Aero Mesh Jersey) along with cooling base layer they recommended to use with it, just to satisfy my curiosity. The day it arrived was a fairly hot day, so I went for a ride wearing the jersey and base layer. On the ride, I thought maybe it was a little cooler than other jerseys, but then a downpour came out of nowhere, and that was the end of the test. I have worn the jersey a couple more times since, with and without the base layer, and I concluded that it was nothing special.

Several months later, I bought a Pearl Izumi Pro Mesh Jersey (because I liked the color, but also because I like having a zipper pocket for my keys, and PI is one of the few brands of cycling kit that knows how zippers are supposed to be installed in the American market). It arrived, and it appears to be made of exactly the same material (I can't speak to "Cocona particles" that Pactimo mentions, whatever those are). Fits great, looks great, fine. Last month, I went on a longish ride in the heat, full sun, no shade, and I wore this PI jersey. About three hours in, I started to get chills, mainly on my forearms. I assumed I was experiencing a symptom of heat exhaustion, so I hunkered down and finished my ride, which was just another twenty minutes. Then last Sunday, I wore this PI jersey for another hot ride, though this time it was mostly overcast. About an hour in, I was feeling okay, but I stopped at a park to rinse sweat out of my eyes, because my headband was already saturated. The water was decidedly warm, and I took no care to keep it from splashing onto my jersey. As soon as I got going, I had chills again, mainly on my forearms. I didn't think I was suffering much from heat yet. Then I noticed that the front of my jersey was wet and cold to the touch. This time, I leaned into it, and rode another two and a half hours. It was a strange feeling. My torso and arms were cool, my head was uncomfortably warm and sweating, and my legs were unbothered. Eventually, my sweat output wasn't sufficient to keep the jersey wet, and it was dryish by the time I reached the end of my ride.

So now I'm thinking this mesh fabric for both jerseys is perhaps quite effective at cooling, as long as it has absorbed enough moisture to work with. More data is needed. I really wish I could find a way to keep my head this cool.