http://nsidc.org/snow/glossary.html

Yet another definition of ablation:
the process of being removed. Snow ablation usually refers to removal by melting

depth hoar:
large (one to several millimeters in diameter), cohesionless, coarse, faceted snow crystals which result from the presence of strong temperature gradients within the snowpack

surface hoar:
the deposition (sublimation) of ice crystals on a surface which occurs when the temperature of the surface is colder than the air above and colder than the frost point of that air

graupel:
snowflakes that become rounded pellets due to riming. Typical sizes are two to five millimeters in diameter (0.1 to 0.2 inches). Graupel is sometimes mistaken for hail.
This stuff is hell to drive on. It feels like you're skating on marbles.