Although described as a windproof skull cap, after a bit of experimentation we discover it is only the forehead section that is completely windproof. The rest is a light and stretchy microfleece, breathable yet warm.
Although slightly wary of this we are soon convinced - anything above around freezing point and we found the Altura too warm to wear under a helmet and have to remove it, so just as well it isn't completely windproof!
It is thin enough to go under any helmet, barely bulking out the adjustment, and certainly does the job without drenching us in sweat. A great winter addition.