Rime ice

Hard rime is a white ice that forms when the water droplets in fog freeze to the outer surfaces of objects.

This fog freezes to the windward (wind-facing) side of tree branches, buildings, or any other solid objects, usually with high wind velocities and air temperatures between −2 and −8 °C (28 and 18 °F).

Scientists at meteorologically extreme places, such as Mount Washington in New Hampshire, often have to break huge chunks of hard rime off weather equipment in order to keep anemometers and other measuring instruments operating.

[1] The frozen droplets on the surface of rimed crystals are hard to resolve and the topography of a graupel particle is not easy to record with a visible-wavelength microscope because of the limited resolution and depth of field in the instrument.

As the riming process continues the mass of frozen, accumulated cloud droplets obscures the identity of the original snow crystal, giving rise to a graupel particle.

Factors that favour soft rime include: small drop size, the slow accretion of liquid water, a high degree of supercooling, and fast dissipation of latent heat of fusion.