Frostbite is the freezing of the skin and also the bodily tissues under the skin. This condition happens when you are exposed to temperatures below the freezing point of skin.

Children are at greater risk for frostbite than adults, both because they lose heat from their skin more rapidly than adults. Frostbite is most likely to happen in body parts farthest from the heart, and those with a lot of surface area exposed to cold.

Causes

In conditions of prolonged cold exposure, your body sends signals to the blood vessels in your arms and legs telling them to constrict (narrow).
Several factors can contribute to its development including
Length of time a person is exposed to the cold,
Temperature outside,
Force of the wind (wind chill factor),
Humidity in the air,
Wetness of clothing, shoes and body coverings,
Ingestion of alcohol and other drugs and
High altitudes.
Other things that may increase the risk of frostbite include: smoking, windy weather (which increases the rate of heat loss from skin), diabetes, peripheral neuropathy, and Raynaud's phenomenon.

Symptoms

Generally, frostbite is shows discoloration of the skin, along with burning and/or tingling sensations, partial or complete numbness, and possibly intense pain. Frostbite usually affects areas that are exposed to the cold, such as the cheeks, nose, ears, fingers, and toes, leaving them white and numb. When skin has thawed out, it becomes red and painful (early frostbite). With more severe frostbite, the skin may appear white and numb (tissue has started to freeze).

Treatment

In the first phase of frostbite, the skin temperature begins to drop, and blood flow to the surface of the skin dramatically decreases. As the cooling process begins, the body initiates the Hunting response, a 5-10 minute cycle where the blood vessels dilate, and then contract, which is the bodys attempt to re-warm.
The most important step in treating frostbite is to make sure you will not refreeze the injured area. If you are really in a big problem due to it and come to a shelter then you will do far worse injury to yourself or an injured friend treating your frostbite and then moving on to the trailhead, only to get frostbite in the same areas when you start to move again.

The person with frostbite should drink hot fluids such as coffee or tea. Never drink alcohol or smoke. Never rub, apply snow to, or put intense, direct heat on the frostbitten areas. If the feet or toes are frozen, do not walk on them and keep the feet raised.