Sweat is the human body's method of preventing overheating. Hypothermia is caused by heat being lost from the body faster than the body can produce it. Normal cause of hypothermia is an external force transfering heat from the body faster than it can be generated. i.e. Wind or water etc. Prevention is relativily easy. Correcting a hypothermic situation is dramaticly more difficult. Requires shelter and effective rewarming. Prevention is facillitated by wearing the proper clothing including a warm hat and by thermostating and regulating your level of exertion. Thermostating is the art of adding or removing clothing, opening or closing sleeves and zips according to your level of exertion. In cold conditions, as in very warm, you should be deliberate in your movements so as to avoid generating too much heat and thus sweat. Vapour barrier waterproof garments are at their most effective in temperate -10° to 25°c temperature ranges. For very cold weather, layers of natural wool and a windproof outer layer are the most effective methods of protection.