Hoar Frost
What we see as feathery white overnight growths on fences, branches and leaves is here resolved into its individual needle like ice crystals.

Images by Jari Luomanen
            (atmospheric photography).
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Hollow prism. Crystals with such 'imperfections' in clouds give poor and indistinct halos or none at all.




As air cools overnight its water vapour can eventually reach saturation. Above 0°C the excess water condenses into dew. Below 0° the vapour changes phase to the solid – ice. The ice grows on surface irregularities as long columnar hexagonal crystals.

The hexagons reflect ice’s molecular structure shown at left as a chemist’s ball and stick model.  Red balls are oxygen atoms.   Along each white stick is a hydrogen atom.

Nature shows great variety within the confines of hexaganality. Crystals differ in length, are sometimes hollow, sometimes with one end pyramidal. Their hexagon prisms can be 'irregular', of unequal side length. What is invariant are the interfacial angles for they must follow the planes of atoms, orders of magnitude below in size.