Editing Frost (section)

===Window frost===
'''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 a bad [[thermal insulation|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 usually forms 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 [[nucleation|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.<ref>{{cite book|last=West|first=Bruce|author2=Mauro Bologna|others=Paolo Grigolini|title=Physics of Fractal Operators|publisher=Springer|date=2003|page=46|isbn=978-0-387-95554-4|url=https://books.google.com/books?id=EgyTpQZOga0C&pg=PA46}}</ref>

If the indoor air is very humid, rather than moderately so, water first [[condensation|condenses]] in small droplets, and then freezes into [[clear ice]].

Similar patterns of freezing may occur on other smooth vertical surfaces, but they seldom are as obvious or spectacular as on clear glass.

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