Compact and superlight, the Arc'teryx Atom SL men's sleeveless vest easily stashes in your pack, ready to grab when summer temperatures drop. Worn as a mid layer or standalone, this insulated vest delivers moderate insulation perfect for early morning runs and taking in the view from a freshly bagged peak. Its Tyono 20 fabric balances breathability and wind resistance, and the Coreloft synthetic insulation is light, compressible and efficient, even when wet. Lightweight stretch fleece side panels improve ventilation and hold the fit. Layering is the key of keeping you dry, warm and using your energy efficiently. Put into the right systems, a good sequence of layers moves moisture away from your skin, conserves or dissipates heat and provides you with weather protection. Here they are. The first layer, worn next-to-skin will provide warmth and quickly move moisture away from your skin. Then, the second layer, is the insulation and fleece, that provides warmth while also dissipating heat during periods of high output. And finally, the third layer, the shell, which is the most weather-protective layer, worn on the outside of all other layers.