For optimal comfort outdoors, your base layer should be made of merino wool. Rather than absorbing moisture, merino wool transports perspiration away from your skin and disperses it on the outer surface where it can evaporate. The result: you stay drier even when you sweat, and your base layer dries quickly afterwards. Keeping dry is important for maintaining a cool body temperature in the summer and avoiding hypothermia in the winter. Merino wool is a high-performance material that excels during activities with fluctuating periods of high and low intensity. Cotton is not. 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.