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.
How It Forms
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.
Hazards and Precautions
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.