The suspended particles may be partly ice crystals 20 to 100 micron in diameter, but chiefly, especially when dense, droxtals 12-20 micron in diameter. Ice fog is formed by direct freezing of supercooled water droplets with little growth directly from the vapor. It occurs at very low temperatures, and usually in clear, calm weather in high latitudes. The sun is usually visible and may cause halo phenomena or luminous pillars. Ice fog is rare at temperatures warmer than -30 °C, and increases in frequency with decreasing temperature until it is almost always present at air temperatures of -45 °C in the vicinity of a source of water vapor. Such sources are the open water of fast-flowing streams or of the sea, herds of animals, volcanoes, and especially products of combustion for heating or propulsion. At temperatures warmer than -30 °C, these sources can cause steam fog of liquid water droplets, which may turn into ice fog when cooled.