I would worry more about helmet color for how
visible it makes you rather than temperature. That's something specific to the Carbon version of my helmet that I don't love - yes carbon fiber looks dope and I'm sure many people who pay the extra $$$ for carbon fiber helmets are doing so for the aesthetic rather than the weight savings. My motivation when I bought it was weight savings, I'll take all I can get to reduce neck fatigue off-road and when touring. I would personally have preferred it if the HJC F70 Carbon came with a hi-viz decal or even just a white overlay to be more visible. Your helmet is big and it's usually the highest part of you and the motorcycle visible in traffic.
Law enforcement versions of the HJC modular carbon fiber helmets sold through 3rd parties do feature some white color for extra visibility, likely added by the companies that adapt them for LEO use and add comms and organization specific decals to them.
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There are quite a few potential features for airflow in a helmet:
- Visor: Opening this is the most significant way to regulate air into your helmet, but be sure you still have adequate eye protection. If you do not have something like a pinlock lens installed for anti-fog, cracking the visor open a notch on the detents pivot is also one of the easiest ways to mitigate fog when you're riding in cold, humid, or rainy conditions. There are lots of temporary anti-fog solutions out there as alternatives to a pinlock, a travel bottle of Head & Shoulder's dandruff shampoo is a surprisingly good option, inexpensive, and widely available.
- Chin vent(s): On some helmets these are always open. Be cautious of that because some of them have poor designs that let rain though (my Biltwell Lane Splitter experience). I ride with them open all year on my helmet, you can close them while moving in the winter to keep cold out but may need to open them when stopped to not have excessive fogging from your breath inside the helmet. On some helmets the chin vents can be annoying and blow air right up into your eyes, I had this issue on the latest generation of Shoei helmets that fit differently for me than previous generations.
- Top/chipney vent(s): These let fresh air in to the top of your head. I leave them closed unless it's scorching hot out, they often feel like someone's taking a straw and blowing on one or two very specific points on your head and that can be a weird feeling if you don't have long hair to diffuse it. That said, they're great to have when the weather is very hot.
- Rear/top rear exhaust vent(s): These let air exit the helmet when moving, that way air actually flows through the helmet rather than just ramming into it and fighting the pressure of what's already in the helmet. Some helmets don't have rear vents. They're uncommon on retro style helmets and even some high end helmets like the Schuberth modular helmets which don't have them because ventilation and airflow inside the helmet cause noise. Airflow noise doesn't matter much if you wear hearing protection.
- Comfort liner and EPS foam: some have pathways and spacing intended to help air flow through the helmet, retro helmets with quilted liners (Biltwell Gringo) usually lack this. The material and density of comfort liner can also help keep you cool, and you can also add a layer of wearing a thin bandana, skull cap, or balaclava made out of a fabric like Coolmax meant to wick moisture and cool.
- Chin skirt: These are sometimes removable or adjustable. In cold weather they're great for reducing how much cold air enters the helmet below your chin, at the expense of trapping your breath condensation in the helmet to create fog. In hot weather they're often something you want to be able to remove or not have at all. Chin skirts also greatly reduce wind noise inside the helmet.
Consider not only the temperatures you'll be riding in, but the speeds and how long of stretches you'll have your helmet on for. Ventilation matters more on long low speed rides, or on trail riding where you're exerting effort, than it does on short rides around town or higher speed (on the CT125 we'll cap this at "mid-speed"
) commutes.
As
@AZ7000' said though, when it's hot it's hot. Protective gear impedes airflow, and airflow doesn't cool you if it's hotter than you are. When it's 100°F+ out some of your best options are to take breaks from riding, that's what siestas are for!, and soak your hair, bandana, clothing, etc in water so you can cool off through wicking moisture (not as effective when it's already hot
and humid).