Window Frost

As promised last week, here’s a slide show that focuses on window frost patterns. All the images are courtesy of Wikimedia Commons. They show some really beautiful patterns. Legend has it that Jack Frost creates these ice frescoes, but as we explained last week, it all comes down to the geometric structure of water crystals.

Window frost is really hoar frost constrained to 2D. If the window glass is below freezing temperature, and there’s any humidity on the inside of the house, ice crystals will assemble following the microscopic irregularities on the surface of the window glass. The irregularities on “smooth” glass are visible in this electron micrograph with magnification 500X:

These irregularities provide nucleation sites for the water molecules to attach to the glass. Once attached, that molecule will provide the electrostatic base for forming its usual hexagonal crystal. With two caveats:

  • Nucleation sites randomly distributed on the glass will make every frost pattern unique — like a snowflake, and modify water’s tendency to grow into a symmetric crystal. The electric charge at nucleation sites will “bend” the crystal from its preferred growth direction. Hence the artistry of Jack Frost.
  • If freezing temperatures exist on the glass itself, there will be thin layer of air adjacent to that glass, maybe 1–2 mm thick depending on inside/outside temperatures, that’s also cold enough to form ice. Molecules that try to crystalize farther from the glass will quickly sublimate, escaping back into the air as water vapor.

The deposition process is the same as with hoar frost, but the temperature gradient constrains the crystals to 2D. Yes, there will be a thin layer of 3D crystals, but the overall frost pattern will be 2D. Next time you find Jack Frost has visited, get a flashlight and shine it parallel to the glass. The 3D layer will be more visible. Touch it with your finger and the frost will melt back down to the glass. It’s a fragile structure. Even your warm breath will disrupt it.

Alas, with energy-efficient double-glazed windows now commonly used in construction, Jack Frost is being shut out. That’s a good thing actually, in terms of the environment. If you really want to to see frost patterns on windows, go outside on a freezing day and breathe on a window pane. They won’t last long, depending on the outside humidity, but you will see them form.

Next Week in Sky Lights ⇒ Solar Eclipse by Phobos