Temperature and water vapor humidity determine crystalline forms. Ice crystals are responsible for various atmospheric optics displays in the atmosphere. The principal axis (c axis) of a single crystal of ice is perpendicular to the axis of hexagonal symmetry. Planes perpendicular to this axis are called basal planes. Ice clouds are composed of ice crystals. The most notable one is cirrus and ice fog.

See also

• snow

External links

• Caltech homepage
• American Meteorological Society Glossary

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 polluted valleys 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 a mile (1600 m).

The depth of the diamond dust layer can vary substantially from as little as 20 to 30 m (60 to 100 feet) to a few hundred metres (1000 feet). 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.

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. At temperatures between 0 °C and about -39 °C (+32 °F and -38 °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 °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, 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 °F).

Diamond dust is often associated with halos around the sun and other related optical phenomena (Greenler, 1999). These result because the diamond dust crystals form directly as ice (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.

While diamond dust can be seen in any area of the world that has cold winters, it is most frequent in the interior of Antarctica, where it is common year-round. Schwerdtfeger (1970) shows that diamond dust was observed on average 316 days a year at Plateau Station in Antarctica, and Radok and Lile (1977) estimate that over 70% of the precipitation that fell at Plateau Station in 1967 fell in the form of diamond dust (once melted, the total precipitation for the year was only 25 mm [1 inch]).

Diamond dust may sometimes cause a problem for automatic weather stations. The ceilometer and visibility sensor do not always correctly interpret the falling diamond dust and report the visibility and ceiling as zero (overcast skies). However, a human observer would correctly notice clear skies and unrestricted visibility.

References and additional material

• Greenler, R. (1999). Rainbows, Halos, and Glories. Milwaukee: Peanut Butter Publishing. — An excellent reference for optical phenomena including photos of displays in Antarctica caused by diamond dust.
• Schwerdtfeger, W. (1970). Climates of the Polar Regions. Elsevier.
• Radok, U. and R.C. Lile (1977). Meteorological Studies at Plateau Station, Antarctica. American Geophysical Union.
• Manual of Surface Weather Observations (MANOBS). 7th Edition, Atmospheric Environment Service of Canada.