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 side of tree branches, buildings, or any other solid objects,
usually with high wind velocities and air temperatures between −2 and −8 °C.
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. Rime is also a weather form.
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 least 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. Formation on snow crystals
Under some atmospheric conditions, forming and descending snow crystals may
encounter and pass through atmospheric supercooled cloud droplets. These
droplets, which have a diameter of about 10 µm, can exist in the unfrozen state
down to temperatures near −40 °C. Contact between the snow crystal and the
supercooled droplets results in freezing of the liquid droplets onto the surface
of the crystals. This process of crystal growth is known as accretion. Crystals
that exhibit frozen droplets on their surfaces are referred to as rimed. When
this process continues so that the shape of the original snow crystal is no
longer identifiable, the resulting crystal is referred to as graupel.
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. However, observations of snow crystals with a
low-temperature scanning electron microscope clearly show cloud droplets
measuring up to 50 µm on the surface of the crystals. The rime has been observed
on all four basic forms of snow crystals, including plates, dendrites,
columns and needles. As the riming process continues, the mass of frozen,
accumulated cloud droplets obscures the identity of the original snow crystal,
thereby giving rise to a graupel particle.
See also Atmospheric icing
Soft rime References
External links AMS Glossary, American Meteorological
Society Weather Facts, WeatherOnline
This article incorporates public domain material from websites or documents of
the United States Government.