Diamond dust is a cloud composed of tiny ice crystals that forms near the ground. It is often reported under clear sky conditions and so is also known as ‘clear-sky precipitation.’
The formation of diamond dust requires very cold temperatures, typically less than minus-13. When the air is very cold the water vapor that is always present condenses to form ice crystals.
Though it forms in air near the ground, diamond dust is not an ice fog. An ice fog initially forms as tiny drops of liquid water that then freeze. Diamond dust forms directly as a crystalline piece of ice. The shape of these ice crystals is similar to tiny, thin, six-sided pencils. The ice crystals are small and few in number so diamond dust is sometimes hard to see.
During daylight, bright sparks of light can be generated as the sun hits the ice crystals. This happens as the light bends as it passes through the crystals. This can make the air sparkle, much like a diamond ring can sparkle if the light hits the ring at the right angle. Because it is shaped like crystals, diamond dust can generate some beautiful optical phenomena, such as halos and sun dogs. We can see halos and sun dogs here in Wisconsin in high altitude cirrus clouds — which are also composed of ice crystals.
Because it needs cold temperatures to form, diamond dust is frequently observed in the interior of Antarctica. At the Antarctica Plateau, diamond dust is observed 316 days a year. Diamond dust also is frequent in the Arctic during winter and can occasionally occur in Wisconsin.