Kevin Logan, age 14., of Cross Creek, N.B., Canada, for his question:
What causes frost patterns on window panes?
We tell small children that Jack Frost adorns the windows with icy patterns or ferny foliage. Later in life we want the exact facts. In this case, the cold facts are even more fascinating than the fantasy of Jack Frost.
Nature is the icy fingered artist. Tjhat looks like delicate brush work is done by changes in the weather conditions. ~Tost of the frosty patterns are caused by slight changes in the temperature of the air, but other factors also contribute to variations in design. Some of these factors are extremely complex. They are related to the formation of snow and other forms of frozen precipitation.
In the meantime, lets enjoy the old fantasy that the frosty window panes are adorned by Jack Frost. For the major work is indeed done by puffs of frozen air at temperatures around the freezing point of water. The frost tends to form very early on cloudless mornings. The earth loses yesterday's heat from the sun during the night. And surrounding air is cooled as it touches the ground and the chilly surfaces of objects near the ground.
The air contains gaseous vapor in the form of seperate water molecules. It must give up some of this moisture as it chills. The moisture, however, must condense on some solid surface. It may condense around dusty nuclei in the air and become misty fog or collect in liquid drops of dew on the grass. If the air is cold enough, vapor molecules may change directly into microscopic crystals of ice.
The artwork on the window panes may begin with a film of condensed moisture. It is then frozen by a puff of cold air, and the glass gets a thin sheet of ice. Perhaps another film of moisture forms and trickles down, cutting ditches and tiny streams through the underlying ice, while another puff of cold air freezes this moisture. New frozen vapor molecules may add microscopic ice crystals. The freezing and melting process repeats again and again. Bit by bit the subtle weather conditions etch the window panes with patterns that resemble ferny foliage.
We learn that liquid water freezes to solid ice at 0 degrees Centigrade, and as a rule we can depend upon this forthright statement. But the exact temperature is a fine fraction of a degree, and under certain conditions the critical freezing point is colder than 0 degrees. These factors help to create the freezing and melting moisture that frosts our winter windows.