More than any other layer, the base layer helps regulate your body temperature by moving perspiration away from your skin. Trapped inside your clothing, perspiration can leave you chilled or damp no matter how well your shell layer fends off rain and snow. Keeping dry is important for maintaining a cool body temperature in the summer and avoiding hypothermia in the winter. If you’ve ever worn a cotton T-shirt under your raincoat while hiking, you probably remember feeling wet and clammy, even though you weren’t getting wet from the rain itself. Cotton is an example of a fabric that retains perspiration and can leave you vulnerable to unwanted chills. For outdoor comfort, your base layer should be made of merino wool. Rather than absorbing moisture, these fabrics transport (or ”wick”) perspiration away from your skin, dispersing it on the outer surface where it can evaporate. The result: You stay drier even when you sweat, and your shirt dries faster afterwards.