Diamond Dust

Diamond dust is a ground-level cloud composed of tiny ice crystals. This meteorological phenomenon generally forms under otherwise clear or nearly clear skies, so it is sometimes referred to as clear-sky precipitation. It is most commonly observed in Antarctica and the Arctic, but it can occur anywhere with a temperature well below freezing. In Polar regions diamond dust may continue for several days without interruption.

To form diamond dust the temperature must be below the freezing point of water, 0 °C (32 °F), or the ice cannot form or would melt. However, diamond dust is not often observed at temperatures near 0 °C (32 °F). At temperatures between 0 °C (32 °F) and about −39 °C (−38.2 °F) increasing the relative humidity can cause either fog or diamond dust. This is because very small droplets of water can remain liquid well below the freezing point, a state known as supercooled water. In areas with a lot of small particles in the air, from human pollution or natural sources like dust, the water droplets are likely to be able to freeze at a temperature around −10 °C (14.0 °F), but in very clean areas, where there are no particles (ice nuclei) to help the droplets freeze, they can remain liquid to −39 °C (−38.2 °F), at which point even very tiny, pure water droplets will freeze. In the interior of Antarctica diamond dust is fairly common at temperatures below about −25 °C (−13.0 °F).

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