Originally Posted by
dougmc
I realize this is a rhetorical question, but the answer is "yes, some do".
A straw hat or sombrero will keep your head cooler -- it does this by providing shade but being still porous enough to allow decent air flow as well.
If a helmet is designed to provide good shade but not restrict air flow much, it could help keep your head cooler. I don't recall ever seeing a bicycle helmet that didn't restrict air flow much (even with all the big air vents, which reduce the shade factor as well), but it could be done.
Yes, a helmet could provide enough extra shade but not restrict enough airflow to keep your head cooler. Any additional airflow restriction is made more moot by travelling at higher speeds.
The issue I find a problem with helmets is with sweat, which people might be interpreting as evidence of being "hotter". Of course, this is more of a issue when going up hill, where one is going slower with more exertion. In such a situation, a helmet might be hotter.