Rime icing is a type of ice accretion that forms on objects exposed to certain atmospheric conditions. It appears as an opaque, granular, and milky-white deposit of ice. This form of ice develops when supercooled water droplets rapidly freeze upon contact with a surface that is at or below freezing temperatures.
Formation Conditions
Rime ice forms under conditions involving supercooled water droplets. These droplets remain in a liquid state even when their temperature is below the usual freezing point of 0°C. When these small, supercooled droplets strike a sub-zero surface, they freeze almost instantaneously upon impact. The rapidity of this freezing process is a direct result of the small size of the droplets.
This rapid solidification traps tiny air pockets within the ice structure. Rime ice is also brittle due to its crystalline structure and irregular shape. It typically forms in stratiform clouds at temperatures ranging from -10°C to -20°C, though it can occur between 0°C and -40°C.
Rime Versus Other Ice Types
Rime ice differs from other forms of atmospheric ice, such as clear ice and mixed ice. Rime ice is opaque, rough, and brittle, forming from small supercooled water droplets that freeze quickly on impact. It typically accumulates on leading edges of surfaces.
In contrast, clear ice, also known as glaze ice, is transparent, smooth, and dense. It forms when larger supercooled water droplets freeze relatively slowly upon impact. This slower freezing allows the water to spread out before solidifying, enabling air bubbles to escape and resulting in a transparent, solid sheet of ice. Clear ice often forms at warmer sub-zero temperatures, typically between 0°C and -10°C, and is generally more tenacious than rime ice. Mixed ice, as its name suggests, is a combination of both rime and clear ice characteristics. It forms when a range of droplet sizes are present or when liquid droplets are intermingled with snow or ice particles.
Consequences of Rime Icing
Rime icing presents challenges, particularly in aviation. The accumulation of rime ice on aircraft surfaces, such as wings and propellers, can disrupt airflow. This disruption leads to a reduction in lift and an increase in aerodynamic drag, which can compromise flight performance and safety. Furthermore, ice accumulation can cause propeller imbalance or block instruments, affecting their accuracy.
While rime ice is generally less dense and easier to remove compared to clear ice, its rapid formation and rough texture can still pose hazards. Even a small amount of ice can alter the aerodynamic properties of an airfoil, diminishing maximum lift. Beyond aviation, rime icing also impacts ground infrastructure, including power lines and communication towers. Ice accumulation on these structures can add weight, potentially leading to material degradation, malfunctions, or even collapse, contributing to widespread power outages.