Snow without Clouds

Snow crystals, called diamond dust, in the form of ice needles can fall from the sky even when clouds are not present.  At temperatures below minus 20 degrees, ice needles can form as long as six-sided deposition nuclei are present for the water vapor to freeze on.  Ice needles form when water vapor changes to ice, skipping the liquid phase.  This is called deposition.

A six-sided deposition nuclei's shape exposes it more directly to water vapor in the atmosphere allowing it to convert the water vapor to ice more easily.