I use one of these, most other things don't cover my ears. My head is 60-61cm in motorcycle helmets. Most other hats I've tried leave little gap at the bottom for cold air to come in.
It is quite thin so fits under a helmet well and also doesn't get too smelly.
http://uk.icebreaker.com/en/mens-hats-neckwear/quantum-headliner/IBM451.html?dwvar_IBM451_color=001&cts=001
I use one of these, most other things don't cover my ears. My head is 60-61cm in motorcycle helmets. Most other hats I've tried leave little gap at the bottom for cold air to come in.
It is quite thin so fits under a helmet well and also doesn't get too smelly.