What weight base layer do I need for winter versus everyday cold?

Midweight options (like the Smartwool Classic Thermal line and Carhartt Force Midweight) target genuine winter and active cold-weather use, while lightweight pieces (the Merino.tech tee, Smartwool Active Ultralite) suit 3-season layering and aerobic activity. Fleece-lined budget sets skew warm and are popular for sedentary cold-weather wear.