Hard Rime

Hard rime is a white ice that forms when the water droplets in fog freeze to the outer surfaces of objects. It is often seen on trees atop mountains and ridges in winter, when low-hanging clouds cause freezing fog. 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 (28.4 and 17.6 °F).

Hard rime formations are difficult to shake off; they have a comb-like appearance, unlike soft rime, which looks feathery or spiky, or clear ice, which looks homogeneous and transparent.

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.

Rime ice can accumulate on the leading edges and control surfaces of aircraft operating in certain meteorological conditions.

Meteorologists distinguish between three basic types of ice forming on vertical and horizontal surfaces by deposition of supercooled water droplets. Of course there are also intermediate formations.

• Soft rime is less dense than hard rime and is milky and crystalline, like sugar. Soft rime appears similar to hoar frost.
• Hard rime is somewhat less milky, especially if it is not heavy.
• Clear ice is transparent and homogeneous and resembles ice-cube ice in appearance. Its amorphous, dense structure helps it cling tenaciously to any surface on which it forms.

Both rime types are less dense than clear ice and cling less tenaciously, therefore damage due to rime is generally minor compared to clear ice. Glaze ice is similar in appearance to clear ice but it is the result of a completely different process, occurring during freezing rain or drizzle.

These three types occur also when ice forms on the surface of an aircraft when it flies through a cloud made of supercooled water liquid droplets. Rime ice is the less dense, milky ice is intermediate and clear ice is the most dense. Dense ice can spoil lift and may have a catastrophic effect on an airborne aircraft.