This is a modern interpretation of the wind shell jacket, the one you keep in your pocket for a long descent or put on for the first hour of the ride until the day warms up. That jacket used to be a simple affair - a nylon shell - but the lack of stretch meant that it had to be cut on the generous side to fit all body shapes, both on and off the bike. Castelli has completely reimagined what the shell jacket should be and how it's constructed. Adding panels on the sides and the sleeves in a very high-stretch yet tightly knit fabric allowed to give the jacket a modern, close-to-body fit for aero gains on any terrain, and it actually keeps more heat in than a jacket that's flapping about. It is very compact so that it can pack away into minimal space in a jersey pocket. The stretch panels are treated for water repellency, which will help in a light mist, but this is not meant to be a true rain jacket.
Coated ripstop nylon front and back and front of sleeves to keep wind off
Tightly knit stretch panels on sides and sleeves allow for perfect fit
High collar to keep drafts out
Reflective details on chest and lower back for low-light visibility