The hardest clothing problem in the outdoors is not extreme cold — it is alternation. A November glassing hunt means sitting motionless on an exposed ridge at 20°F for two hours, then climbing 800 vertical feet fast enough to soak a cotton tee. Gear that is right for one half is wrong for the other. The answer is a system, not a jacket.
Layer one, against the skin: merino or a merino blend, 150–200 gsm. Its job is moving sweat off your skin and insulating even while damp — the property that makes wool worth its price. Cotton is disqualified at this step, every time, no exceptions. Cotton kills comfort at best and people at worst.
Layer two, the workhorse: a 250 gsm merino or grid-fleece mid-layer with a deep chest zip. The zip matters more than the fabric — heat management on a climb is done with zippers, not by stopping to repack your bag. A brushed, quiet face earns its keep the first time you raise binos twenty yards from a mule deer.
Layer three, the shell and the puffy: a quiet-faced softshell for wind, and a packable insulated jacket that lives in your pack and comes out the moment you stop moving. The discipline is the hard part: puffy on immediately at every stop, off just before you feel warm on the climb. Ahead of the temperature, always.
The whole system for most Western seasons is five pieces and under two kilograms. Buy the mid-layer first — it is the piece you will wear the most, hunting or not — then build outward. And test the system on day hikes in October, because opening morning is a terrible time to learn your zipper placement.