|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.
|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.