Rime is the mass of ice crystals that many of us see cover the landscape in white on cold winter mornings. The ice crystals mostly grow by sublimation of supercooled water vapor directly onto cold surfaces. The rough ice crust which sometimes forms on surfaces like tree branches and grass leaves forms out of contact-freezing of supercooled fog, and not by sublimation. This is called hoar-frost, and is less common than rime.
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