On the hottest days, we need every trick in the book to stay comfortable on the bike: frozen bidons, pre-soaked cap, frequent shade stops. Castelli's Active Cooling Baselayer adds to our arsenal of hot-weather techniques with innovative fabric to keep our temperature well-regulated through tough conditions. Castelli uses Polartec's Delta knit fabric, which combines hydrophilic and hydrophobic fibers to regulate the drying for better evaporative cooling. Rather than the baselayer drying immediately, it's a steady drying so you can benefit from the evaporation of sweat and ride harder and further in hot weather.