Whether you're riding in hot weather or cold, your body is producing moisture. Efficient moisture management is extremely important in any weather because your body fatigues much more rapidly with even slight variations in core temperature, which fluctuate more based on perspiration. The Gore Bike Wear WS Thermo Base Layer is specifically designed for use in the cooler months to maintain your core temperature through efficient moisture management and windproof protection.There are really two parts to how the WS Thermo Base Layer helps your body maintain its core temperature while you ride. The first is moisture management. Your body has a much more difficult time regulating its temperature when your skin is wet. The combination of an internal hydrophobic fabric (100% polyester) with an external hydrophilic fabric (100% polyester) pulls moisture away from your body to keep your skin dry and aid in temperature regulation. The second part is protection from the weather. Gore uses WindStopper windproof fabric (100% polyester with a WindStopper membrane) to shield your skin from wind and help your body maintain its core temperature. These two aspects of Windstopper fabric work together to increase your efficiency and performance.The WS Thermo Base Layer increases comfort in other ways, as well. Mechanical stretch panels (88% polypropylene, 8% elastane, 4% nylon) ensure maximum range of motion and increase durability by stretching instead of tearing. Gore designed the WS Thermo with as few seams as possible, since they cause irritation. Where seams were necessary, Gore used flatlock seams, which cause less chafing than the traditional variety. Finally, Gore designed the WS Thermo with a low cut back to give you full coverage in the seated position.The Gore Bike Wear WS Thermo Base Layer comes in White. It's available in sizes Small to XX-Large.