What is riming?

During the wintertime, bodies of water that are warmer than the air temperature allow water to escape into the air. As that humid air moves around and is below 0°C [32°F] the cold water will attach to solid objects that are in its path. The process continues as more water molecules are blown against the surface. The buildup of this frost, sometimes called rime ice, is often described as looking like ice feathers.

Though the two are often confused, riming differs from the process of forming hoar frost. As for the difference between rime and hoar, hoar develops when water vapor freezes. In other words, going directly from the gaseous to the solid state. By contrast, rime forms when supercooled liquid water droplets freeze on contact with cold surfaces.