How warm is yak and merino wool?
Unlike human hair, wool has crimps, which are natural "waves" in the fiber that trap air, insulating in extreme colds and wick vapor, cooling in extreme heat.
Wool can be regarded as an ‘active’ fiber, which keeps body temperature at an equilibrium - it efficiently transfers water vapor from sweat in and away from the body as temperatures change. In fact, as wool transfers water vapor, a chemical reaction occurs that produces heat.
Yak, who grow a thermoregulating fiber called khullu live in a high elevation climate that varies between -40° F in the winters to 90° F in the summers. Even the desert-living Bedouins wear lightweight wool like ours clothing to keep themselves cool (Fleece and Fiber).