The Arc'teryx Kyanite Baselayer Zip Neck W is Arc's warmest base layer top, and the choice for cold days and stop-and-start activities. On the move, the Polartec Power Stretch Pro fleece stretches for unrestricted freedom. During rest phases, or when it’s really cold out, the soft brushed interior traps heat to keep you warm. The fleece’s durable nylon outer has a smooth face for comfort under layers,  the tall collar extends warmth, and the deep front zipper aids heat dump and makes on-and-off easier.