It's not the jeans per se. It's the cotton. Synthetic (or traditionally, wool) garments stay warm when wet. Cotton ones don't. Mountain weather is unpredic

It's not the jeans per se. It's the cotton. Synthetic (or traditionally, wool) garments stay warm when wet. Cotton ones don't. Mountain weather is unpredictable and sweat happens too. "Do it in jeans first" - absolutely. But after going nearly hypothermic in my all cotton stuff in the Presidentials on Mt. Mansfield years ago - nice sunny summer day down below, guess what at the top - I went with the program and got synthetic fleece stuff etc. It doesn't have to cost a mint. As long as you don't care about style, the thrift stores generally have synthetic t-shirts, fleece jackets and pants. Shell pants and especially decent shell jackets are less common but I've found enough there too to keep me covered. Shell vs. warm core - layering! Assuming you have a knapsack to put the stuff you're not wearing in, if you have fleece pants and shell pants over them, you now have three options. Four, if the shell pants can unzip into shorts.