Frost is the solid deposition of water vapor from saturated air. It is formed when solid surfaces are cooled to below the dew point of the adjacent air. Frost crystal sizes differ depending on the time and water vapor available. Frost is also usually translucent in appearance.
Window frost, also called fern frost, forms when a glass pane is exposed to very cold air on the outside and moderately moist air on the inside. If the pane is not a good insulator, such as a single pane window, water vapour condenses on the glass forming patterns. The glass surface influences the shape of crystals, so imperfections, scratches or dust can modify the way ice nucleates.
Frost formation is a complex process, and conditions have to be right for it to occur. It forms on surfaces directly from the vapor state without condensing as dew. If dew forms, frost formation is unlikely, even if the temperature drops below freezing.
Frost is more likely to form on surfaces above ground first, such as house roofs, or automobiles, because the air immediately above the ground is usually a few degrees warmer than air a few feet higher. There is some heat transfer from the ground to the air a few centimeters above it. If there is wind, frost will not form. If the skies are cloudy frost will not form, as the clouds reflect the radiated heat from the ground which helps in keeping the lower layers mixed.
The ideal condition for frost formation is a night with clear skies, light winds, and a temperature forecast to be near or a little below freezing. Temperature and water vapor humidity determine the crystalline forms. Ice crystals are responsible for the artistic displays of window frost. The principal axis of a single crystal of ice is perpendicular to the axis of hexagonal symmetry, and as these crystals are formed the patterns of window frost develop.