What Is Freezing Fog?
Freezing fog is fog made up of tiny supercooled water droplets that remain liquid even though the air temperature is below freezing. When these droplets come into contact with sufficiently cold surfaces, they can freeze and form a coating of ice.
This makes freezing fog different from ordinary fog because it can create two hazards at the same time: reduced visibility and ice accumulation. Roads, sidewalks, trees, power lines, vehicles and aircraft can all develop ice when freezing fog persists.
Freezing fog can occur in Canada during the colder months when moist air becomes saturated near the surface while temperatures remain below 0°C.
Quick Facts About Freezing Fog
Main Characteristic: Fog containing supercooled liquid water droplets
Temperature: Occurs below freezing
Primary Hazards: Reduced visibility and ice accumulation
Ice Formation: Droplets freeze when they contact cold surfaces
Common Season: Late fall through winter and early spring
Travel Impacts: Roads, aviation and pedestrian travel
Related Conditions: Fog, dense fog, ice fog and freezing drizzle
Related Weather: Temperature inversions and subfreezing temperatures
How Does Freezing Fog Form?
Freezing fog begins with the same fundamental process responsible for ordinary fog.
Air near the Earth’s surface becomes saturated with moisture. Water vapour then condenses into microscopic liquid droplets suspended in the atmosphere.
If the surrounding temperature is below freezing, these droplets do not necessarily turn immediately into ice.
Instead, they can remain liquid in a supercooled state.
When a supercooled droplet collides with a sufficiently cold object, it can freeze onto the surface.
Thousands or millions of these droplets can gradually produce a thin coating of ice.
What Is Supercooled Water?
Supercooled water is liquid water that exists at a temperature below its normal freezing point.
Although water normally freezes around 0°C, microscopic atmospheric droplets do not always freeze immediately when temperatures fall below that level.
For a droplet to freeze, ice formation generally needs to begin around a suitable particle or surface.
Fog droplets can therefore remain liquid while suspended in subfreezing air.
Once they strike a cold road, tree, vehicle or other object, they may freeze on contact.
This process is what makes freezing fog potentially hazardous.
Freezing Fog vs. Regular Fog
The primary difference is the temperature and behaviour of the droplets.
Regular fog consists mainly of liquid water droplets suspended near the surface and can occur at temperatures above freezing.
Freezing fog contains supercooled liquid droplets in subfreezing conditions.
Both reduce visibility, but freezing fog can additionally deposit ice onto exposed surfaces.
This means a relatively thin layer of freezing fog can still create problems even when visibility is not extremely poor.
Freezing Fog vs. Ice Fog
Although the names sound similar, freezing fog and ice fog are different phenomena.
Freezing fog consists primarily of liquid droplets that are supercooled.
Ice fog consists primarily of tiny ice crystals already frozen while suspended in the atmosphere.
Freezing fog can produce ice when droplets strike surfaces. Ice fog, by contrast, is primarily a visibility phenomenon associated with extremely cold conditions.
Ice fog is particularly characteristic of very cold northern and interior environments, while freezing fog can develop under less extreme subfreezing conditions.
Freezing Fog vs. Dense Fog
Dense fog describes how severely fog reduces visibility.
Freezing fog describes the physical state of the droplets and their ability to freeze onto surfaces.
A freezing fog event can therefore also be dense.
For example, an area could simultaneously experience very poor visibility from a high concentration of fog droplets and ice accumulation as those supercooled droplets contact surfaces.
Freezing Fog vs. Freezing Drizzle
Both conditions involve supercooled liquid water, but the size and behaviour of the droplets differ.
Freezing fog consists of extremely small droplets suspended near the surface.
Freezing drizzle consists of larger droplets that fall from clouds as precipitation and freeze when they contact cold surfaces.
Both can create slippery roads and sidewalks.
It is also possible for freezing fog and freezing drizzle to occur within the same general weather environment.
Freezing Fog vs. Frost
Frost generally forms when water vapour deposits or freezes onto sufficiently cold surfaces under favourable conditions.
Freezing fog involves droplets already suspended in the atmosphere.
Those droplets then collide with exposed surfaces and freeze.
Both can leave surfaces coated with ice crystals, but their formation mechanisms are different.
Why Doesn’t Freezing Fog Immediately Turn Into Ice Fog?
Temperatures below 0°C do not automatically cause every atmospheric water droplet to freeze.
Very small droplets can remain liquid well below the normal freezing point.
The presence of suitable ice-nucleating particles influences when ice formation begins.
As a result, an atmosphere can contain supercooled liquid fog droplets rather than ice crystals even though temperatures are below freezing.
This distinction is fundamental to understanding freezing fog.
Where Does Ice Accumulate During Freezing Fog?
Supercooled droplets can freeze onto many exposed surfaces, including:
- Roads and bridges
- Sidewalks
- Trees and vegetation
- Power lines
- Vehicles
- Road signs
- Rail infrastructure
- Aircraft
- Buildings and other structures
Objects directly exposed to moving fog droplets can collect more ice than sheltered surfaces.
The amount of accumulation depends on temperature, droplet concentration, wind, surface temperature and how long the fog persists.
Can Freezing Fog Make Roads Slippery?
Yes.
Supercooled droplets can freeze onto road surfaces and create a thin layer of ice.
This can be particularly dangerous because the ice may not be immediately obvious to drivers.
Bridges and overpasses can be especially susceptible to freezing because they lose heat from both their upper and lower surfaces.
A road that initially appears merely damp may therefore have icy patches.
Reduced visibility can make the situation more hazardous by decreasing the time available to identify changing road conditions.
Can Freezing Fog Create Black Ice?
Freezing fog can contribute to thin, relatively transparent ice deposits on road surfaces.
Such ice may be difficult to distinguish from wet pavement and can therefore resemble conditions commonly described as black ice.
However, black ice can form through several processes and is not caused exclusively by freezing fog.
Refreezing meltwater, freezing drizzle and other sources of surface moisture can also produce thin road ice.
Can Freezing Fog Accumulate on Trees?
Yes.
Supercooled fog droplets can freeze onto branches, needles, leaves and other exposed vegetation.
When deposition continues, visible ice can build up.
Wind can influence which side of an object receives the greatest accumulation because droplets are carried toward exposed surfaces by moving air.
In some situations, prolonged freezing fog can create striking ice-covered landscapes.
What Is Rime Ice?
Freezing fog can contribute to the formation of rime ice.
Rime develops when supercooled water droplets freeze onto a surface.
Because the droplets can freeze rapidly, air may become trapped within the resulting ice, giving it a white or opaque appearance.
Rime commonly develops on objects exposed to moving supercooled fog or cloud droplets.
Trees, towers and elevated terrain can all accumulate rime under suitable conditions.
Why Is Freezing Fog Common on Higher Terrain?
Higher terrain can extend into low clouds containing supercooled droplets.
From the perspective of someone on a mountain or elevated ridge, a cloud at ground level effectively becomes fog.
Supercooled droplets can then collide with exposed objects and freeze.
This can produce substantial rime accumulation on trees, towers and other structures.
Temperature and wind conditions at elevation may also remain favourable for freezing fog even when nearby lower elevations experience different weather.
When Does Freezing Fog Form?
Freezing fog requires several conditions to occur together.
Temperatures near the surface must be below freezing.
The air must contain enough moisture to become saturated.
Conditions must also allow liquid droplets to remain supercooled rather than converting primarily into ice crystals.
Stable atmospheric conditions and relatively light winds can support fog formation near the surface.
Can Freezing Fog Form Overnight?
Yes.
Nighttime is favourable for many freezing fog events because the ground loses heat after sunset.
As the surface cools, it lowers the temperature of the air directly above it.
If moist air reaches saturation while temperatures are below freezing, supercooled fog droplets can develop.
The coldest conditions often occur late at night or around sunrise, allowing freezing fog to develop or become more extensive during the early morning.
Can Freezing Fog Form After Snow?
Yes.
Snow cover can help cool the air near the surface.
Snow also provides a source of moisture through sublimation and, under some conditions, melting and evaporation.
When moist air becomes trapped over a cold snow-covered surface, conditions can become favourable for fog.
If temperatures remain below freezing and the fog contains supercooled liquid droplets, freezing fog can occur.
Can Freezing Fog Form After Rain?
Yes.
Wet ground provides a substantial moisture source.
If temperatures subsequently fall below freezing and the air near the surface becomes saturated, fog may develop.
Depending on the atmospheric conditions, droplets may remain supercooled and create freezing fog.
Surface water left behind by rain may also freeze independently, potentially creating multiple icing hazards at once.
What Role Do Temperature Inversions Play?
A temperature inversion occurs when warmer air exists above a colder layer near the surface.
This arrangement creates atmospheric stability and limits vertical mixing.
Cold, moist air can consequently remain trapped close to the ground.
If the trapped air becomes saturated below freezing, freezing fog can develop and persist.
Valleys and other low-lying areas are particularly susceptible to strong surface inversions during calm winter weather.
Why Can Freezing Fog Develop in Valleys?
Cold air is denser than warmer air and tends to settle into low-lying terrain.
During calm winter nights, cold air can drain down surrounding slopes and accumulate within valleys.
Temperatures within the valley may therefore become colder than nearby elevated locations.
If sufficient moisture is present, the trapped air can reach saturation and form fog.
Below-freezing temperatures can then allow the droplets to remain supercooled.
How Does Wind Affect Freezing Fog?
Wind affects both fog formation and ice accumulation.
Light winds can help distribute moisture through a shallow layer without completely disrupting the stable conditions needed for fog.
Stronger winds may mix the lower atmosphere enough to disperse some types of fog.
However, moving air also carries supercooled droplets toward exposed objects.
Wind can therefore increase the rate at which droplets collide with certain surfaces and contribute to rime formation.
How Long Does Freezing Fog Last?
Freezing fog may last for a relatively short period or persist for many hours.
Its duration depends on temperature, humidity, wind, sunlight and atmospheric stability.
Morning freezing fog may dissipate as sunlight warms the surface.
Persistent cloud cover, a strong inversion or continued cold and moist airflow can allow it to remain much longer.
What Causes Freezing Fog to Dissipate?
Freezing fog can dissipate when:
- Temperatures rise
- Relative humidity decreases
- Drier air arrives
- Winds increase atmospheric mixing
- Solar heating warms the surface
- The temperature inversion breaks down
As the atmosphere becomes less saturated, suspended droplets evaporate and visibility improves.
If temperatures rise above freezing, the risk of additional freezing deposition also decreases.
How Is Freezing Fog Forecast?
Meteorologists examine the combination of subfreezing temperatures and near-surface moisture.
Temperature and dew point are particularly important.
When they move close together, the air approaches saturation.
Forecasters also consider wind speed, cloud cover, snow cover, terrain and atmospheric stability.
Computer weather models can identify areas favourable for fog, while surface observations help determine whether fog and icing are actually developing.
Why Is Freezing Fog Difficult to Forecast?
Fog can develop on a very small geographic scale.
A valley may experience freezing fog while a nearby hill remains clear.
Small changes in temperature or wind can determine whether the atmosphere reaches saturation.
Forecasting whether fog droplets will remain liquid or become predominantly ice crystals adds another layer of complexity in cold environments.
As a result, the precise location and duration of freezing fog can be difficult to predict.
How Does Freezing Fog Affect Driving?
Freezing fog creates an unusual combination of hazards.
Visibility may be reduced, making it more difficult to identify vehicles and obstacles.
At the same time, supercooled droplets can freeze onto roads.
Drivers may therefore encounter slippery surfaces without necessarily experiencing snowfall or freezing rain.
Bridges, overpasses, ramps and untreated roads can become particularly hazardous.
How Does Freezing Fog Affect Aviation?
Freezing fog is particularly important in aviation because supercooled droplets can create both low visibility and icing.
Ice may accumulate on aircraft and airport surfaces under favourable conditions.
Reduced visibility can also affect takeoffs, landings and ground operations.
Aircraft icing procedures, de-icing operations and airport weather observations become especially important when freezing fog is present.
Does Freezing Fog Affect Power Lines?
It can.
Supercooled droplets can freeze onto exposed wires, towers and other infrastructure.
Small accumulations are generally less significant than major freezing-rain ice storms, but persistent icing can add weight to exposed structures.
Wind and the duration of the event influence how much ice accumulates.
Freezing Fog in Canada
Freezing fog can occur across Canada during periods when temperatures are below freezing and near-surface moisture is sufficiently high.
It can develop in valleys, around lakes and rivers, over snow-covered terrain and in areas experiencing stable winter weather.
Canada’s long cold season makes freezing fog an important winter visibility and icing condition.
Its occurrence varies considerably by region because local terrain, moisture, temperature and prevailing weather patterns all influence formation.
Freezing Fog and Weather Alerts
Freezing fog is the meteorological condition itself.
Weather alerts are separate products issued when forecast or observed conditions meet the relevant criteria.
For Snowstorms.ca, this distinction is useful: the Freezing Fog condition page can explain the science and hazards, while your alert pages can explain official Canadian alert terminology and criteria.
How to Stay Safe in Freezing Fog
Drivers should reduce speed and increase following distance when visibility deteriorates.
Treat surfaces cautiously because roads that appear merely wet may be icy.
Pay particular attention on bridges, overpasses and untreated surfaces.
Pedestrians should also watch for thin ice on sidewalks, stairs and parking lots.
Air travellers should check flight information because freezing fog can affect both visibility and airport operations.