What are the five main types of fog that affect aviation visibility?

The five main fog types are radiation fog, advection fog, upslope fog, steam fog, and precipitation-induced fog. Radiation fog forms on calm, clear nights when the ground cools rapidly and chills the air near the surface to its dew point. Advection fog occurs when warm, moist air moves over a cooler surface, while upslope fog forms as moist air is lifted and cooled along rising terrain. Understanding these distinctions, as covered in the FAA's Aviation Weather Handbook, helps remote pilots anticipate which conditions are most likely at their operating location.

Why is fog especially dangerous for small unmanned aircraft systems (sUAS) operations?

Fog can reduce visibility to below the 3-statute-mile minimum required under 14 CFR 107.51(a) for most sUAS operations, forcing an immediate ground stop. Because fog can form very rapidly — sometimes within minutes under the right conditions — a remote pilot who launches in clear conditions may quickly lose visual line of sight of the aircraft, which is a separate requirement under 14 CFR 107.31. The FAA Pilot's Handbook of Aeronautical Knowledge notes that low-visibility conditions are among the most hazardous weather phenomena for all aviators, and remote pilots must continuously monitor conditions and be prepared to land immediately if visibility deteriorates.

How do you know if radiation fog is likely to form before a sUAS flight?

Radiation fog is most likely on clear, calm nights with high relative humidity, when skies allow rapid radiative cooling of the surface and winds are calm or light enough to avoid mixing that would disperse the fog. Remote pilots can check forecast products such as Terminal Aerodrome Forecasts and the Graphical Forecasts for Aviation, both described in the AIM and Aviation Weather Handbook, to look for fog advisories or low visibility forecasts near their planned operating area. Monitoring the temperature-dew point spread is also a practical technique — when the spread narrows to around 4°F (2°C) or less and conditions are calm and clear overnight, radiation fog development is a serious possibility by morning.

Fog Types and Visibility Hazards for sUAS Operations

Of all the weather phenomena a remote pilot must understand, fog ranks among the most operationally disruptive. Unlike thunderstorms or high winds that announce themselves dramatically, fog can materialize with little warning, drop visibility from miles to near zero within minutes, and persist stubbornly through the morning hours. For small unmanned aircraft systems (sUAS) operating under Part 107, 14 CFR 107.51(a) requires a minimum ground visibility of 3 statute miles from the control station, and 14 CFR 107.31 separately requires the remote pilot to maintain visual line of sight (VLOS) with the aircraft at all times. Fog directly attacks both of those requirements. A thorough understanding of fog formation, the five primary fog types, and their operational implications is essential not only for the FAA Part 107 knowledge test but for every preflight weather evaluation you conduct.

At its core, fog is simply a cloud at ground level — a visible mass of tiny water droplets suspended in the atmosphere. Fog is reported when visibility is reduced to less than 5/8 of a statute mile; this 5/8-mile threshold is specifically the criterion for reporting "fog" (FG) rather than "mist" (BR) in METARs, though fog can restrict visibility below 7 statute miles at higher levels and need not be uniformly below 5/8 mile everywhere to be reported. Mist (BR) and haze (HZ) are distinct phenomena — mist is composed of water droplets and reduces visibility to between 5/8 and 6 statute miles, while haze is caused by fine dry particles suspended in the air and is reported separately from mist rather than being an interchangeable term. Understanding how different fog types form helps a remote pilot predict when and where fog is most likely to appear, how long it will last, and when conditions are likely to improve.

The Physics Behind Fog Formation

Fog forms when air is cooled to its dew point, when moisture is added to already-saturated air, or when both processes act together. The dew point is the temperature at which air becomes saturated and water vapor begins to condense into tiny liquid droplets. When the temperature-dew point spread (also called the dew point depression) narrows to approximately 4°F (2°C) or less, fog becomes likely. Remote pilots should monitor this spread during preflight weather checks — a small and shrinking gap between temperature and dew point is a reliable fog warning sign. The Aviation Weather Handbook notes that calm or light winds, clear skies (which promote radiative cooling), and high relative humidity are classic fog-producing conditions.

The Five Primary Fog Types

1. Radiation Fog

Radiation fog is a common fog type in the continental United States, alongside advection fog, which is also extremely common, particularly in coastal regions, and it is one of the fog types most likely to affect morning sUAS operations. It forms on clear, calm nights when the ground radiates heat upward into a clear sky. As the ground cools, it chills the overlying air to the dew point, and fog develops near the surface. Radiation fog is characteristically shallow — often only a few hundred feet deep — and forms in low-lying areas, valleys, and basins where cool, dense air settles. It typically appears a few hours after sunset or in the early morning hours, thickens around sunrise, and then burns off as the sun heats the ground after mid-morning. Key characteristic: radiation fog forms under calm or light wind conditions. Completely calm conditions tend to produce shallow, patchy fog, while a light breeze of a few knots can help mix and deepen the fog layer; if winds increase further, they mix and dissipate the fog layer, but also can cause it to lift into a low stratus ceiling before dispersing entirely. For remote pilots, this means a planned morning flight may be grounded even if the afternoon is perfectly clear.

2. Advection Fog

Advection fog forms when warm, moist air moves horizontally over a cooler surface, cooling the air mass to its dew point from below. The word advection means horizontal transport, distinguishing this type from the stationary cooling of radiation fog. Advection fog is common along coastlines, over cool ocean currents (like the California coast), and inland areas adjacent to large bodies of water. Unlike radiation fog, advection fog can form at any time of day or night, can cover vast geographic areas, and — critically — it requires and is sustained by wind. Advection fog generally forms with light to moderate winds, roughly up to 15 knots, and can be lifted into a low stratus layer as winds increase further. Advection fog can persist for days and move inland, making it far more challenging operationally than its radiation counterpart.

3. Upslope Fog

Upslope fog occurs when moist, stable air is forced up a terrain slope, cooling adiabatically (by expansion) until it reaches the dew point. This fog is often cited as occurring on the eastern slopes of the Rocky Mountains and along other gradually rising terrain, though this is a generalized regional example rather than a fixed rule. Like advection fog, upslope fog requires wind — it depends on a sustained flow of air up the slope. It can form quickly when the conditions are right and tends to persist as long as the upslope flow continues. Remote pilots operating in hilly or mountainous terrain should be especially alert to upslope fog, because what begins as clear visibility at the launch point can deteriorate rapidly as air pushes up the terrain toward the operating area.

4. Steam Fog (Evaporation Fog)

Steam fog, sometimes called sea smoke or arctic sea smoke, forms when cold air moves over a relatively warm water surface. Moisture evaporates rapidly from the water into the frigid overlying air, quickly saturating it and causing condensation as wisps that rise from the surface like steam. This type of fog is common over rivers, lakes, and coastal waters during late autumn and winter when air temperatures drop significantly below water temperatures. Steam fog is typically shallow and patchy, but it can reduce visibility substantially at low altitudes — exactly where sUAS operations take place. Remote pilots planning operations over or near water in cold weather should specifically watch for steam fog conditions.

5. Precipitation-Induced Fog (Frontal Fog)

Precipitation-induced fog, sometimes called frontal fog, forms when rain falls from a warm air mass above into cooler, drier air below. The falling rain evaporates, adding moisture to the lower air until it saturates and fog forms. This fog type is associated with warm fronts, where warm air overrides cooler air, and pre-frontal conditions can produce extensive areas of low visibility. Frontal fog can develop over large areas simultaneously and is often embedded within an already-deteriorating weather picture of low ceilings, reduced visibility, and precipitation. Because frontal fog is part of a broader weather system, it is generally well-forecast and identifiable in standard aviation weather products like METARs, TAFs, and area forecasts.

Why Fog Matters for Part 107 Operations

Under 14 CFR Part 107, a remote pilot must have at least 3 statute miles of ground visibility (107.51(a)) and must be able to see the sUAS at all times without the aid of vision-enhancing devices other than corrective lenses (107.31). Fog directly threatens both requirements. Even a patchy fog bank that the pilot can navigate around on the ground may completely obscure the sUAS once it is airborne. Beyond the legal minimums, loss of visual contact with the aircraft means the pilot cannot detect conflicts with manned aircraft, cannot judge the aircraft's attitude and flight path, and cannot avoid obstacles. The safety consequences are immediate and real.

Additionally, fog droplets can accumulate on small UAS components. Rotors, sensors, and camera gimbals are exposed directly to the saturated air, and moisture ingestion into electronic speed controllers or motors can cause malfunctions. Operating in or near fog can also compromise GPS signal accuracy if the aircraft is near the fog layer's top and experiences signal diffraction. Though not always discussed in exam prep materials, these airworthiness considerations reinforce why even partial fog is a no-fly condition for most sUAS operations.

Key Numbers and Rules

Fog defined: visibility less than 5/8 statute mile due to suspended water droplets at the surface (the FG vs. BR reporting threshold in METARs).
Part 107 minimum ground visibility: 3 statute miles under 107.51(a) — fog nearly always violates this requirement.
Dew point spread warning: temperature-dew point spread of 4°F (2°C) or less indicates high fog potential.
Radiation fog trigger: calm or light winds, clear skies, high humidity — common in early morning.
Advection fog wind range: generally forms with light to moderate winds up to roughly 15 knots; stronger winds tend to lift it into low stratus.
Fog vs. mist: visibility less than 5/8 SM = fog; 5/8 to 6 SM = mist (a distinct phenomenon from haze). Both degrade VLOS conditions.
Radiation fog dissipation: typically dissipates within a few hours after sunrise as ground heating resumes.

Memory Aid

To remember the five fog types, use the mnemonic RAUSP:

R — Radiation (ground cools overnight, calm and clear)
A — Advection (warm moist air moves over cooler surface, coastal)
U — Upslope (air forced up terrain, cools adiabatically)
S — Steam (cold air over warm water, evaporation)
P — Precipitation-induced / frontal (rain evaporates into cool dry air)

Pair each letter with its formation mechanism and you can reconstruct the full answer on any knowledge test question.

Common Test Traps

Confusing radiation and advection fog: Radiation fog forms under calm or light wind conditions and clears after sunrise. Advection fog requires wind and can form at any hour, day or night. Test questions often swap these details to catch unprepared students.
Assuming fog burns off quickly: Only radiation fog reliably dissipates after sunrise. Advection fog and frontal fog can persist all day — do not assume morning fog will always clear by your planned launch time.
Forgetting the dew point spread: A small temperature-dew point gap on a METAR or forecast is a fog precursor. The test may show you weather data with a 2°F spread and ask about likely hazards — the answer is fog formation risk.
Mist vs. fog legal distinction: The FAA test may present a scenario with 3/4 SM visibility and ask if Part 107 operations are legal. The answer is no — you need at least 3 SM. Students sometimes confuse mist thresholds with legal minimums.
Steam fog and airworthiness: Some questions address moisture effects on sUAS components. Operating near steam fog over water in cold conditions exposes the aircraft to rapid moisture accumulation, which is a legitimate operational hazard even if visibility technically exceeds 3 SM.