Diamond dust
Encyclopedia
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 region
s diamond dust may continue for several days without interruption.
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 (1,968.5 ft).
The depth of the diamond dust layer can vary substantially from as little as 20 metre to 300 metres (984.3 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.
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 (32 °F). At temperatures between 0 °C (32 °F) and about -39 C 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, 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.
s around the sun 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.
s. 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. The METAR identifier for diamond dust within international hourly weather reports is IC.
Note that images are different from naked eye in that they capture out-of-focus crystals which are shown as large, blurred objects. By naked eye, Diamond dust looks more like the photo below:
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...
composed of tiny ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
crystals. This meteorological phenomenon is also referred to simply as ice crystals
Ice crystals
Ice crystals are a small crystalline form of ice including hexagonal columns, hexagonal plates, dendritic crystals, and diamond dust. The highly symmetric shapes are due to depositional growth, namely, direct deposition of water vapour onto the ice crystal...
and is reported in the METAR
METAR
METAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing, and by meteorologists, who use aggregated METAR information to assist in weather forecasting....
code as IC. Diamond dust generally forms under otherwise clear or nearly clear skies, so it is sometimes referred to as clear-sky precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
. It is most commonly observed in Antarctica and the Arctic
Arctic
The Arctic is a region located at the northern-most part of the Earth. The Arctic consists of the Arctic Ocean and parts of Canada, Russia, Greenland, the United States, Norway, Sweden, Finland, and Iceland. The Arctic region consists of a vast, ice-covered ocean, surrounded by treeless permafrost...
, but it can occur anywhere with a temperature well below freezing. In Polar region
Polar region
Earth's polar regions are the areas of the globe surrounding the poles also known as frigid zones. The North Pole and South Pole being the centers, these regions are dominated by the polar ice caps, resting respectively on the Arctic Ocean and the continent of Antarctica...
s diamond dust may continue for several days without interruption.
Characteristics
Diamond dust is similar to fogFog
Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of stratus cloud, the term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated...
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
Ice fog
Ice fog is a type of fog consisting of fine ice crystals suspended in the air. It can happen only in cold areas of the world since water can remain liquid down to -40 °C...
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
Fairbanks, Alaska
Fairbanks is a home rule city in and the borough seat of the Fairbanks North Star Borough in the U.S. state of Alaska.Fairbanks is the largest city in the Interior region of Alaska, and second largest in the state behind Anchorage...
, Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
, 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 (1,968.5 ft).
The depth of the diamond dust layer can vary substantially from as little as 20 metre to 300 metres (984.3 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.
Formation
These ice crystals usually form when a temperature inversionInversion (meteorology)
In meteorology, an inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to a temperature inversion, i.e...
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
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
than colder air, this mixing will usually also transport water vapor into the air near the surface, causing the relative humidity
Relative humidity
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...
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
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
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 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
Supercooling
Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid....
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, but in very clean areas, where there are no particles (ice nuclei
Ice nucleus
An ice nucleus is a particle which acts as the nucleus for the formation of an ice crystal in the atmosphere.The presence of ice nuclei increase the temperature that ice will form in the atmosphere from around −42°C to about −10°C...
) 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.
Optical properties
Diamond dust is often associated with haloHalo (optical phenomenon)
A halo from Greek ἅλως; also known as a nimbus, icebow or gloriole) is an optical phenomenon produced by ice crystals creating colored or white arcs and spots in the sky. Many are near the sun or moon but others are elsewhere and even in the opposite part of the sky...
s around the sun and other related optical phenomena
Optical phenomenon
An optical phenomenon is any observable event that results from the interaction of light and matter. See also list of optical topics and optics. A mirage is an example of an optical phenomenon....
(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
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
light in specific directions. Some halos can also be seen under a cirrus cloud
Cirrus cloud
Cirrus clouds are atmospheric clouds generally characterized by thin, wispy strands, giving them their name from the Latin word cirrus meaning a ringlet or curling lock of hair...
, but diamond dust can create much more spectacular displays because the ice crystals are all around the observer.
Climatology
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 (0.984251968503937 in)).Weather reporting and interference
Diamond dust may sometimes cause a problem for automatic weather stationAutomatic weather station
An automatic weather station is an automated version of the traditional weather station, either to save human labour or to enable measurements from remote areas...
s. The ceilometer
Ceilometer
A ceilometer is a device that uses a laser or other light source to determine the height of a cloud base. Ceilometers can also be used to measure the aerosol concentration within the atmosphere.-Optical Drum 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. The METAR identifier for diamond dust within international hourly weather reports is IC.
Books
— An excellent reference for optical phenomena including photos of displays in Antarctica caused by diamond dust.External links
- A remarkable video filmed in Hokkaido, Japan. 1min 22sec HQ
- Longer version of the above video. 5min 10sec HD
Note that images are different from naked eye in that they capture out-of-focus crystals which are shown as large, blurred objects. By naked eye, Diamond dust looks more like the photo below: