A little goes a long way.
The days that you feel a slight chill when you step outside are often the toughest to dress for. The sunny skies suggest you'll quickly warm up, but you don't want to spend the first several miles of your ride fighting off a chill. For rides like this, grab Castelli's Pro Issue Base Layer. This lightweight polyester crew-neck top fits under your jersey and helps facilitate moisture transfer from your skin to the surface to regulate your body temperature. It won't be as insulating as Castelli's Flanders base layers, and it won't cool you off as much as Core Mesh, but it will do a little of each to keep you comfortable on the rides that are difficult to categorize.
As far as construction, Castelli eliminated more seaming in this latest version of the Pro Issue to enhance comfort, and it widened the collar to give it a more streamlined fit under aero jerseys. The flat waist hem helps prevent chafing or irritation by laying smoothly against the skin in shorts, and according to Castelli, the base layer functions best in temperatures ranging from 59 to 77 degrees Fahrenheit.