REI Co-op Midweight Base Layer Half-Zip Top in Blue Color: Blue Nights Size Medium Men's Smooth outer face lets layers slide on easily, while a brushed interior provides warmth High neck coverage with a half-zip front lets you open up for venting when you're working hard Fabric wicks moisture and dries quickly to keep you comfortable 4-way stretch delivers a flexible range of motion Flatseam construction reduces bulk and chafing, and rolled-forward shoulder seams prevent abrasion from pack straps UPF 50+ rating protects against sun rays