It took me a while, over the years, to find the right cap to use under the helmet. Some were too thick, some the bill was too long or short. This absorbs sweat well, keeps your head cool. Looks good too if you keep it on off the bike.
Helpful for air flow and wicking, but when the heat & humidity are high it will still be soaking wet with sweat just like any other cap etc... I purchased two last summer and got a third one this summer and two more for a riding buddy of mine. These are easy to hand wash and dry which saves the shape of the brim.
I chose the lightest colorful due to the fact that I live in Tucson,Az. it's great for keeping my head cool now that it is officially steamin'hot! I also wanted to just protect my hair from the sun which it does well. I love the fact that you can flip up the front flap if you want to and I have yet to have it blow off my head, we get some pretty strong spring time winds too.
Giro nails the headgear again with this cap. Synthetic fabric breathes well and air dries quickly after washing, The brim gives a little cover from sun glare and raindrops without hindering your vision. The elastic is lightweight and doesn't irritate my head where the cap interfaces with my helmets' retention hardware. The brim has reflective piping on its edge for added night-riding safety. And the aesthetic-solid color with big stripe down the middle-is simple and classy. How could you ask for more in a cap?
These hats are wonderfully light, fit well under a helmet, look good without a helmet, and have the added bonus of having reflective piping on the edge of the not-too-bignot-too-small brim. The titaniumflash charcoal variety actually has a full reflective upper brim on the top side. The bottom sides of the ones I have bought are all dark no matter what the upper color is, providing better eye protection from glare and sun. Have bought a bunch and given them to my riding siblings, they were much appreciated and well liked. I'm getting more for spares.