Our go-to lightweight layer for active pursuits, Mountain Equipment's Aerotherm Jacket helps our body maintain a comfortable microclimate when we're burning calories in chilly conditions. The softshell fabric provides windproof protection and breathability while being backed by a microfleece lining, so we can stay warm without overheating as we're hiking, running, or climbing in the crisp mountain air. It also stashes away easily in our pack, so we can wear it or shed it as needed as the weather changes or our activity level changes.