Merino is THE stuff for active comfort in the cold. The key is staying dry between rain, snow, and sweat...it's challenging. 100% (95 and 88 also good) merino helps to achieve this goal. It's warm when you need it to be and water (all types) evaporates quickly to keep you from getting chilled from damp clothing. It's got to be part of a system though, and you have to use that system properly or you will get damp and cold. We just came back from five nights in our tent @ an avg. of 26F at night. After hiking all day in these and then, putting on fresh Smartwool tops and bottoms we were warm all night (we could have stayed in the same set if we had to). Medium weight is probably fine for most applications, but do be sure you've got the right system for the conditions. In summer and spring it's less critical, but in winter (THE best time IMO)...having the wrong gear can make for a miserable experience...and a dangerous one. Smartwool has the added benefit of being able to go in the dryer at home...not all merino products can. That's my take.
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