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Freezing Fog: Definition, Causes, and Dangers

Freezing Fog: Definition, Causes, and Dangers
Freezing fog refers to a weather condition that is observed during winters. Here's some more information on the same.
ScienceStruck Staff
Last Updated: Feb 28, 2018
There is also a type of fog known as ice fog that is confused with freezing fog. In case of ice fog, the droplets freeze into extremely tiny crystals of ice in midair. This phenomenon is observed only in the world's very cold areas, such as the Arctic and the Antarctic regions. It requires a temperature below -35°C (-31°F).
A fog refers to droplets of water vapor suspended in the air near the Earth's surface, as a result of which visibility is reduced. There are various types of fogs, as they are known to form in many ways, based on the manner in which the cooling that caused the condensation, has occurred. Furthermore, fogs are also identified based on freezing conditions. One such type is the freezing fog.
What is Freezing Fog?
A freezing fog can be defined as a type of fog that occurs on super-cooling of the water droplets it is composed of. Furthermore, these super-cooled water droplets remain in the liquid state till they encounter a surface on which they freeze. They freeze to form soft or hard rime, which is the accumulation of soft granular ice tufts, visible on the windward side of the surface.
While hard rime refers to white ice with a comb-like appearance, soft rime has a feathery or spiky appearance and may look clear and transparent. Furthermore, it is said that hard rime is tough to shake off.
Freezing fog is mostly observed in the winter months. Also, it can occur at any place where the temperature is below the freezing point. However, it is common on mountain tops that are exposed to low clouds.

It is considered to be similar to freezing rain (or rain that occurs at surface temperatures below freezing), however, not as dangerous.

It is also referred to as pogonip in the Western United States.
What Causes it?
Normally, the formation of fog occurs if there is a temperature difference between the ground and air. When cold air passes over a relatively warm and moist surface (like moist soil or a lake), fog is formed. When the air temperature is below the freezing point i.e. 0°C (or 32°F), it leads to the formation of freezing fog. However, the super-cooled droplets that are formed will only freeze when they come in contact with a solid surface.
For the suspended droplets to freeze, a freezing nucleus would be needed unless the air temperature is very low. Thus, the suspended water droplets tend to remain in the liquid state (between 0°C and 15°C) since there is nothing to crystallize around, until they come in contact with a solid surface.
Dangers of Freezing Fog
Due to a decrease in visibility, it is believed to cause hazardous driving conditions. It can cause vehicle collisions. It also poses a risk of accidents for fast or closely moving vehicles. That is, it can lead to multiple-vehicle-pile-up on high-speed roads like motorways.
As the super-cooled droplets tend to freeze on surfaces, this makes it hard to keep the windscreens of cars clear while driving. Ice also tends to accumulate easily on bridges as there is absence of insulation effect from the ground underneath.

Though not commonly caused by a freezing fog, a dreadful and dangerously unpredictable type of road ice also called black ice may occur due to it. It can be caused over time when a smooth layer of ice builds up on the surface of roads. This worsens the driving conditions, causing injuries and even death in case of accidents. Loss of control is possible even at less speeds and correcting a skid is almost impossible, on such ice.

It can also be dangerous for an aircraft flying relatively close to the ground, as accumulation of ice may take place on the aircraft surface.

Freezing fog is known to break down power lines. This occurs due to the weight of ice that is deposited on the lines.

Being outdoors in such a weather can be unpleasant. Moreover, remaining outdoors for a long time with inadequate clothing can be dangerous.
Precautions
Firstly, it is not advisable to make journeys under such weather conditions. However, if it is important and cannot be avoided, certain precautionary measures need to be taken. They are mainly for the road users. Here are some of them.
  • Make sure that you are not in a hurry and have enough extra time to reach your destination.
  • Check that your car is in a proper working condition (mainly the car lights and fog lights) before you start driving.
  • Be sure to drive in control, preferably at a less speed and maintain considerable distance with the vehicle ahead. The distance maintained should allow you to stop in it. Keep reducing your speed as the fog closes in.
In cases of severe weather conditions, warnings are usually issued. There are warning signs put up in some areas when such weather conditions are predicted. To be on the safer side, keep a check on weather forecasts, especially during winters.
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