Much of my winter clothing is used for hiking and packpacking and has worked well in below zero F temps. The amount of heat a body generates is dependent on the level of exertion as much as on the actual temperature. Most of the clothing is based, like on onion, on layering. Marmot Men's DriClime Windshirt - at Moosejaw.com The Marmot Driclime Windshirt is amazingly warm by itself and very breathable. I use it over a base layer down to the 20s F. When it gets really cold I wear another layer underneath, usually a micro fleece shirt.