Originally Posted by
cyccommute
Ski helmets also suffer from being
too warm. While they would be nice for a 20 degree commute, they would be way too hot when the temperature rises to 40, 50, 60 or even 70

which is always a possibility around here.
Take a look at helmets like the Giro Fuse Ski Helmet. The GF has one.
It has a set of vents much like a bike helmet, with a sliding button which opens/closes the vents.