Diamond Dust

The air is bitterly cold, and as you step outside, you notice a fine twinkling and sparkling in the air, beautiful and ephemeral. A faint, cold, wet prickling on bare skin may also accompany this light show, as tiny crystals of ice sift through the air to melt on the warm surface of your body. This fine, shimmering mist of ice crystals is called diamond dust, and it usually only occurs in very cold regions during the winter.

Diamond dust occurs when there is a winter temperature inversion -- with the air coldest near the ground and a warmer, moister layer of air lying over it. The diamond dust forms when the warmer air blends with the colder, and the moisture it contains condenses out in the colder air, freezing into tiny motes of ice. These fine motes drift and sparkle, and eventually settle out, making them a type of precipitation as well as mist.

Although water freezes at 32 degrees Fahrenheit, this is much too warm for diamond dust to form. Water droplets can remain liquid, though supercooled, far below the freezing point of water. If there is a lot of dust in the air to form nuclei for the freezing of this water, diamond dust may occur at 14 degrees Fahrenheit, but clean, unpolluted air can need to grow as cold as -38 F before diamond dust appears.

The significance of diamond dust

Diamond dust is usually too thin in the temperate latitudes, even in a cold region like the Upper Midwest or Siberia, to obscure vision or to accumulate more than a fraction of an inch on the ground. Diamond dust is mostly significant at these latitudes because it makes haloes around the sun, moon, or outdoor lamps. By contrast, in the eternally-frigid interior of Antarctica, diamond dust is constant and sometimes intense, and makes up the majority of the precipitation that falls there. The Antarctic ice cap is maintained by diamond dust, and without it, the south polar region would be even more parched than it already is.

For most people, however, diamond dust is only an intriguing curiosity -- a tiny twinkling of light and motion in the deep winter air, caused by flecks of drifting ice too small to be seen except when they reflect the sun.