Light jacket or warmest jersey, rain or shine.Fitting somewhere on the spectrum between a light jacket and your warmest jersey, Castelli’s most technical jersey gets a significant performance upgrade this year. ROS stands for Rain or Shine. It’s a label Castelli give their most advanced products that excel in both dry and wet conditions. For dry conditions, you’re looking for comfort along with breathability to prevent you from getting wet and chilled from the inside. But with wet-weather clothing, you’re looking for protection that will help you stay warm by keeping the rain out and letting sweat escape. The Castelli Alpha ROS Soft Shell Jersey tick both of those boxes. Both the front and back materials are brand new this season and represent a significant step forward in rain protection. These fabrics are used with reduced seaming, and seam sealing where necessary. Castelli call this "functional waterproofing" because while they don’t guarantee that no drop will ever enter the jacket, they can promise you that this is their best effort to keep you warm and dry in the widest range of conditions.