A type of precipitation composed of slowly falling, very small, unbranched crystals of ice which often seem to float in the air; it may fall from a high cloud or from a cloudless sky. Diamond dust forms under very low air temperatures in strong, surface-based temperature inversion layers. Either vertical mixing within or radiational longwave cooling of this layer causes the air to become supersaturated with respect to ice, so that small ice crystals form. These mostly unbranched crystals are seemingly floating in the air, slowly falling from an often cloudless sky (AMS, 2000). Columns (ppco) and plates (pppl) are the dominant shapes found in diamond dust (Walden et al., 2003), but stellar dendrites (ppsd) may also be observed. Long-prism columns having a ratio of length to width 5 are defined as ‘Shimizu crystals’.
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