Frostbite

Frostbite occurs when the skin and surrounding tissue freezes due to low temperatures. Frostbite can happen to anyone who is exposed to temperatures below freezing, and doesn't have adequate protection. Those who spend long periods outdoors in cold weather conditions, such as climbers, hikers, skiers and sailors are particularly at risk of getting frostbite.

Frostbite can affect any part of your body, but the extremities, such as the fingers, toes, penis, ears and nose are most likely to be affected. The damage to the skin and tissue is caused by the formation of ice crystals, which expand and rupture cells. Damage to tissue is also caused by the closure of arteries (blood vessels) carrying vital oxygen and blood supply to the affected area.

The severity of frostbite varies depending on the length of exposure and how badly damaged your tissue is. If only the skin and surface tissue are affected, the damage will be superficial, and it is likely that you will make a full recovery. However, if your blood vessels are affected, the damage is more likely to be permanent. If gangrene (dead tissue) occurs, amputation may be necessary.

http://www.nhs.uk/Conditions/Frostbite/Pages/Introduction.aspx