Freezing fog can cause ice caps to form on trees.
Freezing fog consists of water droplets that freeze on contact with objects they encounter, forming a thin layer of ice or frost. This is because the water is supercooled: its temperature is below the normal freezing point, but it has remained liquid. The phenomenon differs from ice fog, which is made up of small ice particles. Many regions of the world can experience freezing fog, especially during the winter months when air temperatures can drop below freezing, creating the conditions necessary to create supercooled water droplets.
how to form
Icy fog creates very dangerous driving conditions.
Fog typically forms where there is cool air over a moist, relatively warm surface, such as a lake, moist soil, or moist land. Frost fog forms when the air temperature is below freezing: 32°F (0°C). Oddly enough, water doesn’t always freeze at the same temperature that ice melts. This will happen if it is in contact with something solid: it can be a surface or a container. In the case of a suspended droplet, a “freezing nucleus” is required, usually a small ice crystal, unless the air temperature is very low.
Freezing fog is more common during the winter months.
Between 32°F (0°C) and 5°F (-15°C), the suspended water droplets that form a mist, or cloud, are generally in a supercooled state. They remain liquid because there is nothing for the ice to crystallize from, but when they touch something solid, they immediately freeze. Surfaces exposed to freezing fog quickly become covered with ice.
A fog consisting of ice particles, “ice fog” or “frozen fog,” can form at temperatures below -30°F (-35°C). This is rarely seen outside of the Arctic and Antarctic regions. Ice fog forecasts outside of these regions do occur, but are quite rare as the conditions necessary for them to occur are not very common.
Frost fog can occur anywhere in the world where the air temperature can drop below freezing. It is common on top of mountains, but can form at sea level if conditions are right. It is more likely to form at night, when heat escapes from the atmosphere into space. This happens more easily when there are no clouds: clouds provide an insulating layer that tends to keep heat inside.
Water, soil, and solid surfaces tend to retain heat longer, and in calm conditions, a layer of icy air can build up on top of a warmer surface. If moisture is present, it can evaporate and condense into water droplets in the cold air above. If the temperature is low enough, the droplets will cool below the freezing point.
Sometimes freezing drizzle follows freezing fog. This has larger droplets of super cooled water that freeze on contact. Since the air temperature is often below freezing, the ice deposits that form can remain and accumulate quickly.
dangers and precautions
Icy fog creates very dangerous driving conditions. In addition to the poor visibility associated with normal fog, ice buildup on vehicles can cause dangerous breakdowns. Ice is also likely to deposit on road surfaces, adding to the danger. Ice tends to form on bridges first, as they are exposed to cold air from both below and above, and do not have the insulating effect of the ground below. In some areas, warning signs to this effect may be posted when these conditions are anticipated.
The use of sand and salt can help keep roads from freezing, but accidents often occur due to a combination of freezing fog and poor driving. People are advised to avoid traveling by car in these conditions if possible, but if a trip is necessary, it makes sense to slow down and keep an adequate distance from the vehicle in front. Severe weather warnings may be issued if such conditions are expected, so check the forecast before heading out in winter.
Freezing fog and drizzle can create problems with objects freezing and not working due to cold and ice. Power lines have been known to collapse under the weight of ice deposited on them. It is also often unpleasant to be outdoors in these weather conditions due to the cold and the formation of ice crusts on clothing. It can be dangerous to be outside for long periods of time, especially when wearing inappropriate clothing. Other precautions include wrapping exposed pipes, if not already insulated, and covering vulnerable plants in the garden.