Early last week I started a thread about helmet design and cooling; I really didn't know beans about it before, now I know beans and not much else. But I'm learning. I really didn't wear a helmet last year but increasing awareness of the implications of Russian Roulette had me in the helmet through the winter, spring and into hard summer when brain temps approaching poached egg have had me often riding helmetless again.
My previous helmet (the black Schwinn shown below) was made by Pacific Cycle and cost a total of $25. It works well in the winter but happily I haven't tested it for its main purpose yet. As a quick guess, I would venture that aprox 25%- 30% of the surface area of the helmet is vented. The new helmet is a Bell Influx (sell price $65-$75) and even a quick visual will tell you there is massively more vent area than the old helmet, pushing or exceeding double. I've around 400,000 miles motorcycle experience, all in Bell helmets so I like them; my secondary search characteristic was reviews saying the helmet was very well vented. The Influx reviews consistently remarked on that point. It came with a visor that I immediately yanked because I tend to climb head down and the reduction in forward vision had caused me to almost come to grief on several occasions.
It just came in but I haven't ridden with it yet since it is very close to 100 degrees here now and it probably doesn't cool that well; tomorrow AM will be fine.
I have Shimano sandals coming too, not here yet. I suspect that there won't be a magic bullet to beat the heat, just a collection of small tricks.
Pictures of old (black) and new below. Look at how small the vents are in the black helmet as they get to skull level. Also no vents at all on the side. Can you imagine wearing that in high humidity 100 degrees?
BTW: No relevance to the aircraft pict I took today, just thought one or two of you might like it.