The Arc'teryx Atom SL Jacket delivers strategic warmth exactly where you need it most during high-output mountain pursuits. This ultralight synthetic insulation jacket features Coreloft Stretch insulation in the body core while leaving the sleeves uninsulated, creating the perfect balance of packable warmth and temperature regulation for trail runners and alpine enthusiasts. Breathable Octa fleece side panels enhance ventilation during intense climbs, while the weather-resistant Fortius Air 20 fabric with PFAS-free DWR finish sheds moisture when conditions turn nasty. At just 280g, it compresses down to nearly nothing in your pack, making it an essential piece for variable mountain weather where you need the flexibility to layer up or strip down as the trail demands.