Q: You sometimes hear the expression “diamond dust” in regard to weather. What is it?A: Diamond dust occurs usually when the air is very cold, says Brent McRoberts of Texas A&M University. “The dust is not dust at all, but tiny ice crystals,” he explains. “The dust is so fine that it hangs in the air, and when light is reflected off it, it produces some very strange looking beams that appear to go from the Earth straight up into the atmosphere.”Q: What are these light beams called?A: The most common name for them is light pillars, McRoberts adds. “When light is bounced off of these ice crystals — light from car headlights, streetlights, outdoor signs or other means — the light forms a pillar of light that is very unusual looking. Diamond dust can also produce a halo around the moon or sun on cold days. This dust often occurs in very cold regions and can frequently be observed in Alaska, Canada and Siberia and is most often seen at night.”