A cold rain can be the most dangerous weather for hikers, because it can cause hypothermia (or "exposure") even when conditions are well above freezing. When the wind blows, its chill effect can make you much colder than the temperature would lead you to suspect, especially if you're sweaty or wet. Hypothermia occurs when wind and rain chill the body so that its core temperature drops if the condition is not treated in time.
Avoid hypothermia by dressing in layers of synthetic clothing, eating well, staying hydrated, and knowing when to take refuge in a warm sleeping bag and tent or shelter. Cotton clothing, such as blue jeans, tends to chill you when it gets wet from rain or sweat, increasing your risk of hypothermia. Natural wool and artificial fibers such as nylon, polyester, and polypropylene all do a much better job of insulating your body in cold, wet weather.