In frostbite, skin becomes hard black in about -
The localized effects of cold are frost-bite, trench foot, and immersion foot.
Trench foot and immersion foot (wet cold) result due to prolonged exposure to severe cold (5-8° C) and dampness. Blister formation with ultceration and localized dry gangrene occurs.
Frostbite is infarction of peripheral digits with oedema, redness and later necrosis of tissue beyond a line of inflammatory demarcation. It occurs due to exposure to greater extremes (-2.5° C) of cold. Necrosis with blister formation and gangrene occur. Skin becomes hard and black in about two weeks. The sequence of events is:-
Erythematous patches → few hours
 Soft blisters → 24 - 36 hours
Hard and black → 2 weeks
Deep up to the none → 1 month
3 days
7 days
2 weeks
4-6 weeks