Category: "Film frost"
Black cars remain my favorite place to observe frost patterns. Here are a few I saw on one car today.
The straight lines of frost are more common when it is drier. And it has been quite dry here due to the very low temperatures.
These pictures show an interesting mixture of straight lines and curved boundaries.
And finally, the windshield had a pattern that resembled a hilly landscape.
But the pattern is actually quite flat. The frost is playing mind games on us, presenting an optical illusion of 3D topography.
As with all cases of frost on surfaces, the ice initially got started when a thin layer of liquid (melt) froze in various spots. The ice that grew, first grew in the melt layer, then grew on top, essentially "sucking"** the vapor out of the surrounding air, thus drying out surrounding regions. This is why we see bare surfaces near the larger frost crystals. Those frost crystals grew from the vapor, just like snow, but are anchored to the surface because that's where the film froze. So, two types of crystallization are important: freezing of the melt (melt --> ice) then vapor deposition (vapor --> ice).
** The actual process is diffusion (the way perfume molecules reach our noses), but this term is a little more vivid.