OK, on to evaporation. Evaporation can only happen when there’s less moisture in the air than on your skin. So if you are in a big hot stinky swamp pit, evaporation ain’t gonna do much for you. If there is no evaporation happening, your body will stop sweating. This is very bad, and you will soon be very unhappy as your body overheats.
“But Poof!” you are now saying, “won’t closing your vents ensure that your suit becomes a big hot stinky swamp pit? Aren’t you ensuring that you’ll raise your core temperature too much because your sweat can’t evaporate?” If you are hard-core enough to ride in ass weather like Mr Ed did there in Arkansas, when the humidity is that high, let’s face it: nothing ain’t gonna help you. At that point, you get yourself shade and water, and often. Also, assuming you’re touring, try riding at night or at higher elevations. However, in most of our daily lives, this isn’t going to be an issue. No one’s jacket is windproof (I wish mine was, in the wintertime!), so unless your idea of gear is Saran Wrap, your skin is going to be able to breathe and your sweat is not going to stop evaporating 100%.
So, on to our second idea: insulation. Earlier, we established that sweat is the body’s way of transferring heat from itself to the air. This can only happen if the air is cooler than the body. Otherwise, the skin will draw heat from the air. Why is this a problem? It’s called vasodilation. The idea here is that as the body heats up, blood vessels enlarge to circulate more blood to the skin. Normally, this is good because the evaporative cooling process cools down the skin, and therefore, the blood. However, if your sweat evaporates too quickly and dries out, the skin absorbs heat from the air, which then actually heats up your blood. Mmm, nice hot blood circulating all over your body—especially up into your brain.
By zipping up your vents, you provide a layer of insulation between your skin and that hot air. By keeping your clothing wet, you augment your sweat and keep your skin (and therefore blood) cool. One thing I mentioned in particular is a bandana. I actually use a Cool Tie, which is a bandana-like tube filled with paraffin crystals that hold water much longer than cotton. In desert conditions, I soak this Cool Tie and wrap it around my neck while riding. It keeps the blood flowing to my brain cool, and helps me keep my head on straight. It’s very easy to become confused when in the early stages of heatstroke, and keeping your blood cool is one big way to combat this.
I’d like to reiterate that this is only really applicable when the ambient temperature is above your body temperature. Also, I have nothing at all against mesh jackets. I own one myself and am not trying to discourage anyone from purchasing one. I’m just saying that in extreme, 99F+, conditions, you have to be prepared to go into desert survival mode, which, for me, includes zipping up my vents and keeping the hot air and sun off of my skin. No matter what your opinion on the vents, it should also include frequent stops, lots of water (and/or gatorade, like Mr Ed mentioned), and lots of shade. If you’re not willing or able to make those sorts of preparations when riding in 99F+ weather, take the car or stay home. That isn’t being a big pansy; it’s being smart and safe.
Don’t forget that once the temperature gets above your body temperature (~99F), you don’t want to be wearing a mesh jacket. You want to zip up all of your vents and keep as much of your skin covered as possible.
Hot air hitting your skin at a temperature higher than your body temperature will heat up your skin and dehydrate you faster than you’ll know it’s happening.
Cover your neck with a soaking wet bandana, wet down your T-shirt, and stop often to re-soak both. Drink way more water than you think you’ll need.
Heatstroke is a very real possibility on a motorcycle, and at high temperatures, mesh clothing will not help with this. If you do wear a mesh jacket in these temps, make sure you have a Camelbak or some sort of hydration system, and drink water constantly.