Frost can sometimes create beautiful patterns on the inside of windows. The delicate shapes, called fern frost because of their resemblance to fern plant, form when water vapour condenses into tiny droplets on the window pane. Ice crystals form, making water freeze on to the sharp points of the ice crystals, creating a chain reaction that creates the patterns.
Window frost (also called fern frost or ice flowers) forms when a glass pane is exposed to very cold air on the outside and warmer, moderately moist air on the inside. If the pane is not a good insulator (for example, if it is a single pane window), water vapour condenses on the glass forming frost patterns. With very low temperatures outside, frost can appear on the bottom of the window even with double pane energy efficient windows because the air convection between two panes of glass ensures that the bottom part of the glazing unit is colder than the top part. On unheated motor vehicles the frost will usually form on the outside surface of the glass first. The glass surface influences the shape of crystals, so imperfections, scratches, or dust can modify the way ice nucleates. The patterns in window frost form a fractal with a fractal dimension greater than one but less than two. This is a consequence of the nucleation process being constrained to unfold in two dimensions, unlike a snowflake which is shaped by a similar process but forms in three dimensions and has a fractal dimension greater than two.
If the indoor air is very humid, rather than moderately so, water will first condense in small droplets and then freeze into clear ice.
Fern frost can form on windowpanes when the air outside is very cold and the air inside is moist. The outside air temperature on the winter’s day when this photo was snapped was 15 degrees F (-9 C) and the inside air temperature was 66 F (19 C). Crystal formation is affected by surface features of the glass, like dust and dirt particles, which serve as nucleation points for crystalline growth. Towels had been left to dry near the old and weathered bathroom window shown here provided just the right level of moisture conducive to frost formation. If there had been considerably more moisture in the air, for instance after a steamy shower, or if the windowpane had not been extremely cold, the water vapor would have merely formed ice or water droplets on the glass. The very low temperature of the glass allowed the moisture to go directly from a gaseous state to a solid state— in the form of frost.
Picture Credit : Google