If you’re planning a multi-day trek through the wilderness, or even just an epically long day-hike, you will need to carefully consider what to wear. As any seasoned hiker would tell you, jeans and a T-shirt will only get you so far. Serious hiking requires a more thoughtful approach to clothing, including an accurate assessment of the conditions you’ll be encountering, and a thoughtful layering system that will not weigh you down. Weight is an especially important issue for hikes that extend over multiple days. You can’t bring a suitcase filled with half your wardrobe on this type of excursion – you will need to pack very lightly. One of the most crucial things to consider when deciding on your hiking wear is moisture. Moisture can come from outside, like rain, or from inside, usually from perspiring. Being wet all the time is no fun, and can also be very dangerous. Water on our skin traps heat, and when the water evaporates, the heat goes with it. This means that, if you are wet in windy or cold conditions, the evaporating moisture could cool you down faster than your body can expend energy to keep itself warm. It’s a recipe for shivering discomfort, and can lead to hypothermia, in extreme conditions. To stay dry during long hikes, it’s a good idea to have, at minimum, a water-resistant outer layer, like a rain jacket. This will help keep you dry, by repelling most of the moisture that comes from the outside. Water-resistant jackets are usually also breathable, which means that, they let air pass in and out. This is a good thing, because it can help regulate your temperature if it’s warm, and can help keep you dry, by letting your perspiration evaporate efficiently. Unfortunately, breathability also means that the jacket will be less water-resistant. A waterproof garment or poncho, which is not breathable at all, can be totally waterproof, but that’s when moisture on the inside can become a problem. If you wear a waterproof jacket or poncho, you are almost guaranteed to stay dry if it rains. If you sweat, however—and you will sweat if you are exerting yourself on a hike—that moisture will have nowhere to go. There are several problems with this. First, it’s uncomfortable. But this is the least-important problem. More importantly, if you are soaked with sweat, you could lose more heat than you need to, leading to chills. In warm conditions, the opposite problem occurs. The purpose of sweating is to cool your body down, but if the sweat can’t evaporate, the heat it contains will stay against your skin, and you could overheat. You’ll be producing more perspiration to compensate, which could lead to dehydration. Some products claim to be both waterproof and breathable, but many experts argue that this claim doesn’t make sense, and that waterproof garments are, by definition, not breathable. Still, waterproof, breathable clothing is very popular among people who go hiking for long distances in wet conditions. Another way to manage moisture from the inside is to choose base layers appropriately. A base layer is the first layer that you wear while hiking. Casual hikers on short outings might simply choose a cotton T-shirt as a base layer, but experienced hikers know better. As is the case in most sports, synthetic fibers like polyester are the best base layers for hikers. Synthetic fibers can be very lightweight, and they absorb very little moisture. A cotton shirt is very absorbent, so it traps wetness against the skin. Synthetic base layers do not have this problem, and are an important part of the equation for staying dry. Choosing appropriate clothing doesn't necessarily have to be expensive. All you need to do is ensure that you're making the right choice, instead of the easy or uninformed one.