If you are lost in the early spring or late fall, or are at a higher altitude, keeping warm is one of the most critical requirements for survival. Hypothermia, the cooling of the bodies core, can set in even when it is in the 60'ies, and the effect of body heat loss is greatly amplified when wet or in windy conditions. Once hypothermia starts to set in, it becomes very difficult to re-warm your core, so it is very important not to allow yourself to get cold.
Signs of hypothermia include feeling profoundly cold, blue lips and pale skin, uncontrollable shivering, typically accompanied by muscle aches, a feeling of drowsiness, and a lack of ability to think clearly. In severe cases of hypothermia, where the body temperature has fallen below 94 degrees, dementia, seizures, and unconsciousness followed by eventual death will occur. It is critical to get the core temperature of yourself or the victim up and stay warm.
One of the first rules of staying warm is realizing that dampness kills. If you are in wet clothes you need to get out of them and get them dry as soon as possible. If you are already shivering, even if you have all of your equipment, getting into your sleeping bag will not get you warm. You need to build a fire or come to some other heat source. When traveling through the back country, you should keep a small survival kit on your person at all times. Waterproof matches, two survival candles, a pocket knife and a large lawn and garden trash bag should be part of that kit. While preparing your fire you can do exercises like running in place to keep your body warm. A trash bag filled with leaves can provide surprising warmth when worn like a jacket. Cutting a hole in the bottom of the bag for your head, and two for your arms, stuff the trash bag with leaves and tuck the bottom into your pants.
You should gather enough fuel for your fire to get through the night. Even in the desert, a dead yucca plant can provide at least some fitful flames for warmth. Try to find dry dead wood, smaller pieces work best although burn through quickly. Larger wood will be harder to come by without a small axe or portable saw. If all else fails a small survival candle in a soup can will throw a surprising amount of warmth in a tight space.
The most critical piece is to stay dry and not allow your core temperature to get too low. As soon as you start to feel cold do everything you can to stay warm. You should also make every effort if you get cold not to fall asleep. Our core temperature drops when we sleep and drowsiness is a sign of hypothermia.
Keeping hydrated and finding water...