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Is the briefing accurate?

Your briefing uses the same official observations and forecasts every pilot relies on: METARs, TAFs, PIREPs, SIGMETs and model guidance. It can only be as accurate as those forecasts, and every forecast carries some uncertainty. Every weather briefing, from any provider, also has the same built-in limit. It describes one departure time, one route, one altitude and one arrival time. Real flights change. You leave late, get rerouted, or get assigned a different altitude, and each change puts you in weather the briefing didn't look at. That's why PlaneWX can keep updating your briefing for monitored flights as the forecast changes, and why your decision-making continues in the air.

How can I make my briefing more accurate?

Enter the flight the way you'll really fly it: a realistic departure time, the route you expect to file or be cleared, your planned altitude, and the right aircraft profile. If any of those change, update the trip and brief again. Check again close to departure, when the forecasts are freshest.

Yes, in specific places. Weather reports, forecasts, model data and NOTAMs are fetched and decoded by code. AI reads forecaster discussions and writes the plain-language briefing, the hazard summary and most section ratings. It also proposes a starting WX Score and some deductions, such as for convection. Ceiling and visibility are always scored by code. Within 12 hours of departure, your personal minimums and aircraft limits are checked by rule-based code, not the AI. Further out, AI also helps weigh icing and turbulence against your limits. You always make the final call.

What is the WX Score?

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How far in advance can PlaneWX forecast weather?

PlaneWX provides weather intelligence from 14 days out through departure. Confidence improves as your flight approaches. We are transparent about uncertainty at every time horizon.

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PlaneWX connects you with experienced pilots who can review your full briefing (WX Score, aircraft profile, personal minimums) and offer a second set of eyes grounded in shared data, not guesswork. Mentor sits alongside the risk-management loop when you want it.

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Fog for Pilots: Radiation, Advection, Upslope, and Freezing Fog

PlaneWX · Last reviewed October 1, 2026

Fog is a cloud based at the surface that reduces visibility to less than 5/8 statute mile. It forms when the air cools to its dewpoint or gains moisture until the two match, and it seldom forms when the temperature-dewpoint spread is greater than 2°C (4°F). How it forms decides when it clears: radiation fog often lifts by noon, while advection fog can persist for days [1, para. 18.1.1].

Key takeaways

Watch the spread at your destination and your alternates, not just at departure [1, para. 18.1.1][3, para. 8.2].
Know the type. Radiation fog has a clock; advection and frontal fog don’t [1, paras. 18.1.1.1.1 to 18.1.1.1.4].
Plan the out before you go. The NTSB: “Plan ahead with cancellation or diversion alternatives.” [2]

Key facts

Fog is a cloud based at the surface that cuts visibility below 5/8 SM [1, para. 18.1.1].
Fog forms when temperature and dewpoint become identical, or nearly so [1, para. 18.1.1].
Fog seldom forms with a spread greater than 2°C (4°F) [1, para. 18.1.1].
AC 91‑92: be especially cautious at a spread of 3°C or less [3, para. 8.2].
Radiation fog is a land, night, clear sky, light wind fog [1, para. 18.1.1.1.1].
Advection fog is more extensive and persistent, and can arrive at any hour [1, para. 18.1.1.1.2].
Upslope fog can form under cloudy skies and is often dense and deep [1, para. 18.1.1.1.3].
Freezing fog occurs at 0°C (32°F) or below and can ice an airplane while taxiing [1, para. 18.1.1.1.6].
Most AWOS stations don’t report fog, mist, or haze [1, Table 3-9].
About two-thirds of GA accidents in reduced visibility are fatal [2].

Hover, focus, or tap any dotted term for a definition and its official source.

Fog is the classic setup for VFR flight into instrument conditions: a good departure, a destination that was fine an hour ago, and a layer that forms underneath you. The NTSB’s warning is stark: “Historically, about two-thirds of all general aviation (GA) accidents that occur in reduced-visibility weather conditions are fatal.” [2] This page covers how each type of fog forms and clears, what the temperature-dewpoint spread really tells you, how fog shows up in METARs and TAFs, and two snow-country cousins, whiteout and flat light.

What is fog?

“Fog is a visible aggregate of minute water droplets that are based at the Earth’s surface, and it reduces horizontal visibility to less than 5/8 SM (1 km); unlike drizzle, it does not fall to the ground.” “Fog differs from a cloud only in that its base must be at the Earth’s surface, while clouds are above the surface.” [1, para. 18.1.1]

“Fog forms when the temperature and dewpoint of the air become identical (or nearly so).” That happens by cooling the air (radiation, advection, and upslope fog) or by adding moisture (frontal and steam fog) [1, para. 18.1.1]. Above 5/8 SM the same droplets are called mist: visibility less than 7 SM but at least 5/8 SM [1, para. 18.1.2].

What are the types of fog?

“Fog types are named according to their formation mechanism.” [1, para. 18.1.1.1] The mechanism tells you how it will behave:

Fog type	How it forms	What to expect
Radiation fog	Over land, when the ground cools the air to its dewpoint on a clear night with light wind [1, para. 18.1.1.1.1]	A nighttime fog that often doesn’t clear until after sunrise. “Other radiation fog generally clears before noon unless clouds move in over the fog.” [1, para. 18.1.1.1.1]
Ground fog	A shallow form of radiation fog [1, para. 18.1.1.1.1]	“Ground fog usually burns off rather rapidly after sunrise.” [1, para. 18.1.1.1.1]
Mountain or valley fog	Cold air drains into valleys overnight and cools to saturation [1, para. 18.1.1.1.1.1]	Most common in autumn and spring, and densest around sunrise [1, para. 18.1.1.1.1.1]
Advection fog	Moist air moves over a colder surface, often from the sea [1, para. 18.1.1.1.2]	“advection fog is usually more extensive and much more persistent than radiation fog. Advection fog can move in rapidly regardless of the time of day or night.” [1, para. 18.1.1.1.2]
Upslope fog	Moist, stable air cools as wind pushes it up rising terrain [1, para. 18.1.1.1.3]	“Unlike radiation fog, it can form under cloudy skies.” “Upslope fog is often quite dense and extends to high altitudes.” [1, para. 18.1.1.1.3]
Frontal fog	Rain falling from warm air into cold air near its dewpoint saturates it [1, para. 18.1.1.1.4]	“This fog may extend over large areas, completely suspending air operations.” Most common with warm fronts [1, para. 18.1.1.1.4]
Steam fog	Very cold air moves over relatively warm water [1, para. 18.1.1.1.5]	Usually shallow, but “pilots can expect convective turbulence flying through it.” [1, para. 18.1.1.1.5]
Freezing fog	“Freezing fog occurs when the temperature falls to 32°F (0°C) or below.” [1, para. 18.1.1.1.6]	Ice can form on an aircraft in flight or even while taxiing [1, para. 18.1.1.1.6]
Ice fog	Supercooled droplets freeze directly into ice crystals in very cold air [1, para. 21.4.1]	“Ice fog is rare at temperatures warmer than -30°C” [1, para. 21.4.1], so it is mostly an arctic hazard

How wind changes fog

Wind works differently on each type. Radiation fog “is shallow when wind is calm. Winds up to about 5 kt mix the air slightly and tend to deepen the fog by spreading the cooling through a deeper layer.” Stronger wind disperses it or lifts it into stratus [1, para. 18.1.1.1.1]. “Advection fog deepens as wind speed increases up to about 15 kt.” [1, para. 18.1.1.1.2] Light wind is not a reason to relax.

Rain, then clearing

“When rain soaks the ground, followed by clearing skies, radiation fog is not uncommon the following morning.” [1, para. 18.1.1.1.1] A clearing evening after a wet day is a fog setup for a dawn departure or arrival.

What does the temperature-dewpoint spread tell you?

The METAR temperature and dewpoint group gives you the spread directly. Two FAA documents give two thresholds: the Handbook says fog seldom forms above 2°C (4°F) [1, para. 18.1.1], and AC 91‑92 tells pilots to “Be especially cautious when the temperature and dewpoint spread is 3 °C or less: fog may form.” [3, para. 8.2] They don’t conflict: one describes when fog seldom forms, the other sets a wider caution band. A worked reading:

METAR temperature/dewpoint	Spread	What the FAA sources say
`08/01`	7°C	Outside both thresholds. “Fog seldom forms when the temperature-dewpoint spread is greater than 2°C (4°F).” [1, para. 18.1.1] Advection fog can still arrive with moist air from elsewhere [1, para. 18.1.1.1.2]
`14/11`	3°C	Inside AC 91‑92’s caution band: be especially cautious at 3°C or less, because fog may form [3, para. 8.2]
`12/11`	1°C	Inside the Handbook’s 2°C band too. If the night is clear and calm, radiation fog is a real possibility [1, paras. 18.1.1 and 18.1.1.1.1]
`M01/M02`	1°C, below freezing	Any fog that forms is freezing fog [1, para. 18.1.1.1.6]

Two cautions. A spread is a snapshot, and it shrinks as the evening cools. And the spread at your departure says nothing about a coastal destination where moist air is moving in. Check the trend over several reports, and check the TAF for fog groups.

How is fog reported and forecast?

Fog appears in observations, advisories, and forecasts, but not everywhere [1, Table 3-9]:

Code	Meaning	Source
FG	Fog, reported when visibility is less than 5/8 SM [1, Table 3-9]	METAR and TAF
BR	Mist: visibility less than 7 SM but at least 5/8 SM [1, para. 18.1.2]	METAR; in a TAF for 5/8 to 6 SM [1, Table 3-9]
FZFG	Freezing fog [1, Table 3-9]	METAR and TAF
MIFG, BCFG, PRFG	Shallow fog, patchy fog, and partial fog [1, Table 3-9]	METAR and TAF
VCFG	Fog in the vicinity, 5 to 10 SM from the center of the airport [1, Table 3-9]	TAF
VV	Vertical visibility into the fog, which counts as the ceiling [1, para. 24.4.3.9]	METAR and TAF

AWOS gaps. “Fog/mist/haze is not included in METARs/SPECIs from most AWOS, nor most AWOS broadcasts, depending on the type of AWOS.” [1, Table 3-9] A low visibility with no weather code may be fog.
TAF threshold. “A visibility threshold must be met for fog to be included in the TAF (visibility of less than 5/8 SM).” [1, Table 3-9] See the TAF guide, and TAF vs MOS, LAMP and NBM for forecast guidance at airports without a TAF.
AIRMET Sierra. Issued when fog or mist is occurring or expected that could restrict visibility below 3 SM over a wide area [1, Table 3-9]. See the advisories guide.

How low visibility and an indefinite ceiling set the flight category is covered in the ceiling and visibility guide.

Whiteout and flat light

In snow country, visibility can vanish without any fog. The Handbook describes blowing snow lofted to about 50 ft, “obscuring the sky, and reducing surface visibility to near zero, it is called a whiteout.” “Visibility improves rapidly when the wind subsides.” [1, para. 18.1.6] The NTSB adds flat light: “Flat light occurs when the sky is overcast, especially over snow-covered terrain and large bodies of water.” No shadows are cast, so depth, distance, and altitude are hard to judge [4]. Its advice: “If visibility drops, use your instruments and land at the nearest suitable airport.” [4]

Where are the official fog references?

Official fog references

Links checked October 1, 2026: all returned HTTP 200.

Where do fog reports mislead pilots?

The station that can’t see fog. Most AWOS don’t report it [1, Table 3-9].
The TAF threshold. Fog under the TAF’s 5/8 SM line is forecast; thinner fog may only show as mist or a lower visibility [1, Table 3-9].
Calm assumptions. A little wind deepens radiation fog [1, para. 18.1.1.1.1].
Clouds as protection. Upslope fog can form under cloudy skies [1, para. 18.1.1.1.3].
Valley reports in the mountains. A valley station can report VFR while the ridges sit in fog [1, para. 18.2.2].

Source note: in Table 3-9 the Handbook says FG is reported for visibilities from 5/8 SM to less than 7 SM when the temperature-dewpoint difference is 40°F (about 20°C) or less. That contradicts its own definition of fog (below 5/8 SM) and of mist [1, paras. 18.1.1 and 18.1.2], so this page doesn’t use it. The same table says FZFG is reported below 0°C, while Section 18.1.1.1.6 says at or below 0°C.

How do you use fog information in a go/no-go decision?

The decision is yours as PIC. Habits drawn from the FAA and NTSB guidance above:

Read the spread trend at every airport you might need. Destination and alternates, over several hours [1, para. 18.1.1][3, para. 8.2].
Time around radiation fog. If the setup is clear, calm, and moist, a later departure may solve it; it generally clears before noon unless clouds move in [1, para. 18.1.1.1.1].
Don’t wait out advection fog on a hunch. It is more persistent and can move in at any hour [1, para. 18.1.1.1.2].
Carry a fuel and airport plan. Have a contingency plan for alternates if unexpected circumstances arise [3, para. 8.1], and brief passengers on the alternatives before the flight [2].
Turn around early. AC 91‑92: don’t continue VFR into IMC; wait it out or turn around if en route weather drops below your personal limits [3, para. 8.2].

For a VFR cross-country, the PlaneWX guide to VFR weather go/no-go covers building the decision.

Where does fog fit in a disciplined decision?

PlaneWX is decision support for general aviation pilots, built around one idea: “Fly like it’s your job.” Treat the weather decision the way a professional crew would, with the same process every time. That process is a loop: Brief, then FRAT, then Fly or Stay, then Debrief, with a Mentor if you want one. Fog feeds the Brief. Here’s how PlaneWX handles it, as its help center describes [5]:

Brief: visibility against your limits. Forecast visibility below your hard limit at departure or arrival is a deduction of up to 40 points (Scoring).
Brief: freezing fog. FZFG is on the list of conditions that are “always unfavorable regardless of your score math” (Scoring).
Brief: fog risk is awareness, not a score. The temperature-dewpoint spread and fog risk alone don’t change the WX Score unless visibility is below minimums. Near arrival, up to two airports within about 30 nm reporting fog, mist under 3 SM, or IFR in the last 90 minutes can appear as “Not scored” lines when the destination has no TAF or its TAF shows no fog at your ETA (Scoring, Understanding Your Briefing).
FRAT. The flight risk assessment opens in the last 4 hours before departure (FRAT).
Fly or Stay. “PlaneWX never recommends GO or NO-GO. You make the call as PIC.” (The Risk Loop)
Debrief. If fog formed earlier or later than forecast, note it. Self Debrief is a PlaneWX Labs feature for Pro Plus (The Risk Loop).

The help center is plain about the limits: “PlaneWX is not a substitute for a complete, independent pre-flight weather briefing. Pull one from a source you already trust before you fly.” (The Risk Loop)

Check visibility at your destination and nearby airports in a PlaneWX briefing, or read how PlaneWX scores ceilings and visibility.

Frequently asked questions

What is fog?

What temperature-dewpoint spread means fog is likely?

The Handbook says “Fog seldom forms when the temperature-dewpoint spread is greater than 2°C (4°F).” [1, para. 18.1.1] AC 91‑92 tells pilots to be especially cautious when the spread is 3°C or less [3, para. 8.2]. A small spread is a warning, not a forecast.

When does radiation fog burn off?

Ground fog usually burns off rather rapidly after sunrise, and other radiation fog generally clears before noon unless clouds move in over it [1, para. 18.1.1.1.1].

Why is advection fog harder to plan around than radiation fog?

It is usually more extensive and much more persistent, and it “can move in rapidly regardless of the time of day or night.” [1, para. 18.1.1.1.2]

Does wind clear fog?

It depends on the type. Winds up to about 5 kt tend to deepen radiation fog; stronger wind disperses it or lifts it into stratus [1, para. 18.1.1.1.1]. Advection fog deepens as wind increases up to about 15 kt [1, para. 18.1.1.1.2].

Do AWOS stations report fog?

Often not. “Fog/mist/haze is not included in METARs/SPECIs from most AWOS, nor most AWOS broadcasts, depending on the type of AWOS.” [1, Table 3-9]

What is the difference between mist and fog in a METAR?

Visibility. FG is reported below 5/8 SM [1, Table 3-9]; mist (BR) covers less than 7 SM down to 5/8 SM [1, para. 18.1.2].

What is flat light?

“Flat light occurs when the sky is overcast, especially over snow-covered terrain and large bodies of water.” No shadows are cast, so depth and altitude are hard to judge [4].

Does PlaneWX score fog?

Its help center says forecast visibility below your hard limit is scored, and freezing fog (FZFG) is always unfavorable. The temperature-dewpoint spread and fog risk alone are awareness only, and nearby-airport fog reports appear as “Not scored” lines. PlaneWX never recommends GO or NO-GO; you make the call as PIC [5].

Glossary

Every term below links to the FAA, NWS, or NOAA document that defines it. The same definitions appear when you hover, focus, or tap a dotted term on this page.

Advection fog: Fog that forms when moist air moves over a colder surface and cools below its dewpoint; common along coasts. Source: FAA Aviation Weather Handbook, 18.1.1.1.2
BR (Mist): Fog that restricts visibility to between 5/8 and 6 statute miles; below 5/8 SM it is coded FG. Source: NWSI 10‑813, Appendix B2.6.3
FG (Fog): A visible aggregate of tiny water droplets based at the surface that reduces visibility to less than 5/8 statute mile. Source: FAA Aviation Weather Handbook, 18.1.1
Flat light: Diffuse light under an overcast, especially over snow or water, that removes depth cues and the horizon. Source: NTSB SA‑052, SA‑052
FZFG (Freezing fog): Fog at a temperature of 0°C or below, whose droplets can freeze on contact with surfaces. Source: FAA Aviation Weather Handbook, 18.1.1.1.6
Ground fog: A form of radiation fog confined to a thin layer near the ground. Source: FAA Aviation Weather Handbook, 18.1.1.1.1
Ice fog: Fog made of tiny ice crystals that forms when supercooled droplets freeze directly; rare warmer than about minus 30°C. Source: FAA Aviation Weather Handbook, 21.4.1
IMC (Instrument meteorological conditions): Visibility, distance from cloud, and ceiling below the minimums set for visual meteorological conditions. Source: FAA Pilot/Controller Glossary, INSTRUMENT METEOROLOGICAL CONDITIONS
MIFG (Shallow fog): A METAR code for a thin layer of fog near the ground. Source: FAA Aviation Weather Handbook, 3.4.2 Table 3-1 acronyms
Radiation fog: Shallow fog that forms over land on clear nights with light winds as the ground cools the air to its dewpoint. Source: FAA Aviation Weather Handbook, 18.1.1.1.1
Steam fog: Fog that forms when cold air moves over much warmer water; it sits in unstable air, so expect convective turbulence in it. Source: FAA Aviation Weather Handbook, 18.1.1.1.5
Temperature-dewpoint spread: The difference between air temperature and dewpoint; fog seldom forms when it is greater than 2°C, or 4°F. Source: FAA Aviation Weather Handbook, 18.1.1
Upslope fog: Fog that forms as moist, stable air cools while it is pushed up rising terrain; often dense and deep. Source: FAA Aviation Weather Handbook, 18.1.1.1.3
VV (Vertical visibility): How far up you can see into a surface-based obscuration such as fog, in hundreds of feet; it counts as the ceiling. Source: NWSI 10‑813, Appendix B2.7.2
Whiteout: Blowing snow lofted to about 50 ft that hides the sky and cuts surface visibility to near zero. Source: FAA Aviation Weather Handbook, 18.1.6

This page explains published FAA and NTSB guidance. It is not a substitute for a current weather briefing or for instrument training. Always get a current briefing before you fly.

References

FAA‑H‑8083‑28B, “Aviation Weather Handbook.” Federal Aviation Administration, Flight Standards Service. April 2, 2026. Paragraphs cited: Table 3-9 (para. 3.4.2.6), 18.1.1 to 18.1.1.1.6, 18.1.2, 18.1.6, 18.2.2, 21.4.1, 24.4.3.9. https://www.faa.gov/regulationspolicies/handbooksmanuals/aviation/faa-h-8083-28b-aviation-weather-handbook. PDF: https://www.faa.gov/sites/faa.gov/files/FAA-H-8083-28B.pdf
NTSB Safety Alert SA‑020, “Reduced Visual References Require Vigilance.” National Transportation Safety Board. March 2013, revised December 2015. https://www.ntsb.gov/Advocacy/safety-alerts/Documents/SA-020.pdf
AC 91‑92, “Pilot’s Guide to a Preflight Briefing.” Federal Aviation Administration. March 15, 2021 (editorial update March 19, 2025). Paragraphs cited: 8.1, 8.2. https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_91-92_Ed_Upd_(3-19-25).pdf
NTSB Safety Alert SA‑052, “Visual Illusions: The Ground May Be Closer Than It Appears.” National Transportation Safety Board. March 2016. https://www.ntsb.gov/Advocacy/safety-alerts/Documents/SA-052.pdf
PlaneWX Help Center: “Scoring,” “Understanding Your Briefing,” “FRAT,” “The Risk Loop.” PlaneWX. Accessed October 1, 2026. Scoring · Understanding Your Briefing · FRAT · The Risk Loop