If it's hot out, and you're not wearing a base layer, you're doing it wrong. A proper base layer, like the Castelli Seamless Sleeveless Base Layer, will efficiently manage moisture and actually keep you cooler. To achieve the aforementioned, Castelli constructed the base layer from two fabrics, polypropylene and nylon. As you might imagine, the materials express a duality to the base layer's function pulling and drying. Accordingly, the inner layer is comprised of the polypropylene material. This fabric effectively pulls moisture away from the skin, transferring it to the outer layer of nylon. Why nylon Because its lightweight, airy composition is an efficient evaporator, drying quicker than most materials. So, instead of simply soaking in your sweat, the base layer keeps you cooler by evaporating moisture next to the skin. This has been furthered by the three different mesh patterns on the base layer. Taking anatomy into account, this system maintains a consistently open mesh in order to provide the most ventilation possible. Accordingly, this keeps you cooler, as well. Don't believe us It's time for an experiment. Blow on the back of your hand while dry, and now blow on the same spot after applying a minimal amount of moisture Cooler right Well, that's basically the principle behind this base layer. The Castelli Seamless Sleeveless Base Layer is available in the color White and in the sizes Smallmedium, Largx large, and XX Large.