The first thing you need to know about frostbite is do NOT rub snow on it. Plenty of old wivery is good, but not that one, say the experts on frozen tissue.

Dr. Michael Rolnick, Georgetown University Medical Center's director of emergency services, warns that frostbite can affect different people different ways. (Or the same people at different times.)

Frostbite, like burns, has several stages or degrees, says Rolnick:

* Frostnip: Freezing of the top layer of skin, sometimes indicated by whitening. Usually on top of the nose or the ears. Disappears with rewarming.

* Superficial Frostbite: Partial or full-thickness freezing of the skin. Usually reversible, but itching, burning and coarse sensation return with rewarming. Sometimes there is quite severe pain.

* Second-degree Frostbite: Thick, clear or pink blisters within 24 to 48 hours after exposure. It can involve tendons, muscles and bone and may cause loss of limb. Recovery, says Rolnick, "can be slow and painful and must be supervised by a physician."

Prevention, of course, is best. Keep hands, feet, face and head in warm and waterproofed covering. Ears are especially vulnerable targets. Don't try to thaw bitten portions outdoors, because of the danger of refreezing. Joggers, especially, should take care. Even cases of penile frostbite have been reported.

Pain or blistering should get prompt medical attention.

And if you're warm today, just wait a minute . . .