Precipitation in the form of slowly falling, singular or unbranched ice needles, columns, or plates. They make up cirriform clouds, frost, and ice fog. Also, they produce optical phenomena such as halos, coronas, and sun pillars. May be called 'diamond dust.' Precipitation of ice crystals in the form of needles, columns or plates sometimes so tiny, they seem suspended in air. They are mainly visible when they glitter in sunshine and occur only at very low temperatures and stable air masses.