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.