Details Arc'teryx Men's Size Chart Designed for high output in cooler temperatures, the Phase AR Zip Neck is a midweight base layer top made from next generation Phasic™ AR. Engineered to provide insulation and offer performance stretch, Phasic™ AR uses different polyester yarn structures on its interior and face to rapidly wick and disperse moisture. The tall collar adds warmth, and the deep front zip provides ventilation. Lighter weight Phasic™ SL panels improve thermal regulation. With updated style lines and an improved fit.