A hat certainly protects the wearer from the sun’s rays. I’m not so sure about keeping the wearer cooler - because the hat warms up pretty quickly. The experience of not being sunburned feels subjectively cooler, but I’m not sure it actually keeps your head at a lower temperature.
Again, personal experience has never been feeling too hot in a helmet, but then lots of insulating hair is … um … not a problem for me. 
Still I wonder about the cooling impact of a helmet with well designed vents.
Accept that the actual contacts between the interior surface of the helmet and head are relatively minimal.
Volume X of air flowing across the surface of the helmet (which is head surface plus some fraction) guided to flow through the vents will, in a hypothetically ideally designed helmet, be flowing at a higher velocity by virtue of being funneled into smaller channels. Assume the size of the space between helmet interior surface and head and the egress from the space are properly sized to maximize that the flow rate at speeds consistent with heavy effort. It seems to me to be quite possible that you can, at least theoretically, have more air flowing across the scalp inside a helmet so designed than bare-headed, and convection with evaporation of moisture are the effective means of heat dispersal in a heavy exertion cycling condition.
The question is air flow through the space between the helmet interior and the surface of the head.
Can a helmet (which has greater forward-facing surface area than the head alone has) be designed to funnel a larger volume of incoming air across the surface of the head than would occur in an unhelmeted condition?
I don’t know if any do but my crude understanding would be that yes it could and that being cooler in a helmet at high speeds and thus high effort is not impossible.
The interesting side debate on (or is that by?) over-heated heads notwithstanding …
I’d sorta like the OP to come back and expand on his OP and cryptic follow-on post.
I’d like a coherent explanation of whatever theory he has (had?) about the harm a bike helmet might cause, to whom under what circumstances, and most importantly how it would do so.
Would a white hat over a dark head of hair keep you cooler?
It’d reduce the solar flux impinging onto your head. It’d also reduce the body heat flow out from your head.
Without specific numbers on hair, wind, sweat volume, sunny or cloudy, outside temperature, etc. we can’t say which effect would predominate. My WAG is either effect could be much larger than the other within reasonable situations. We don’t need to go to the Sahara or the Arctic to flip the balance.
Is this related to your earlier threads about how pressure on the head affects your looks?
Thanks for that tip.
Reading this Sleeping on the side and cheekbones - Factual Questions - Straight Dope Message Board
and this Why don't public schools teach good looks? - In My Humble Opinion - Straight Dope Message Board
was real informative.
Normally it might be considered an insult to say somebody is soft in the head. It appears from those threads that our OP believes his own head is soft and pliable. Given that belief he would be wise to stay away from tight-fitting helmets, hats, bandanas, etc. And a balaclava is right out.
That’s an… interesting point of view. Crikey, rubbing your eye makes your jawbone stick out, or in, or… something.
Whilst I don’t believe our bones to be squishy like putty (at least not in adulthood), they are continually being remade (a good thing because it means broken bones can heal) - I do wonder if slight, constant pressure over a very long time might not bend bone, but rather, cause it to change shape by being reworked in a slightly different position of lesser resistance.
(In practice, I think pressure sores would tend to be a problem first).
A lot of this seem to be due to behavior and not the physical properties of helmets.
Losing blood flow to the head? I was taught to keep the strap lose enough to fit two fingers between it in your chin. If you can shake it off you need a smaller helmet, not a tighter strap.
Cyclists taking more risks? That’s not the fault of helmets, but rather cyclists.
Drivers being closer to cyclists with helmets? Again, not the fault of helmets.
The way to defeat risk compensation isn’t to not wear helmets, it’s to change the way people behave.
(My father was in an accident where he filled over his handlebars and landed head-first on the road. Split his helmet down the middle, he had some scrapes. A friend was in a similar accident where he broke an arm but smashed his helmet into pieces. A busted helmet = not a busted skull.)
It’s hard to argue that a bike helmet won’t result in a warmer scalp, but it’s not as if your scalp is the only means available for your head to reject heat. It’s not even the biggest (unless you happen to be bald). Your face, neck, and ears offer a lot of surface area for heat loss, and your jugular veins also carry heat out of your head in the same way that coolant carries heat out of your car’s engine.
If a well-designed bicycle helmet makes the difference between “uncomfortably warm” and “dangerously high brain temperature,” then you were already living on the edge.
Cyclists take more risks when wearing a helmet … simplest risk reduction is to remove the helmet from the equation.
Drivers pass closer to helmeted cyclists … simplest risk reduction is to remove the helmet from the equation.
The way to defeat people engaging in risky behavior while wearing a helmet is to remove the helmet from the equation.
The fallacy of the cracked helmet
"Wearing a bicycle helmet can make us feel safer.
However feeling safe is different than being safe.
We tend to look at a cracked helmet and assume it is “proof” it saved a life. Actually, a cracked helmet has failed to work as intended:
The next time you see a broken helmet, suspend belief and do the most basic check – disregard the breakages and look to see if what’s left of the styrofoam has compressed. If it hasn’t you can be reasonably sure that it hasn’t saved anyone’s life.“
Yeah, people sometimes see or read about such accidents and assume that, because the helmet broke, it didn’t work. Quite the contrary: They’re supposed to break, and that’s how they work.
EDIT: Of course the break in the helmet isn’t proof that it worked as intended. But it’s even less proof that it didn’t work, which is how it’s most often (incorrectly) presented.
Even the safest of cyclists (and drivers around them) are going to have accidents.
Would they be safer wearing a helmet, or not wearing one?
None of those will help when your front wheel gets caught in a rut and you endo, like I did.
The way to defeat risky behavior is to train away the risky behavior, just like motorcycles. That still won’t obviate accidents. Even if you’re somehow the perfect cyclist, there’s the driver that plain doesn’t see you or your helmet as they hit you. If that somehow miraculously never happens to you, and you live without road hazards; we live in a world of mechanical failure, and the bike can lay you down all by it’s lonesome through breaking.
Mine compressed, then split slightly. The helmet that has split has still absorbed energy that would have damaged your skull, even if it failed in the process. In the cases where it split, you’ve delivered more energy than it was designed to take.
Say all you want, but the cracked helmet still limited the blow to your skull. I’ve taken more than one of those, I’d like mine to be limited to the protected variety in the future.
This is the “if you want to make cars safer, remove the airbag and replace it with a metal spike” argument. Which of course is fallacious - for most effective safety devices, the improved odds of surviving an accident more than makes up for the risky behavior that results from the safety device.
Rubbish. It takes energy to break things apart. A helmet has absorbed energy by cracking.
That said, one can argue that cycling is already extremely safe. And that helmet laws discourage people from riding, and thus discourage people from getting the exercise they need. I think the risk calculation greatly depends on where you ride - huge difference in cycling risks between Amsterdam and Alabama.
That’s my point – there is no evidence that wearing a helmet provides any overall benefit and it may actually increase the chance of having an accident and/or increase the severity of an injury.
http://lovelybike.blogspot.com/2012/01/did-not-wearing-helmet-save-gene.html
“Bicycle helmets have some protective properties under some conditions, but these properties are limited and do not extend to colliding with moving motor vehicles. Bicycle helmets also have some drawbacks, including their ability to cause rotational injuries. After reading lots and lots and lots of studies (the studies themselves, and not the media’s digested, distorted, misquoted and sensetionalised versions of the studies), I believe that the evidence pertaining to bicycle helmet effectiveness is mixed and inconclusive. And this is talking about effectiveness itself, without even delving to the larger, social implications of the helmet debate.”
Some of you guys really should read some of the actual science I linked to earlier in the thread.
And the risk isn’t nearly as different as the ‘perception of risk’ between Amsterdam and Alabama. Riding a bicycle is simply not a high risk activity.
What exactly does that mean? I have many cyclist friends here in Alabama, and every one of them can tell you stories about the times they’ve been hit by cars. I can tell you 3 stories myself.
I’ve been hit by a car twice while on a bike, and at least once while I was in another car (might be other cases I don’t remember). I’ve also been in other accidents of various sorts in both sorts of vehicle. Bicycles might be a bit more or less dangerous than cars, but given that we accept cars as safe enough, we probably ought to do so for bikes, too.
I don’t think the OP’s concern has any merit whatsoever. People have been wearing hats and helmets of various types for millennia and the human race is still here.
I do find the side discussions interesting though: is a hat capable of keeping your head cooler and under what conditions, and: is the skeleton of a human adult at all malleable.
Having looked at the science, I’m firmly in the camp of “mandatory bicycle helmets are bad idea”, but you are getting a little hyperbolic here.
There is certainly evidence that if your head is moving rapidly towards a solid object, the outcome is significantly better if that head is protected with a helmet. It would be overstating the case to dispute that.
The relevant questions are, of course, what is the relative frequency of the type of accident that a helmet would protect against, and what is the evidence for overall outcomes factoring in risk compensation and poorer general health outcomes if people are discouraged from aerobic activity by the prospect of wearing a helmet.
I’d also like to see evidence for your earlier claim that pro cyclist injury and death rates are worse since mandatory helmets were introduced for racing. I’m an ex-racer with a strong interest in cycling, and I’ve looked and I can’t find any data myself. Deaths are so rare that there may not be a big enough sample. But a priori I’m skeptical about your claim, because pro cyclists have always ride in an incredibly dangerous manner with or without helmets, so some of the risk compensation processes may not apply. In any event, of course, what happens in pro racing is probably not really relevant to general wearing of helmets.