The natural phenomenon commonly called “frozen fog” involves water droplets suspended in the air at below-freezing temperatures, which is the definition of freezing fog. Fog is simply a cloud resting on the ground, and when the air temperature drops below 0°C (32°F), the tiny liquid droplets within it become supercooled. This supercooled liquid state is possible because the droplets lack an ice-nucleating particle on which to freeze immediately. The visible, frosty accumulation that results when this fog contacts a surface is known as rime ice.
Nomenclature of Frozen Fog
The scientific terminology for frozen fog is divided into three distinct categories based on the state of the water. Freezing fog is the most common term, referring to a cloud of supercooled liquid water droplets suspended in the air at temperatures at or below freezing. The droplets remain liquid until they strike an object, such as a tree branch or a power line, at which point they freeze instantly.
Rime ice is the solid deposit that forms on a surface exposed to freezing fog. It is the physical consequence of the supercooled droplets colliding with and freezing onto a cold object.
The third term, ice fog, describes a fog that is already solid, composed entirely of tiny, airborne ice crystals. This typically occurs only in extremely cold, Arctic or Polar air masses, usually when temperatures fall to around -10°C (14°F) or colder. Unlike freezing fog, ice fog does not deposit rime ice on contact because the water is already in its solid form.
Meteorological Conditions Required for Formation
The formation of freezing fog begins with the presence of supercooled water droplets, which are pure water remaining in a liquid state despite having a temperature below the freezing point. This supercooling is maintained in the atmosphere because the droplets lack an impurity to act as a nucleus for ice crystal growth. The conditions necessary for freezing fog are often found in a temperature range between 0°C and approximately -10°C.
A high level of atmospheric moisture is necessary, which is then cooled to its dew point, typically resulting in a very narrow temperature-dew point spread. Freezing fog often forms under conditions of light or calm surface winds, stable air, and a temperature inversion, where cold air is trapped near the ground beneath a layer of warmer air. This type of fog is frequently a radiation fog, forming overnight as the ground rapidly cools the air directly above it, or an advection fog, where moist air moves across a sub-freezing surface. The combination of a strong moisture source and sustained cold temperatures allows the phenomenon to persist for long periods.
Characteristics of Rime Ice Deposits
Rime ice is the opaque, crystalline deposit that results from freezing fog, building up on surfaces facing the wind. The physical appearance of the ice is directly related to the speed of accretion and the amount of liquid water present. Soft rime forms under light wind conditions and lower liquid water content, resulting in a milky, feathery, or spiky texture.
Soft rime is relatively fragile and less dense, often accumulating as white, needle-like formations that can be easily dislodged. Hard rime, by contrast, forms when wind speeds are moderate to high and the concentration of supercooled water droplets is greater. The rapid freezing of the droplets leaves small air gaps, creating a dense, opaque, and comb-like structure that clings tenaciously to the windward side of objects. Rime ice accumulations can transform landscapes, coating tree branches, wires, and vertical structures with a white, crystalline layer.
Practical Hazards Associated with Frozen Fog
The combination of reduced visibility and ice formation makes freezing fog a safety concern for both ground and air travel. Like regular fog, it severely limits visibility, but the presence of supercooled droplets adds the danger of rapid ice buildup on surfaces. On roadways, the freezing fog droplets instantly freeze upon contact with pavement, creating a thin, often invisible layer of slick ice known as black ice.
This hazard is particularly pronounced on elevated structures like bridges and overpasses, which cool more quickly than the ground. For aviation, freezing fog poses a threat because rime ice rapidly accumulates on the leading edges of wings and control surfaces. The rough, granular texture of rime ice disrupts the smooth airflow over the wing, which reduces lift and increases aerodynamic drag, potentially leading to performance issues. Additionally, the weight of rime ice buildup can overload and cause failure in infrastructure, such as power lines, transmission towers, and communication equipment.