So I went into my local HD dealer and picked up a mesh jacket the other day. Paid about $140 for it out the door.
The first day I used it was about 84 degrees and I really believed it made a difference.
So I jumped at the chance to use it a day or so later in 87 degree heat and experienced a completely different result.
In both cases, I could feel the breeze over my arms. But on the warmer of the two days I definitely didn't get as much core calling across my chest and stomach as I expected. So much so that I actually had to take the jacket off; and it was much cooler without the jacket.
I suspect that the batwing on the triglide simply blocks too much of the art that we want to flow over the jacket. So I am considering taking the jacket back.
For me this is a big thing since I really hate riding in the heat. If the Heat Index is 90 or above them I am pretty much out the the picture.
I rode through Arizona, New Mexico on a Route 66 ride and was completely comfortable.
I don't know what the temp has to do with a mesh jacket keeping you cool - it's not the temp but the blocking of the sun that keeps you cool. If it 90* out and cloudy = no mesh jacket - you'll be cooler without the jacket and warmer with it on.