During winter, it is common for ice crystals to cover the ground and, occasionally, trees and just about everything else. This is not precipitation, which is water or ice that falls from clouds; this is deposition, which is water or ice that is deposited onto surfaces directly from the air. There are two basic methods of this deposition. Essentially, frost comes from water vapor whereas rime comes from water droplets.
Frost, also called hoarfrost, happens when ice crystals grow on surfaces directly from water vapor in the air. A combination of high humidity and temperatures below freezing make this happen. Rime happens when tiny suspended water droplets in a liquid fog are cooled to below the freezing point and then form ice crystals on exposed surfaces. Both frost and rime grow more prolifically on rough surfaces, such as those found on grass and trees, because these sharp edges actually attract and encourage the crystalline formations to start and grow.