What is the difference between freezing fog and ice fog?

Freezing fog is composed of supercooled liquid water droplets — water that remains liquid even though the temperature is at or below 32 °F (0 °C). Ice fog, by contrast, is composed of tiny ice crystals and typically requires much colder temperatures, often well below 0 °F (approximately −20 °F / −29 °C or colder). Both reduce visibility, but freezing fog poses a greater structural icing risk because its liquid droplets freeze on contact with cold aircraft surfaces.

How dangerous is freezing fog for aircraft on the ground?

Freezing fog can deposit a nearly invisible layer of glaze ice on parked aircraft surfaces — wings, tail surfaces, control surfaces, and pitot-static probes — even with no precipitation occurring. The clean aircraft concept, codified at 14 CFR 91.527 for large and turbine-powered multiengine airplanes and 14 CFR 135.227 for commuter and on-demand operations, prohibits takeoff with frost, ice, or snow adhering to critical surfaces, and this same principle is treated as essential safety practice across all operations. A thorough preflight inspection and proper de-icing are required before flight after exposure to freezing fog conditions.

At what temperature does freezing fog form, and what causes it?

Freezing fog forms whenever any standard fog type — radiation, advection, upslope, frontal, or steam — occurs at a surface temperature at or below 32 °F (0 °C). The fog mechanism itself is the same as for ordinary fog: air cools to its dewpoint or moisture is added until saturation is reached (fog seldom forms when the temperature-dewpoint spread exceeds 2 °C / 4 °F). The freezing qualifier simply means that the water droplets making up the fog are supercooled and can freeze instantly on contact with surfaces.

Freezing Fog and Ice Fog Hazards in Cold Weather

Fog is one of the most operationally disruptive weather phenomena pilots encounter, and its cold-weather variants — freezing fog and ice fog — carry hazards far beyond the simple visibility reduction that ordinary fog produces. When the thermometer dips to or below 32 °F (0 °C), fog transforms from an inconvenient nuisance into an active icing threat capable of coating an aircraft with a glaze of ice before the first power lever is ever advanced. Every instrument-rated pilot, and frankly every student preparing for any FAA knowledge test, needs a firm understanding of how these phenomena form, how they differ from each other, and what operational decisions they demand.

The FAA Aviation Weather Handbook (FAA-H-8083-28B), Chapter 18, defines fog as a visible aggregate of minute water droplets based at Earth's surface that reduces horizontal visibility to less than 5/8 statute mile (1 km). Critically, fog does not fall to the ground the way drizzle does — the droplets remain suspended. When those droplets exist at temperatures at or below freezing, the fog is called freezing fog. When the fog is instead composed entirely of ice crystals, it is called ice fog. Both terms describe specific physical states, not just cold fog, and the distinction matters for understanding the hazard each presents.

The Physics Behind Freezing Fog

Water does not always freeze the instant it reaches 32 °F. In the atmosphere, cloud and fog droplets can remain in a liquid state at temperatures well below freezing — a condition called supercooling. Supercooled liquid water droplets (SLDs) are metastable: they are liquid only because they lack the ice nucleus needed to trigger crystallization. The moment a supercooled droplet contacts a solid surface — an aircraft wing, a fuselage antenna, a taxiway light — it loses that metastability and freezes almost instantaneously. This is exactly how freezing fog creates its hazard. According to Chapter 18, tiny supercooled liquid water droplets in freezing fog can freeze instantly on exposed surfaces when surface temperatures are at or below 32 °F (0 °C).

The ice deposit produced is typically clear glaze ice — dense, hard, and transparent — which conforms tightly to airfoil contours and is notoriously difficult to detect visually. Unlike rime ice, which builds a rough, opaque layer that is at least somewhat obvious, glaze from freezing fog can be nearly invisible until it has already disrupted the boundary layer airflow over the wing. The weight penalty and aerodynamic degradation can be severe even from a thin, uniform coating.

Ice Fog: A Different Physical State

Ice fog is composed of tiny ice crystals rather than supercooled liquid droplets. It typically forms at temperatures significantly colder than freezing — generally at temperatures well below 0 °F (approximately −20 °F / −29 °C or colder), though the exact threshold depends on the availability of ice nuclei and moisture. At those extreme temperatures, water vapor deposits directly onto existing particles as ice crystals, bypassing the liquid phase entirely in a process called deposition.

Ice fog is most common in the Arctic and subarctic regions, over snow- and ice-covered surfaces, and near populated areas in extreme cold where combustion and industrial processes add moisture to frigid air. Because ice crystals scatter light differently than liquid droplets, ice fog can produce brilliantly colored optical phenomena such as halos and light pillars — beautiful from the ground but indicative of extremely hazardous conditions for aviation.

Unlike freezing fog, ice fog crystals do not typically adhere as aggressively to aircraft surfaces upon contact because solid ice crystals do not spread and bond the way a supercooled liquid does. However, the visibility reduction can be just as profound, and any airframe that has been sitting in ice fog for any period of time must be carefully inspected for accumulated frost or surface contamination before flight.

How Freezing Fog Forms — The Fog Formation Context

Freezing fog is not a separate fog type in terms of formation mechanism — it is any of the standard fog types (radiation, advection, upslope, frontal, or steam fog) that simply occurs at sub-freezing temperatures. Understanding the parent formation mechanism therefore matters. Fog forms whenever the temperature and dewpoint of the air become equal or nearly equal — the FAA handbook notes that fog seldom forms when the temperature-dewpoint spread exceeds 2 °C (4 °F).

Radiation fog forms over land at night through radiational cooling. In cold climates — especially after a rain followed by clearing skies — the ground cools rapidly, and if temperatures are already near or below freezing, the resulting fog is freezing fog from its onset.
Advection fog forms when moist air moves over a colder surface. Along coastlines in winter, relatively moist marine air advecting over frozen inland terrain can produce widespread freezing fog that is more persistent than radiation fog.
Upslope fog forms as stable moist air rises and cools adiabatically. In the high plains east of the Rockies, upslope flow in winter can produce dense, extensive freezing fog reaching high altitudes.
Frontal (precipitation-induced) fog forms when precipitation from warm air aloft evaporates into cold, near-saturated air near the surface. In winter, this process can rapidly produce freezing fog over vast areas associated with warm fronts.
Steam fog (Arctic sea smoke) forms when very cold air moves over relatively warmer open water. While the fog itself involves rising vapor that quickly recondenses, in extreme cold the resulting droplets quickly enter the supercooled or ice-crystal regime.

Why These Hazards Matter Operationally

The practical danger of freezing fog begins before the aircraft ever leaves the ground. An aircraft parked on the ramp overnight in freezing fog conditions can accumulate a transparent layer of glaze ice on all exposed surfaces — wings, tail, control surfaces, pitot tubes, static ports, and antennas. This is a critical preflight concern: the "clean aircraft concept" prohibits takeoff with frost, snow, or ice adhering to critical surfaces, and is specifically codified for large and turbine-powered multiengine airplanes at 14 CFR 91.527 and for commuter/on-demand operations at 14 CFR 135.227; even where no identical numbered rule applies to smaller Part 91 piston operations, thorough deicing before flight is essential safety practice. Even a thin, seemingly insignificant layer of ice on a wing's upper surface can disrupt laminar airflow, increase stall speed, reduce lift, and increase drag in a way that can be fatal on takeoff.

In flight, an aircraft descending through a layer of freezing fog during an instrument approach is exposed to rapid structural icing accretion. The droplet sizes in fog tend to be smaller than those in freezing rain or freezing drizzle, but the accretion rate at low altitude with limited escape options makes even moderate icing intensity operationally serious. Pitot-static system icing, carburetor icing in piston aircraft, and windshield obscuration are all additional concerns.

On the ground, freezing fog is a taxiing hazard. Ramp surfaces, taxiways, and runways can be coated with a thin, nearly invisible glaze that dramatically reduces braking effectiveness and directional control. Ground crews face slip hazards, and jet blast or propwash can create additional icing on nearby surfaces or equipment.

Key Numbers and Rules

Fog is defined as visibility less than 5/8 statute mile (1 km) at the surface.
Freezing fog occurs when fog exists at temperatures at or below 32 °F (0 °C).
Fog seldom forms when temperature-dewpoint spread exceeds 2 °C (4 °F).
Advection fog can intensify with winds up to about 15 kt; stronger winds tend to lift it into low stratus.
Radiation fog winds up to about 5 kt may deepen it; stronger winds disperse or lift it to stratus.
Upslope fog favors wind speeds of 5 to 15 kt.
Shallow (ground) fog is fog so shallow it does not obstruct vision at 6 ft (2 m) above the surface.
Ice fog typically requires temperatures well below 0 °F (approximately −20 °F / −29 °C) to form from direct ice-crystal deposition, though exact thresholds vary.

Common Test Traps

Confusing freezing fog with ice fog. Freezing fog contains supercooled liquid water droplets; ice fog contains ice crystals. They are not the same thing and the distinction appears on FAA knowledge tests.
Assuming cold fog only reduces visibility. Freezing fog is an icing hazard even on the ground — structural ice can accumulate on a parked aircraft without any precipitation occurring.
Misidentifying the wind threshold for advection vs. radiation fog. Advection fog persists and deepens up to ~15 kt; radiation fog is dispersed by winds stronger than ~5 kt. Exams test these thresholds.
Overlooking preflight contamination checks in clear METAR conditions. A METAR may not report freezing fog if visibility has improved, yet ice deposited overnight can remain on cold-soaked airfoil surfaces.
Confusing steam fog turbulence with stable conditions. Steam fog forms in a shallow unstable air layer — pilots should expect convective turbulence when flying through it, unlike most other fog types which form in stable air.