PURPOSE: To illustrate how two types of ice crystals formed high in the atmosphere float downward through the air with preferred orientations.

DESCRIPTION: Ice crystals may be formed in the upper atmosphere and float downward through the air. Pencil shaped hexagonal crystals fall with their axis of symmetry horizontal, while flat crystals float with their axes vertical (flat area horizontal). This can be seen by throwing the crystals in the air and observing how they fall.

Pencil shaped and flat hexagonal ice crystals made of paper and foam are provided for this demonstration.

Scattering and refraction of light by these crystals creates several important natural atmospheric phenomena: (1) The sun pillar is formed by reflections of the sunlight by the flat surfaces of flat crystals when the sun is near the horizon, (2) The 22 degree halo around the moon is formed by refraction with the minimum angle of deviation from pencil ice crystals when the moon is very high in the sky, (3) The sun dog is formed to the side of the sun by refraction from flat ice crystals when the sun is low in the sky.


REFERENCES: (PIRA unknown.) See demonstration N1-31: SUN DOG - MODEL as an example of one effect of these crystals. See Falk, Brill and Stork: Seeing The Light for further explanation and discussion of many of these natural optical phenomena.

EQUIPMENT: Paper and foam pencil and flat ice crystal models.


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