I like my Rev' It mesh jacket in heat and especially humidity. (about $170 on sale) California has LOW humidity but I've ridden in humid places so know the challenges. I rode cross country to NYC years ago, caught a brutal heat wave with high humidity, luckily only lasted a few days.
Please NOTE that in super HOT weather (like over 100F) Mesh can start to work against you. The rush of super hot air coming through the Mesh at high speeds feels like a blow torch on you, best to wear a long sleeve shirt, it's not good direct on skin.
Your sweat evaporates instantly, like riding with no jacket at all. This not a good thing ... as dehydration is a real risk.
I've ridden a lot in hot weather ... and for me, just SOME venting, IMO, is the best policy. Hottest was Death Valley in August at 118F. No problem at all with my Spidi jacket with just 4 moderate vents. Surprised how well it worked. Trick? Keep moving!
Mexico around Puerto Vallarta was harder due to humidity. My answer there is ride Early AM ... or at night. (yes, I break Cardinal rules!)
Experts say ... "keep the sweat inside!" I agree. You want that sweat on you and on your clothes ... then with a bit of air flow (not mesh) it can cool you nicely and prevent heat stroke. Mesh can provide TOO much air, meaning too much evaporation, sweat dries too quickly before it can cool you off. Mesh is fine of short rides if not TOO hot. (like under 95F) and if speeds aren't too fast. High speed (over 75 mph) and brutal heat will Par Boil you with a Mesh jacket and you may drop dead from Heat Stroke.
Sure, with Mesh the air flow may feel better ... but if truly HOT, it can make things worse in my experience. So, for me, NO MESH if over about 95F doing longer rides.
I've learned a few good survival tricks for riding in high heat:
1. Drink water constantly (I use a 3 liter Camel Back) carry another jug.
2. When you stop ONLY stop in the shade and remove jacket immediately.
3. After Gassing up, go into mini mart, fill your Camel Back with ICE.
Avoid Sodas and sweet drinks ... some Gator Aide is OK, but no more than about 2 pints of it per day.
If truly hot you will consume at least 3 gallons of water a day, perhaps more. Gator Aide helps retain water and gives you
electrolytes which are good for staying alive.
So ... Water Water Water. If really hot ... you will
never Pee.
4. Try to start riding very early AM, quit by 2 or 3 pm. By Sunset things may cool slightly (not much in Arizona unless at altitude)
5. I avoid BLACK riding gear, instead use my light colored dirt riding gear.
6. In super heat you'll want to keep your face shield down. Opening it will cook you if truly hot. Sweat is a good thing. Welcome it, try to "Zen" through the misery of the heat.
Don't fight it. Relax. Think good thoughts.
7. I used to go on long walks mid day when I lived in El Salvador, walking on the HOT road. We would drink hot tea or hot coffee, which helped a lot. Somehow this works. It's counterintuitive but it works.
Getting your head around the heat is key. May take some time. Relax and let it happen. In Mexico, the cure to high heat is BEER! Lots of Beer, once you are drunk, much easier to handle heat. (yes, another Cardinal rule broken! ... drunk and riding on a dirt bike!
)