Diamond dust

Diamond dust is a unique meteorological phenomenon consisting of tiny ice crystals. It is a ground-level cloud that forms under clear or nearly clear skies. Because of this, scientists often call it clear-sky precipitation. In international weather reports, it is identified by the METAR code IC. This phenomenon is most frequent in the polar regions of Earth. However, it can occur anywhere the temperature is well below freezing.

This type of cloud is often confused with fog, but they are very different. Fog is usually a dense cloud made of liquid water droplets. In contrast, diamond dust forms directly as ice crystals. Diamond dust is also much thinner than fog. Because there are fewer crystals in the air than water droplets in fog, visibility is often not affected. Sometimes, diamond dust is called ice mist because it is more transparent than fog. However, it can still reduce visibility to less than 1 kilometer in some cases. The depth of the diamond dust layer can vary from 1 meter to 100 meters.

To understand how diamond dust forms, we must look at how air layers interact. It often happens when a temperature inversion exists at the surface. This is when warmer air sits on top of much colder air near the ground. As these layers mix, the warmer air moves water vapor toward the surface. This mixing increases the relative humidity of the air near the ground. If the humidity increases enough, ice crystals will begin to form. For this to work, the temperature must stay below the freezing point of water, which is 0°C. If the air is too warm, the ice will melt.

Temperature plays a vital role in whether water stays liquid or turns to ice. In some environments, very small water droplets can remain liquid even below freezing. This state is known as supercooled water. In areas with high pollution or natural dust, droplets freeze at around -10°C. This is because particles act as ice nuclei to help the freezing process. In very clean areas, droplets can stay liquid until they reach -38°C. Only when the temperature drops that low will even pure water droplets freeze into ice.

Diamond dust crystals have very specific physical properties. They form slowly as simple hexagonal ice crystals. These crystals usually take the shape of hexagonal plates or columns. Because they form so slowly, they have very well-defined shapes. These shapes act like a prism. They can reflect or refract light in specific directions. This creates a glittering effect as the crystals tumble through the air. This light reflection is often how people first notice the phenomenon. It looks as if many tiny diamonds are flashing in the sky.

These optical properties lead to beautiful displays in the sky. Diamond dust is often associated with various halos. These can include sun dogs, light pillars, and 22° halos. Researchers have documented many of these displays in the polar regions. For example, a display at the South Pole in 1980 showed a parhelion and a parhelic circle. It also featured an upper tangent arc and a Parry arc. These complex light patterns are all caused by the way the hexagonal crystals interact with sunlight.

Historical studies have helped us understand how common diamond dust is in extreme climates. In 1970, a researcher named Schwerdtfeger studied its frequency. He found that at Plateau Station in Antarctica, diamond dust was observed an average of 316 days per year. Later, in 1977, researchers Radok and Lile provided more data. They estimated that over 70% of the precipitation at Plateau Station in 1967 was diamond dust. Interestingly, even though it fell so often, the total water content was quite low. The total precipitation for that year was only 0.3 mm.

Diamond dust can also be found in human-made environments. At ski resorts, snow machines blow ice crystals into the air, creating artificial diamond dust. It can also appear downwind from manufacturing facilities or chilled water plants that produce steam. While it is a beautiful sight, it can sometimes interfere with technology. Automated airport weather stations, like ceilometers and visibility sensors, may struggle to read it. These machines might incorrectly report zero visibility or overcast skies. A human observer, however, can easily see that the sky is actually clear.