Medicine for Mountaineering has a short section on frostbite (Wilkerson, The Mountaineers); a more definitive description is in Hypothermia, Death By Exposure (Forgey, ICS Books). Also, see Wikipedia -the article has a long list of citations.
http://en.wikipedia.org/wiki/Frostbitten

Forgey states that frostbite occurs when skin temp falls to +22-+24 (-5.5C - - 4.4C). Frostnip is a milder form of frostbite. So, based on my reading, outside temp needs to be below freezing for frostbite. There are degrees of frostbite as well and it does not always cause irreversible damage. High altitude and dehydration contribute to the severity.
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