What causes frost to form on the inside of a window?

Frost is formed on the inside of a window, when the air on the outside of the glass is cold enough to cause the interior water vapor to condense and freeze against the sufficiently cold inside surface of the glass. This is no longer a common occurrence, because most homes now have double-pane windows that are much better thermal insulators. Older glass windows, however, could produce elaborate interior frost patterns because the ice crystals would form around scratches, streaks or other imperfections on the interior glass surface.

Window frost is sometimes called fern frost, because of the particular leaf-like patterns it can form on a window. It assumes the form of a fractal, but due to the flat surface on which it unfolds, it remains in two dimensions instead of the three dimensions that snowflakes form in the open air.

If the interior of a home or building is extremely humid, the water vapor will form small droplets when it condenses. These will then freeze directly into formations of clear ice rather than create frost patterns.