I realize that to an extent, but even in August up here when temps can swing from 75 degrees to 25 degrees I bring a puffy coat over vest. I am sure September in CO at 10K' is the same deal so the temps are very similar I would imagine and climates aren't that different at that point, but most of you prefer vests it seems so I am just wondering what you guys do when it gets down to 40 or so degrees in the mornings which can happen at anytime in CO at that elevation from what I gather. A base layer and vest certainly wouldn't cut it for me. Granted the much heavier system of vest and softshell jacket might, but the warmth to weight of a puffy far exceed a simple vest and softshell IMO.