Is it good for cold weather? Also is it good for a petite woman head? I don't think this has a W version, does it?

I measure 55 cm... I have a small and it fits, but a medium might be a bit more comfortable. I would never be able to fit even the thinnest beanie under the small. Also its one of the lowest profile helmets I've tried on, which is nice since there is no women's version. It fits perfect with the smith I/O and the smith I/OS, not the heiress though. I've never been cold with it and I get cold very easy.

If you have a smaller head, the small should fit well in this size, especially because it has an adjustable fit system which you can tighten to keep the helmet in place on your head. The ability to adjust this helmet is also wonderful for those really cold, blustery days when a beanie is needed underneath. (I'm talking temps in the teens and snowing with wind. This thing is as warm as you'd want and as cool as you'd want with the vents.)

This will keep you plenty warm, but throw a beanie under if it gets really cold. Check the sizing chart or try these on - different helmets fit different heads better than others.