What is Freezing Fog?

Freezing fog consists of water droplets that freeze upon contact with objects they encounter, forming a layer of thin ice or rime. This happens because the water is supercooled: its temperature is below normal freezing point, but it has remained liquid. The phenomenon differs from ice fog, which is made up of tiny particles of ice. Many regions of the world can experience freezing fog, especially during the winter months, when the air temperature can drop below freezing, creating the conditions necessary to create supercooled water droplets.

Fog normally forms where there is cold air over a relatively warm, moist surface, such as a lake, wet ground or moist soil. Freezing fog forms when the air temperature is below freezing point — 32°F (0°C). Strange as it may seem, water does not always freeze at the same temperature at which ice melts. It will do so if it is in contact with something solid: this could be a surface or container. In the case of a suspended droplet, a “freezing nucleus” — usually a tiny ice crystal — is required, unless the air temperature is very low.

Between 32°F (0°C) and 5°F (-15°C) the suspended water droplets that make up fog, or a cloud, are normally in a supercooled state. They remain liquid because there is nothing for ice to crystallize around, but on touching anything solid, they will immediately freeze. Surfaces exposed to freezing fog therefore quickly become coated with ice.

A fog consisting of ice particles — “ice fog” or “frozen fog” — can form in temperatures below -30°F (-35°C). This is seldom seen outside the arctic and antarctic regions. Forecasts for ice fog outside these regions do happen, but they are quite rare, as the conditions needed for it to occur are not very common.

Freezing fog can occur in any part of the world where air temperatures can drop below freezing point. It is common on mountaintops, but can form at sea level if conditions are right. It is most likely to form at night, when heat is escaping from the atmosphere into space. This happens more easily when there is no cloud cover — clouds provide an insulating layer that tends to keep the heat in.

Water, soil and solid surfaces tend to retain heat longer, and in calm conditions, a layer of freezing air can build up over a warmer surface. If there is moisture present, it can evaporate, then condense into water droplets in the cold air above. If the temperature is low enough, the droplets will be cooled below freezing point.

Sometimes, freezing drizzle follows freezing fog. This has larger droplets of supercooled water which freeze on contact. Since the air temperature is usually below freezing, the ice deposits which form can remain, and may build up rapidly.

Freezing fog creates very hazardous driving conditions. In addition to the poor visibility associated with normal fog, a build-up of ice on vehicles may cause dangerous malfunctions. It is also likely that ice will be deposited on road surfaces, adding to the danger. Ice will tend to form on bridges first, as they are exposed to cold air from beneath as well as from above, and do not have the insulating effect of the ground below. In some areas, warning signs to this effect may be put up when these conditions are forecast.

The use of grit and salt may help prevent icing of roads, but accidents often occur through a combination of freezing fog and inappropriate driving. People are advised to avoid making automobile journeys in these conditions if possible, but if a journey must be made, it makes sense to curtail speed and maintain an adequate distance from the vehicle in front. Severe weather warnings may be issued if such conditions are expected, so it is advisable during the winter to check forecasts before setting out.

Freezing fog and drizzle can create problems with objects that freeze over and become nonfunctional because of the cold and ice. It has been known for power lines to collapse under the weight of ice deposited on them. It is also usually unpleasant to be outdoors in these weather conditions because of the cold and the formation of ice crusts on garments. It may be dangerous to remain outdoors for extended periods, especially with inadequate clothing. Other precautions include wrapping exposed pipes, if they are not already insulated, and covering vulnerable garden plants.

Freezing fog is a weather phenomenon that occurs when the water droplets in fog become supercooled, remaining liquid even though temperatures are below the freezing point. When these droplets come into contact with surfaces like roads, trees, or bridges, they instantly freeze, forming a layer of ice known as rime. This can create hazardous conditions, especially on roadways.

Regular fog consists of tiny water droplets suspended in the air at or near the Earth's surface, reducing visibility. Freezing fog contains these water droplets in a supercooled state, which means they are below freezing temperature but still liquid. Upon contact with objects, these droplets immediately freeze, unlike in regular fog where no icing occurs.

Freezing fog typically forms under calm and cold conditions with high relative humidity. The temperature must be at or below the freezing point of water (0°C or 32°F). Clear skies at night can lead to rapid cooling of the surface, which, combined with moist air, creates the perfect environment for freezing fog to develop.

Freezing fog is less common than regular fog because it requires specific conditions to form—temperatures below freezing and high humidity. It is more likely to occur in regions with cold climates during the winter months. However, it can happen in various parts of the world, depending on local weather patterns and geographic features.

The primary danger of freezing fog is the formation of 'black ice' on roads, which is a thin, transparent layer of ice that is nearly invisible to drivers and can cause vehicles to lose traction. It also poses risks for aviation and can lead to the closure of airports due to icy runways and poor visibility.

Freezing fog can have an impact on wildlife and vegetation by coating plants, trees, and grasses with ice, which can lead to damage from the weight of the ice. For wildlife, it can reduce visibility and make foraging for food more difficult. However, many species are adapted to cope with such conditions in their native habitats.