Frostflowers: Fractal, Feathered or Fern-like?

This is the time of year for Jack Frost and his ice patterns!

The swirls in the photo above graced the roof of my car some days ago and made me wonder how they came to form in the way that they do.

I am, alas, no scientist, but it seems that small environmental changes, for example in air temperature, air movement, in the number of water molecules or in the quantity of dust particles, cause an ice pattern to grow, perhaps by developing long frond-like arms, thereby deviating from its original flower-like design.

But what causes these formations in the first place? Well, in the case of windows, it seems that frost patterns emerge when a pane of glass is exposed to sub-zero temperatures on the outside and moist air on the inside. Water vapour in the atmosphere condenses and becomes frost on the internal surface of the glass. These days with central heating, we rarely see frost patterns inside our homes, but I recall the high sash windows of my teenage years and the chilly patterns that delighted us on winter mornings.  

• The Story of Snow: what we sometimes miss
• Fern pattern
• Frost photos