Supercooled water droplets that freeze to objects in exposed terrain, forming icy deposits on the windward side. Rime can also form on snowflakes as they fall through the sky, giving them a fuzzy appearence.
Rime is that crunchy, rough snow that looks like popcorn or styrofoam that you notice plastered onto trees on windy mountaintops (making “snow ghosts”). Rime forms on the surface of the snow when super-cooled water in clouds freezes onto the snow surface, trees, chairlift towers or any solid surface. When the super-cooled droplets touch something solid, they instantly freeze; thus the spikes grow INTO the wind (as opposed to wind loading in which drifts form on the downwind side).