Any time you're outdoors — away from such luxuries as thermostats, solid walls around you, and a roof over your head — clothing is your primary shelter. You might find yourself hunkering down for a cold, wet night in the forest with only your clothing and some kind of rudimentary shelter made from natural materials to protect you. If I were to choose the top 5 priorities in survival clothing, they would be as follows:
Stay Dry — If you can't stay dry, you can't stay warm. Wet clothing acts like a "swamp cooler" air conditioner, as evaporation inexorably drains the warmth from your body core. A simple pocket poncho can be a life saver in wet conditions. Waterproof/breathable clothing is even better, though it is expensive. The ability of the clothing to release perspiration into the atmosphere is a huge benefit not afforded by a plastic poncho. Hypothermia doesn't care whether you got wet by falling in a river or by sweating. All that matters is you're wet. So if you rely on plastic to keep you dry, pay attention to ventilation to release perspiration from your body and clothing.
Insulation — Trapping a layer of body-warmed air next to your skin is the purpose of insulation. It's the same principle as a diver's wetsuit, wherein a thin layer of body-warmed water helps protect the diver from the colder water around him. The best insulation is natural, such as wool or down. The problem with down is that when it gets wet, it quits insulating and becomes an almost worthless lump. Wool tends to wick moisture away from your body and continue to insulate even when wet. I like merino wool because it isn't itchy. Man-made insulation materials are almost as good as the natural, and are generally much less expensive. I like fleece as an insulation layer beneath a shell that turns away the wind and precipitation.
Good Footwear — Your feet might be your only mode of transportation, so you must take good care of them. The worst thing is a pair of new boots that haven't been broken in yet, as they can cause disabling blisters. Trail shoes or boots that are nicely softened and fit your feet perfectly are the right choice. But even the best boots aren't enough. Good socks are essential. My favorite socks are made by a company named Point6 (www.point6.com). They fit like a second skin, are made of merino wool, and are very durable. It's a combination of qualities that help prevent blisters, and keep the feet at a comfortable temperature in both warm weather and cold.
Head Covering — Most of your body heat is lost through your scalp. An old woodsman saying is, "If your feet are getting cold, put on your hat." Sounds goofy, but it's true. The body starts to shut down the delivery of warm blood to the extremities such as the feet and hands as the body core cools. If you lose too much core warmth through your scalp, the feet will be the first to know about it. Any head covering is better than none, but a hat with a wide brim and the ability to shed water is the best, as it provides protection against both the sun and the rain. Hint: if your hair gets wet, dry it as soon as possible, or else you'll lose heat quickly due to evaporative cooling.
Leather Gloves — Second only to your feet, your hands need protection. Injure your hands in a survival situation and you will seriously limit your ability to perform life saving tasks. Not only that, but if you get an infection through a cut, abrasion, or a splinter, it can become a life-threatening injury as your weakened body and immune system are not able to fight it off. George Donner, leader of the ill-fated Donner Party, didn't succumb to starvation the way so many of the pioneer group did. Nope, he died because of a small injury to his hand that became infected. He was still alive when rescuers arrived, but was too weak from the infection to be saved. A pair of lightweight leather gloves can become treasured survival gear.