Jack Frost is an elf-like creature common in the folklore of many countries. His origins are Scandinavian - he was the son of Kari, the Norse god of the winds, and was variously known as Jocul, meaning "icicle", or Frosti, meaning "frost", or some times both. Jocul Frosti was an artist and liked to ply his craft in the cooler portion of the year; it was he, it was said, who turned the leaves brown with the approach of winter, and his talent was also evident in the elaborate patterns which appeared on window panes on nights when it was very cold.
The temperature inside a modern house is rarely, if ever, allowed to fall low enough for Jocul Frosti to perform his secret ministry. Those of us old enough to have lived before the dawn of central heating, however, will clearly remember the "ferns" of ice that appeared on the insides of our bedroom window panes on clear, frosty winter mornings.
"Jack Frost's garden", as it was sometimes called, is a phenomenon unfamiliar to our children. On a very cold night, the temperature of the inside surface of a window pane in an unheated house may fall well below the zero mark. Moreover, in the case of bedrooms, the moisture exuded by the occupants increases the humidity of the air, often to the point of saturation. As the temperature falls, the air seeks to rid itself of its excess moisture by depositing it in the form of ice crystals, creating delicate patterns on the glass. The crystals are usually shaped like columns or plates, and like snow, they have hexagonal cross-sections that are pleasing to the eye.
The beautiful jewel-like shapes, incorporating clusters of "rosettes" or "ferns", grow outwards in all directions from the point of first formation, creating intricate branches as they creep across the frozen surface of the glass. The individual crystals, if examined under a microscope, do not have the perfect symmetry found in the snow crystals that form in free air. But it is this lack of symmetry that allows the patterns to take on such a great variety of magical, surreal shapes.
This was, allegedly, the work of Jocul Frosti. But in other countries there were different explanations. In Russia, the icy artist is a white old man called Father Frost; in Japan, the Frost Man is the slightly malevolent brother of the Mist Man; and in Germany, the agent of the patterns is a woman - a cold, austere figure known as Mother Frost.