Huge Dendrites ~ Winter frost crystals cluster like summer mosquitoes.  Imaged by Bill Purdy near New York City.

"This was very early morning frost on my wife�s car. It was crystal clear all night and very cold.  I came outside to find our cars covered in these large frost crystals."
©Bill Purdy, shown with permission.


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That the crystals were well separated hints that the car surface was recently well cleaned and had few nuclei on which crystal growth could start. Hoar frost forms when a surface is cooled at night by radiation to a clear dark sky.   Water vapour molecules attach to cold growth nuclei and commences crystal growth directly from the vapour phase.

Ice - everyday ice '1h' - has hexagonal symmetry and these crystals have grown very slowly overnight along the directions of the crystals' three main 'a' axes to give the tree-like shapes. Growth along the hexagonal prism axis - the 'c' axis - was partly inhibited because that would be towards warmer air away from the cold car roof.