Summer weather is warm, until it isn't. So compact and light, there is no reason to leave it behind, Arc'tyrex's lightest Atom is there for you when the temperature drops. Strategically placed at the chest and back, the Coreloft insulation is efficient, light, and compressible. The Tyono 20 outer balances airflow with wind resistance and has a DWR finish to repel moisture. Stretch fleece side panels ventilate and an uninsulated hood adds coverage and buttons out of the way when not in use.