Frost Pattern on Cold Window

April 02, 2021

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Photographer: Mary Kelly
Summary Authors: Mary Kelly; Jim Foster

The photo above shows an elaborate frost pattern on a storm window in my home in Des Moines, Iowa. It was snapped around midday on February 6, 2021. Fern-like frost patterns like this will often form on windowpanes once the outside air temperature drops below about 14 degrees F (-10 C) if there's sufficient moisture (usually transferred from the inside of the house) to allow frost crystals to grow.

Outside, the air temperature was 9 degrees F (-13 C). The inside temperature was about 66 degrees F (19 C), but on the second floor, where the window frost was prominent, it was probably cooler because the heating vents here aren’t very efficient. Worth noting, the humidity may have been somewhat higher than normal because the bathroom shower had been used earlier in the day. Also, it should be mentioned that the upstairs windows don’t completely latch. So it’s likely that the cold temperature of the indoor glass allowed moisture in the upstairs rooms to crystalize directly onto the windowpanes.