This is the jacket for going fast on cold days or being perfectly dressed on cool days. Our Alpha double-layer construction is the core of what makes this jacket work so that you can be in full flight mode when the season starts. The stretch woven fabric lets a small amount of air in to keep you from getting too sweaty during those efforts on colder days, because if you get wet you get cold. The Polartec® Alpha second layer insulates while still providing plenty of breathability, and it's so light the jacket has hardly any bulk to it. The back is made from our Nano Flex Extra Dry fabric, which provides a good level of insulation yet has no membrane so allows for ample amounts of sweat to escape. The fit is close to body so there is no fabric flapping around, while the little details make the difference: easy-sliding YKK® zippers, reflective drain holes on the pockets, a lie-flat silicone gripper band around the waist. The close fit makes this our most aerodynamic jacket as well. Not that aerodynamics are terribly important in the winter, but we're a firm believer that Fast Is Fun!