Fog Formation

What fog actually is

Fog is a cloud at ground level. Meteorologically it's identical to stratus — tiny suspended water droplets — except that it's touching the surface. The FAA definition for METAR purposes: fog (FG) is reported when visibility drops below 5/8 statute mile. When visibility is 5/8 to 6 SM with near-100% relative humidity, it's mist (BR). Same phenomenon, different severity.

Fog forms in one of two ways:

• Cooling: the air temperature drops until it reaches the dew point. Relative humidity hits 100%, water vapor condenses.
• Moisture addition: enough water vapor is added to the air that RH reaches 100% without significant temperature change. Less common; steam fog is the main aviation example.

Either way, the key number is the temperature/dew point spread — the gap between T and Td. When that gap closes to zero, you have fog (or a cloud, if it's elevated). This single value is the most useful fog predictor available in a standard METAR or hourly observation.

METAR quick read: In METAR KORF 120855Z 00000KT 1/4SM FG OVC002 11/11 A3006, the 11/11 tells you T = Td = 11°C — spread is zero. Visibility is 1/4 SM with fog. The surface is saturated. This is textbook radiation fog on a calm morning.

The five fog types

The FAA (AC 00-45H §16.1.1.1) defines fog types by their formation mechanism. There are five: radiation, advection, upslope, frontal, and steam. Each requires different conditions, forms in different places, and — critically — dissipates differently. Confusing them leads to bad go/no-go calls.

Fog descriptors — not formation types: Freezing fog (METAR: FZFG) is any fog type occurring in below-0°C air — supercooled droplets freeze on contact with aircraft surfaces and pavement. Ice fog forms when fog is composed of ice crystals rather than liquid droplets, in very cold Arctic air. Ground fog (METAR: MIFG) is fog so shallow it does not obstruct vision at 6 ft above the surface — typically a form of radiation fog. These describe the fog's physical state or depth, not how it formed.

The most important distinction for pilots: Radiation fog almost always clears by late morning. Advection fog may persist all day regardless of clear skies and sunshine — the only thing that fixes it is a wind shift bringing drier air. If the forecast says "fog burning off by 1000L" and you're in a coastal area with an onshore flow, ask yourself: what changes the wind direction? If nothing does, that fog isn't going anywhere.

Radiation fog — in depth

Radiation fog is the most common fog type in the continental U.S. and the most predictable. It happens because the earth's surface is constantly losing heat to space via longwave infrared radiation. During the day, solar heating more than compensates. At night — especially on clear, calm nights — the surface cools faster than the overlying air, and that cooling is conducted upward layer by layer.

Three things must come together:

1. Clear sky. Clouds act like a blanket, absorbing outgoing IR and radiating some back toward the surface. A cloudy night significantly slows surface cooling. Clear sky = maximum cooling.
2. Calm winds (< 5 kt). Wind mixes the boundary layer, stirring cold surface air up with warmer air aloft. A gentle wind distributes the cooling through a deeper layer and slows the rate at which surface air reaches saturation. Too much wind and the fog never forms — it dissipates before it can establish.
3. Sufficient moisture. The closer T is to Td before sunset, the less cooling required overnight. A surface T/Td spread of 5–7°F at sunset is a reliable warning flag.

Radiation fog is restricted to land because water surfaces cool too little from nighttime radiation to produce fog this way. It tends to pool in low terrain — valleys, river bottoms, basins — because cold, dense air drains downslope and collects. A ridge-top airport may be completely clear while a valley field 10 miles away is 0/0. Ground fog is a form of radiation fog confined to near-ground level: so shallow it does not obstruct vision at 6 feet above the surface (MIFG in the METAR).

Key operational fact: radiation fog is almost always thickest at dawn (maximum cooling has occurred; solar heating hasn't started yet) and typically clears within 1–3 hours of sunrise. The exception is when humidity is very high — then the fog may not burn off until mid-morning or may re-form the following night.

Advection fog — in depth

Advection is just weather-speak for horizontal transport. Advection fog forms when a warm, moist air mass moves over a surface that is cold enough to cool it to the dew point. The cooling comes from the bottom, through contact, not from the surface radiating heat away. That distinction drives everything about how this fog behaves.

Advection fog is most common along coastal areas but often moves deep into continental areas. When it occurs over open water, it is called sea fog. The classic setup: California summer. The North Pacific High pushes warm, moist marine air onshore. But the California Current offshore is cold — upwelling brings deep, frigid water to the surface. As that warm air crosses the cold current, it cools from below, reaches saturation, and produces fog that rolls in off the water. This is the infamous San Francisco marine layer. In winter, advection fog over the central and eastern United States forms when moist air from the Gulf of Mexico spreads northward over cold ground — it can extend as far north as the Great Lakes.

What makes advection fog different from radiation fog in terms of behavior:

• Wind doesn't prevent it. Radiation fog requires calm air. Advection fog can form with surface winds of 10–15 kt and sometimes more. The wind is what's transporting the air mass — it's part of the mechanism.
• Solar heating doesn't fix it. Radiation fog clears when the sun warms the ground, which warms the air above it, pushing T above Td. With advection fog, the underlying cold surface is still there. Even if the sun heats the fog layer itself slightly, the fog just reforms as the air advecting over the cold surface keeps cooling.
• It can set in rapidly over a wide area. An onshore flow can push a marine layer 50+ miles inland in hours.

Advection fog is more hazardous operationally than radiation fog because it's harder to predict its clearing time. A TAF that says "BECMG 1200/1400 VFR" when the cause is a marine layer is based on a forecast wind shift. If that wind shift is late, the fog isn't lifting at 1200. Check the surface analysis and the area forecast to understand what's driving the fog and what conditions would actually dissipate it.

Upslope & steam fog

Upslope fog

When moist, stable air is pushed up a slope by a persistent wind, it cools adiabatically (dry adiabatic lapse rate if unsaturated). If the slope is long enough and the air moist enough, it cools to the dew point before reaching the summit — and fog forms. This is identical to cloud formation via forced lifting, just at the surface.

Wind speeds of 5 to 15 knots are most favorable — winds stronger than 15 knots tend to lift the fog into a layer of low stratus clouds rather than letting it hug the terrain. Importantly, upslope fog can form under cloudy skies, unlike radiation fog which requires a clear sky for surface cooling. Upslope fog is common on the eastern slopes of the Rockies and somewhat less frequent east of the Appalachians. It is often quite dense and can extend to high altitudes. Dissipation requires the wind to die down or shift direction.

Steam fog (evaporation fog)

Steam fog is the opposite of advection fog in one key way: instead of warm air over a cold surface, it's cold air over a warm surface (water). Water evaporates rapidly into the cold air and immediately saturates it. The result looks exactly like steam rising from a hot spring or a pot of water — wisps of fog curling upward from the surface.

You'll see steam fog on lakes and rivers in fall and early winter, typically after a cold front brings cold air over water that is still warm from summer. It is also observed over the ocean during winter when cold air masses move off the continents and ice shelves. It's usually shallow — 50 to a few hundred feet deep — but can reduce visibility to near zero right at the surface. Steam fog is associated with a shallow layer of unstable air; because the warm water keeps driving convection upward, pilots should expect convective turbulence when flying through it. On occasion, columns of condensed vapor rise from the fog layer forming whirling steam devils, similar in appearance to dust devils on land.

Icing risk with steam fog: In subfreezing air, steam fog can produce supercooled droplets — liquid water in air below 0°C. A low-altitude pass through steam fog in cold temperatures can deposit rime ice quickly. It's one of those situations where the fog itself isn't the only hazard.

Frontal fog & freezing fog

Frontal fog is the fifth FAA fog type, named for its formation mechanism. Freezing fog is not a fog type — it is a temperature modifier that can apply to any of the five types when air is at or below 0°C.

Freezing fog (FZFG) — icing modifier

Freezing fog is not a fog formation type. It is ordinary fog — radiation, advection, or any other type — that happens to be in air at or below 0°C. The formation mechanism is the same. What changes is the state of the droplets: at subfreezing temperatures they become supercooled liquid water, remaining liquid despite being below the freezing point. Supercooled droplets are metastable. They stay liquid until they contact a surface or an ice nucleus, at which point they freeze instantly.

This creates two hazards simultaneously:

• Aircraft icing. Flying through freezing fog or taxiing on a foggy subfreezing ramp deposits rime ice on every exposed surface — wing leading edges, tail, pitot tubes, antennas, control hinges. The droplets are small and produce a rough, opaque white deposit that builds faster than most pilots expect.
• Runway and taxiway icing. Freezing fog coats pavement with a glaze that looks dry under normal lighting. This is one of the sources of "black ice" — the surface appears clean but has friction comparable to wet ice. Braking action reports and NOTAMs are your primary tools here; visual inspection alone is unreliable.

In a METAR, freezing fog is coded FZFG. The FZ prefix is always an icing indicator — you'll also see FZRA (freezing rain) and FZDZ (freezing drizzle). Any FZ code in the present weather group means liquid water is freezing on contact.

Frontal fog — Type 4 of 5

Frontal fog — also called precipitation-induced fog (the FAA's preferred term in AC 00-45H) — is most commonly associated with advancing warm fronts, though it can occur along any frontal boundary. As a warm front approaches, warm moist air glides up and over the cooler air mass sitting at the surface ahead of it. That warm air produces precipitation — rain or drizzle — that falls through the cooler air below the front.

As the warm rain descends into the cold surface air, it partially evaporates. Evaporation adds moisture directly to the cold air, raising its dew point. If that surface air is already moderately humid — as it often is ahead of a warm front — the added moisture is enough to close the T/Td spread to zero. Fog forms, often 100–300 nm ahead of the frontal surface.

The operational trap with frontal fog: the weather looks like it's improving. The front is approaching, warmer temperatures are on the way, the skies might be brightening slightly — but visibility can be near zero. IFR conditions can set in hours before the front arrives and persist until after frontal passage. Solar heating does essentially nothing to clear frontal fog because the evaporation source (warm precipitation) is still running.

What about "precipitation fog"? The FAA classifies precipitation-induced fog as the same formation type as frontal fog — the mechanism is identical regardless of whether the precipitation source is a warm front, cold front, or convective cell. When evaporating precipitation saturates cold surface air and fog forms, that is frontal (precipitation-induced) fog. The FAA does not list precipitation fog as a separate formation type. For METAR decoding: patchy fog is BCFG (uneven distribution) and partial fog is PRFG (covers part of the airport) — both indicate that conditions may vary significantly across a short distance.

METAR fog codes at a glance: FG = fog (vis < 5/8 SM) · BR = mist (5/8–6 SM, high humidity) · FZFG = freezing fog · MIFG = shallow fog (dense fog layer < 6 ft deep — can be VFR overhead) · BCFG = patchy fog (uneven distribution) · PRFG = partial fog (covers part of airport). Knowing these qualifiers lets you reconstruct what the airport actually looked like — not just that "fog" was present.

Predicting fog from observations

The most useful pre-flight fog predictor is the temperature/dew point spread from the current METAR or hourly observation. If you can estimate overnight cooling from your route's surface temps, you can make a reasonable fog prediction without any special tools.

Why fog burns off — or doesn't

"Fog burns off" is aviation shorthand for solar insolation raising the near-surface air temperature back above the dew point, causing droplets to evaporate. This works reliably for radiation fog because the mechanism that formed it — surface cooling — is reversed once the sun rises. As the ground warms, so does the air above it. T climbs back above Td. Fog lifts and dissipates.

The timeline depends on fog thickness, humidity, and solar angle:

• Thin fog on a late spring/summer morning at low latitude → clears within 30–60 minutes of sunrise.
• Thick fog in November at high latitude → may not clear until 1000–1100 local, or at all on short winter days.
• Dense fog with Td very close to surface temperature → requires more heating, clears later.

Advection fog does not burn off with sunshine alone. The underlying cold surface is still there. Even when the sun warms the fog layer from above, the cold surface below keeps cooling the air in contact with it. The only thing that clears advection fog is a change in the air mass — a wind shift bringing drier continental air, or an offshore flow replacing the onshore marine layer.

Upslope fog clears when the upslope flow stops — wind shift, frontal passage, or diurnal wind reversal (slope winds often shift downslope by afternoon as surface heating decreases upslope pressure gradient). Radiation fog can also "burn from the bottom up" — the sun heats the surface, which erodes the fog from below, often producing a shallow 200–300 ft ceiling as the main fog layer lifts slightly while the tops consolidate.

The most common pilot error: planning a morning departure based on a forecast that says "fog clearing by 0900" without asking why it clears. If it's radiation fog, the mechanism (solar heating) is reliable and the timing is reasonable. If it's advection fog along a coast, "clearing by 0900" depends entirely on the wind shift — which may or may not happen as forecast. The TAF is only as good as the wind forecast it's based on.

Red flags

Checkride questions you'll actually be asked

Fog comes up on nearly every Private and Instrument oral. Try to answer each before reading the response.

Decision scenario

Educational example only. Not for real flight planning. Real go/no-go decisions require official sources, current data, and your own pilot-in-command judgment.

Pilot takeaway