plate crystal

plate crystal

[′plāt ¦krist·əl]
(hydrology)
An ice crystal exhibiting typical hexagonal (rarely triangular) symmetry and having comparatively little thickness parallel to its principal axis (c axis); as such crystals fall through the clouds in which they form, they may encounter conditions causing them to develop dendritic extensions, that is, to become plane-dendritic crystals.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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When plate crystals are sampled, they tend to orientate with their basal faces pointing upward (i.e., in the optimal plane to be imaged), whereas columns tend to orientate with only their prismatic faces visible.
For example, with hexagonal simple Plate Crystals (P1a) found in 451 samples (out of the total number of 938 evaluated samples) were simultaneously identified 285 Columnar Crystals (C1e).
d) Aggregates of stellar and large plate crystals were the predominant snowflake type (Type 4).
The CO2 plate crystals also combined to form a very simple halo: a single, brightly colored arc directly over the Sun.
The division is rooted in the prevailing presence of specific snow crystals in aggregates and contains the following six types of aggregated snowfall elements: Needles and sheaths; small columnar and plate crystals; medium size plate and stellar crystals; large dendritic and stellar crystals; aggregates of minigraupels and frozen drops; mixtures of large drops (often from melted snowflakes) and snow crystals.
Sundogs are produced by plate crystals, which look as if they were stamped out with a hexagonal cookie cutter (if you can imagine one smaller than a millimeter across).
Actually, plate crystals usually have a slight tilt, which smears the sundog, giving it an enhanced vertical extent.