Soft rime is a type of ice that forms when water droplets from fog freeze to the outer surfaces of objects. This formation usually occurs with light winds and cold conditions. The rime forms to the windward (facing the wind) side of solid objects. Soft rime is very similar in appearance to hoarfrost, but is formed differently.
Rime can produce shapes like needles or scales. It is a milky white color and can be easily shaken off branches or other objects with ease. Conditions for the formation of rime include small water droplet size, slow accumulation of water particles, high rate of super-cooling, or rapid heat loss.
Hoarfrost and rime have many of the same characteristics, but are told apart by the process in which they form. Hoarfrost happens on cold, humid nights under clear skies, and the water vapor is directly deposited onto solid ice. It is only when the cause is fog freezing over a surface that it is considered soft rime.
The formation of soft rime and hoarfrost also differ by the process the water droplets go through. For soft rime, water vapor is first condensed into a liquid (such as in fog or mist) before attaching to a surface. Hoarfrost is a result of the deposition of water vapor directly into ice, or solid form.
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