I do a lot of riding in 100+ temps and under 15% humidity, and have crossed the Mojave many times during summer months. The cooling vests work wonderfully in these conditions, and will stay wet/damp for about 90 minutes, IF you restrict airflow a little, as others have said. I always wear a heavy jacket (not mesh), but one which has ARM ZIPPERS -- an absolute MUST for hot weather riding. Chest zips (IMHO) usually sit behind a windscreen, but arms/elbows are out in the breeze. Not sure why more manufacturers don't catch on to this trick.
Each to her/his own on using mesh riding gear, but if you use it, you must drink a lot of water to compensate for skin evaporation. Those guys who live in the Sahara and ride camels? No mesh gear there -- ATGATT, head-to-toe, and their lives depend on it, as do ours.
THE most important thing, as previously mentioned, is to STAY HYDRATED. In the west, this means a Camelbak or equivalent. You cannot stay hydrated in high heat and low humidity unless you are drinking small amounts every 15-30 minutes. [I]You will get dehydrated [/I]if you rely on drinking at gas stops only, and many Western roads have 50+ mile stretches between gas, anyway.
The first thing that goes away when you get 10% dry is the "higher" thinking part, as in "making decisions," and you might find yourself trying to recall which lever is for stopping, and which for shifting. . .and why is my hand moving sooooo slow???
Excellent Link Voni!!!!!
Everyone should read this completly.
Again, I can't stress enough LD Comfort Underwear!!!
Important I think to add eye protection to discussion of riding in hot weather.
It's NOT COOL when your eyes water over or just become really painful.
There are others, but I wear (men's version)
+1 for the Veskimo and another for Voni's point on start/finish times.
I get about 4 hrs out of a Veskimo fill. It only holds about 6 lbs of cracked ice but can be filled in the morning from a motel ice machine and from any gas station with ice. I use their plastic container inside a BMW small seat bag with some added styrofoam insulation so it can sit a few hours in the early AM before I turn it on.
When operating out of home base or where you have a freezer, ice blocks work best.
Been thinking about transplanting the pump guts from their "6 lb of cracked ice" plastic container to a larger insulated cooler that can hold an entire 10 lb bag of ice- the usual gas station size.
The Veskimo can adjust your thinking about riding in the heat for sure. Now for that helmet with A/C......
A few years ago I found a really heavy cotton sweat shirt ($2) at a thrift store. Had an idea this might make a more effective "cooling vest" than the one I had purchased. This was a pullover with short zipper and the collar really covers the neck. I amputated the sleeves and also the bottom part at about navel level with pinking shears.
Soaked in either a gas station sink or river then slightly wrung out, it weighed a LOT. Under my mesh jacket it kept me comfortable in 90+ degree temperatures for over three hours. Might be even better under my newer vented jacket - at least the cooling would last longer.
Of course, evaporative cooling works best in low humidity.
I wear a mesh jacket and I do take a few sips of water from my camelbak (filled with ice and water) every 10 minutes when riding in the heat. If it should be extremely warm, I'll soak my helmet liner and T-shirt at rest stops, which is usually good for about two hours. I do these things in warm weather because I know that I should.
However, I have a problem where I don't feel much heat and almost never notice any humidity. I also seldom sweat (the exception being the closing ceremonies at Bloomsburg 2011). Below 80F when riding, I am usually wearing a long sleeve turtle neck because I am cold. Over the years, there have been heated debates around the house as to why the air conditioning should or should not be on.