Freezing fog is fog comprised of supercooled water droplets that deposit rime ice on contact with surfaces having temperatures below 0°C (32°F).
Understanding Freezing Fog
Freezing fog occurs when liquid water droplets in the air are cooled below freezing point (0°C or 32°F) but remain in a liquid state. This is known as supercooling. When these supercooled droplets come into contact with a surface that is also at or below freezing, they immediately freeze, forming a coating of ice.
Key Characteristics
- Supercooled Water Droplets: The primary component of freezing fog is liquid water droplets that are colder than 0°C but haven't frozen.
- Rime Ice Formation: Upon contact with a sub-freezing surface, the supercooled droplets freeze, creating rime ice. Rime ice is a white or opaque coating of ice formed by the rapid freezing of supercooled water droplets.
- Visibility Reduction: Like regular fog, freezing fog significantly reduces visibility, often to less than 1000 meters (approximately 0.62 miles).
- Temperature Range: Freezing fog typically forms in temperatures ranging from 0°C (32°F) down to -20°C (-4°F), although it can occur at even lower temperatures.
Formation Process
Freezing fog forms through a combination of factors:
- High Humidity: A high concentration of water vapor in the air is necessary.
- Cooling: The air temperature must drop below freezing.
- Condensation Nuclei: Tiny particles in the air (like dust or pollution) provide surfaces for water vapor to condense upon, forming fog droplets.
- Lack of Ice Nuclei: A relative lack of ice nuclei (particles that facilitate the formation of ice crystals) prevents the water droplets from freezing in the air, resulting in the supercooled state.
Impact and Hazards
Freezing fog can create hazardous conditions:
- Icy Surfaces: Roads, sidewalks, and other surfaces become coated in ice, creating a significant slipping hazard for vehicles and pedestrians.
- Reduced Visibility: Severely limited visibility can make driving extremely dangerous.
- Aircraft Icing: Ice accumulation on aircraft can disrupt airflow and reduce lift, posing a serious threat to aviation safety.
- Damage to Infrastructure: Ice accumulation can weigh down power lines and tree branches, potentially causing damage or collapse.
Examples
Freezing fog is common in certain geographic regions and weather conditions:
- Alaska and Northern Canada: During winter, prolonged periods of cold temperatures and high humidity can lead to widespread freezing fog.
- Mountain Valleys: Cold air can become trapped in valleys, leading to temperature inversions and freezing fog formation.
- Industrial Areas: Pollution from industrial sources can provide condensation nuclei, increasing the likelihood of fog formation, which can then freeze under the right conditions.
