The Wonder of Fern Frost

Did you know that window frost is also known as fern frost? I’d never heard that until I decided to do a little research to find out how it’s possible that frost creeping across the poorly insulated windows of my house creates such intricately beautiful and leaf-like patterns.

As you probably know, frost appears on windows when it’s below freezing outside but fairly moist inside—which is why our single-pane bathroom window puts on a particularly good show.

Patterns are formed as water vapor condenses as frost on the inside of the window and ice crystals inch off in different directions, following scratches, bumps, dust, streaks, whatever gets in their path.

Whoever thought to dub this magical winter display “fern frost” was spot on because that’s exactly what it looks like. Though I might have tried to come up with a term that expresses the wonder of having something so lovely come from the combination of freezing temperatures and old, dirty windows.