Although Jack Frost nipping at your nose can be the subject of happy, winter songs, actual frostbite is nothing to smile about.
Frostbite occurs when your body can no longer keep your skin warm, and your living tissues begin to freeze. Here's what happens. As the air temperature starts to drop, your body conserves its heat by restricting the flow of blood to your skin, concentrating your warm blood inside.
You'll appear paler, and you'll wrap your jacket more tightly around your shoulders. If the air temperature continues to drop below freezing, your exposed skin or extremities might become too cold. When this happens the blood vessels in the endangered areas open wide again, and blood flow increases.
This extra dose of warm blood from inside your body keeps your skin from freezing, although it cools your overall body temperature. At this point your endangered skin, like your ears and nose, or maybe your toes, will turn a bright red, and you'll start wishing you were indoors.
If the air gets just too cold, however, or an icy wind is blowing, the flow of blood to the frostbitten tissue stops altogether, and the tissue begins to die. This is a desperate measure for your body; your blood stays inward to keep your vital organs as warm as possible, sacrificing the extremities.
The most severe cases of frostbite can lead to gangrene and amputation. In any case, you should seek medical attention. You should never follow the folk remedy of rubbing frostbitten skin; that will only cause further damage. Nor should you rub snow on it; that, too, will make it worse.