If you have leaky windows with semi-moist air on their interior and very low temperatures on their exterior, “fern flowers” or fern frost can form on the inside of the windows.
Such was the case this morning after a sub-zero overnight low:
And my kitchen window had grown a lovely crop of fern flowers overnight.
Easy to see why it’s called fern frost, since the ice crystal patterns resemble the leaves of a fern. But why does it take this shape?
Moisture from the air condenses on the cold window, but instead of forming a liquid pool, the very cold window temperature causes the water to go directly from gaseous to solid state. Ice crystals form around surface imperfections in the glass, like scratches, dust, or dirt (I’m sure there is plenty of that on these windows). These initial ice crystals then serve as “organizing” points for the further deposition of ice, and eventually, a lacy pattern appears.
The cold morning air didn’t seem to bother the intrepid avian visitors to the feeders though. They just fluffed up and toughed it out.