FZN stands for Five Zero Nine, not frozen, and that's exactly how you'll feel in the 509 FZN Merino Hoodie. The low-profile balaclava hood fully seals out biting wind, while the built-in mask tucks away inside the collar until you need it. Naturally breathable, fast-drying, and odor-repellent—comparing this base layer shirt to grandpa's wool knickers is like upgrading your sled from his 1978 Panther to a PRO-RMK.