1. Ice crystals (usually cup-shaped, faceted crystals) of low strength formed by sublimation within dry snow beneath the snow surface; a type of hoarfrost. Associated with very fast crystal growth under large temperature gradients. This is one way in which firn formation may begin. Depth hoar is similar in physical origin to crevasse hoar. 2. Hoarfrost composed of crystals that have built up a three-dimensional complex of faceted, rather than rounded, crystals.