Winter weather
What Is Freezing Fog? How It Forms and Why It Is Dangerous
Freezing fog is ordinary-looking fog made hazardous by supercooled droplets that freeze on contact. Here is how it differs from ice fog, frost, and frozen fog.
Quick answer
Freezing fog is fog made of liquid water droplets that remain unfrozen even though the air temperature is at or below 32°F (0°C). These supercooled droplets freeze when they touch roads, trees, power lines, aircraft, and other cold surfaces. The resulting glaze or rime can create dangerous travel with very little warning.
Fog already reduces how far you can see. Add ice that may be difficult to spot, and a quiet-looking morning can become one of winter's trickier weather situations. That combination is why it helps to understand the phrase freezing fog before it appears in a forecast or advisory.
What does freezing fog mean?
Fog is a cloud at ground level. Its tiny suspended droplets scatter light, reducing visibility. In freezing fog, the droplets are supercooled: they are still liquid below the normal freezing point. Small, clean droplets can remain liquid because freezing usually needs a suitable particle or surface on which an ice structure can begin.
Once a supercooled droplet strikes a cold windshield, bridge rail, tree branch, or runway, contact provides that opportunity. The droplet freezes and contributes to an ice deposit. Many droplets arriving over time may make a rough white coating called rime, a clear glaze, or a mixture depending on droplet size, temperature, wind, and the surface.
The WMO International Cloud Atlas defines freezing fog by its supercooled droplets and their deposits. The important point is that the word freezing describes what the droplets do on contact. It does not mean the fog is already composed entirely of frozen particles.
How does freezing fog form?
Freezing fog can develop through the same cooling and moisture processes that form other fog, except the temperature falls below freezing while enough droplets remain liquid. Clear, calm nights are one common setup. The ground radiates heat, the shallow air next to it cools to saturation, and radiation fog forms. In valleys and basins, cold dense air may settle and trap moisture near the surface.
It can also form when moist air moves over a colder surface, when wind pushes low cloud against higher terrain, or when evaporation adds moisture to very cold air. Light wind helps keep a shallow fog layer in place, but some mixing is useful because it supplies moisture. If the air becomes cold enough or suitable ice nuclei are abundant, more droplets freeze in the air and the fog may transition toward ice fog.
Freezing fog vs. frozen fog, ice fog, frost, and regular fog
| Term | What is in the air | What happens at surfaces |
|---|---|---|
| Regular fog | Mostly tiny liquid droplets at temperatures generally above freezing | Surfaces become damp. Visibility is reduced |
| Freezing fog | Supercooled liquid droplets at or below freezing | Droplets freeze on contact and may create glaze or rime |
| Ice fog | Tiny ice crystals suspended in very cold air | Crystals can settle, but the cloud is already frozen |
| Frost | Water vapor deposits or freezes on a surface | Ice grows on the surface without fog droplets striking it |
Frozen fog is used informally and is not as precise as freezing fog or ice fog. Some people use it for freezing fog because the landscape becomes frozen. Others use it for ice fog because the particles in the air are frozen. When conditions matter, use the official term in the forecast and ask whether the cloud contains liquid droplets or ice crystals.
Ice fog is most characteristic of extremely cold environments. It may sparkle in lights and can be influenced by moisture from combustion or open water. Freezing fog, by contrast, can occur at temperatures just below 32°F and often looks no different from liquid fog until ice begins coating objects.
Why is freezing fog dangerous?
The hazard is a two-part problem: reduced visibility and ice accretion. Drivers may enter dense fog, slow down, and then discover that a bridge deck or untreated road is slick. A thin transparent layer can look merely wet. Bridges, overpasses, ramps, sidewalks, and elevated objects usually cool faster than insulated ground and may ice first.
Aircraft are also vulnerable. Supercooled droplets can freeze onto wings and other surfaces, changing aerodynamics, adding weight, and affecting instruments. Aviation decisions belong to trained pilots and official weather services, ground observers should never treat a look outside as a substitute for an aviation forecast.
Rime can weigh down trees and lines if freezing fog persists. It may also coat an outdoor weather station. A blocked wind sensor, ice-covered rain gauge, or shielded temperature sensor can produce misleading readings. If you inspect equipment, wait until access is safe and follow the manufacturer's cleaning guidance rather than striking or heating delicate parts.
What is a Freezing Fog Advisory?
The National Weather Service may issue a Freezing Fog Advisory when freezing fog is expected to cause travel difficulties. Exact headline practices can vary by office and situation, so read the message rather than relying only on its name. Check the affected area, timing, expected visibility, and any mention of ice on roads and bridges.
A Dense Fog Advisory focuses on widespread poor visibility, typically when visibility is expected to be very low. Freezing fog may occur with or without that separate headline. An advisory is not a promise that every road will ice, and the absence of an advisory does not prove every local surface is safe.
Clues a weather observer may notice
- Air temperature at or below freezing while visibility is reduced by fog.
- A white coating building mainly on the windward side of twigs, fences, and signs.
- Ice on elevated objects even when sheltered ground remains only damp.
- A thermometer or station housing that gradually becomes coated.
- Unexpectedly slick steps, decks, or untreated pavement.
Do not infer pavement temperature from air temperature alone. Road material, recent sunshine, traffic, treatment, elevation, and exposure all matter. Record visibility only if you have known landmarks at measured distances. Otherwise use a plain description such as “nearby ridge obscured.” Accurate, modest observations are more useful than false precision.
How to stay safer in freezing fog
If travel can wait, allow road crews and daylight time to improve conditions. When you must drive, consult the latest official forecast and road agency reports. Use low-beam headlights, reduce speed gradually, leave much more following distance, and avoid cruise control. High beams reflect from fog droplets and can make it harder to see. The National Weather Service's fog driving guidance also advises using pavement markings to guide you and minimizing distractions.
Assume bridges and untreated walks may be icy. Wear footwear with traction, use railings, and give yourself time. Keep a reliable warning source such as NOAA Weather Radio available overnight, especially where fog and subfreezing temperatures are forecast together. A home sensor can tell you what is happening at one point, but official warnings and direct surface conditions should guide safety decisions.
Frequently asked questions
Is freezing fog the same as black ice?
The answer is no. Freezing fog is an airborne cloud of supercooled droplets. Black ice is a thin, transparent ice layer on a surface. Freezing fog is one process that can create it.
At what temperature does freezing fog occur?
It occurs when fog droplets remain liquid while the air is at or below 32°F (0°C). It is especially concerning near the freezing point because a wet-looking surface may be icy.
Can freezing fog happen without snow?
The answer is yes. Snowfall is not required. A shallow moist layer can cool below freezing and form fog under otherwise quiet conditions.
Does freezing fog always make roads icy?
The answer is no. Surface temperature and treatment determine whether ice accumulates, but drivers should treat the possibility seriously because elevated and untreated surfaces can freeze first.