|Hoar Frost Crystals by Eva Seidenfaden (Paraselene Optics Site). ©Eva Seidenfaden,
shown with permission.
Found on a frozen lake near Trier in Germany during Europe's very cold weather last week (Jan '09). The largest crystals were about an inch across.
On a clear night the ground radiates its heat into space and becomes colder than the surrounding air. When it is not too cold, dewdrops form as water vapour condenses onto the cold surfaces. When the surface is below freezing, water vapour from the air converts directly to ice to form the hoar frost. Subzero air can also contain supercooled water droplets. When at the same temperature, droplets have a higher vapour pressure than that of ice and hence they evaporate to provide more vapour for frost formation.
As the day progresses and the hoar frost is heated by the sun's rays it often sublimes directly back to water vapour. The vapour laden air cools once away from the surface leading to the formation of water droplets, 'steam', which iridesce strongly.
Hoar frost covering trees, foliage and the ground confers a white magic to icy dawns. The feathery multiple crystals with branching, dendritic, structures look complicated. And yet they have the same basic hexagonal symmetry as the geometrically much simpler plates and columns of the airborne crystals that form halos. Their hexagonal structure is a manifestation at our level of existence of the structure of ice at a scale a billion or more times smaller - that of atoms.