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. It usually forms when clouds rapidly rise over a mountain range. The air rises so fast that tiny water droplets can't form snowflakes (or graupel in this case) fast enough and the water droplets actually cool well below the freezing level. When they 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).
Rime usually forms a strong layer on the surface of the snowpack. At least within the new snow, It usually is a sign of stability because, as Jill Fredston likes to say, it's like throwing a cargo net over the snow. But once again, is it a sign of stability within the old snow? Not necessarily. The weight of the new snow accompanying the rime, plus the weight of the rime itself, may overload buried weak layers.