With abundant fog around this winter, the Inland Northwest has seen its share of rime development. These photos, depicting quite large growth of the frost was taken outside our office during the third week of January.

By definition, rime is a deposit of interlocking ice crystals formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches.

The deposition of rime is different than the process in which frost is formed. In the case of rime, the object and the air are both below freezing and liquid drops (e.g. fog) must be present. For frost, the object itself must be below freezing but the air can be above freezing. Also, water vapor (not droplets) are the source of moisture, so frost forms on clear cold nights.