Diamond dust is a ground-level cloud composed of tiny ice crystals. This meteorological phenomenon is also referred to simply as ice crystals and is reported in the METAR code as IC. Diamond dust 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.
Diamond dust is similar to fog in that it is a cloud based at the surface; however, it differs from fog in two main ways. Generally fog refers to a cloud composed of liquid water (the term ice fog usually refers to a fog that formed as liquid water and then froze, and frequently seems to occur in valleys with airborne pollution such as Fairbanks, Alaska, while diamond dust forms directly as ice). Also, fog is a dense enough cloud to significantly reduce visibility, while diamond dust is usually very thin and may not have any effect on visibility (there are far fewer crystals in a volume of air than there are droplets in the same volume with fog). However, diamond dust can often reduce the visibility, in some cases to under 600 m (2,000 ft).
The depth of the diamond dust layer can vary substantially from as little as 20 to 30 m (66 to 98 ft) to 300 metres (980 ft). Because diamond dust does not always reduce visibility it is often first noticed by the brief flashes caused when the tiny crystals, tumbling through the air, reflect sunlight to your eye. This glittering effect gives the phenomenon its name since it looks like many tiny diamonds are flashing in the air.
These ice crystals usually form when a temperature inversion is present at the surface and the warmer air above the ground mixes with the colder air near the surface. Since warmer air frequently contains more water vapor than colder air, this mixing will usually also transport water vapor into the air near the surface, causing the relative humidity of the near-surface air to increase. If the relative humidity increase near the surface is large enough then ice crystals may form.
Diamond dust is often associated with halos around the sun (sun dogs) and other related optical phenomena (Greenler, 1999). These result because the diamond dust crystals form directly as simple hexagonal ice crystals — as opposed to freezing drops — and because they generally form slowly. This combination results in crystals with well defined shapes, usually either hexagonal plates or columns. These shapes, like a prism, can refract light in specific directions. Some halos can also be seen under a cirrus cloud, but diamond dust can create much more spectacular displays because the ice crystals are all around the observer.
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