Weather
Fog and visibility for drone pilots
Fog forms when air cools to its dew point near the ground, and it is likely when the temperature and dew point are within about 2 degrees C. Under Part 107 you need at least 3 statute miles of flight visibility from the control station, so fog, mist and haze reported below that mean no flight.
How does fog form?
Fog is a cloud whose base is at the surface. The Aviation Weather Handbook defines it as a visible aggregate of minute water droplets that reduces horizontal visibility to less than 5/8 statute mile. It forms when temperature and dew point become the same or nearly so. Air gets there in two ways: it cools to its dew point (radiation, advection and upslope fog), or moisture is added so the dew point rises (frontal fog and steam fog).
The Handbook says fog seldom forms when the temperature-dewpoint spread is greater than 2 degrees C (4 degrees F). In a METAR the pair appears as 12/11, which is a spread of 1. That is a warning sign, especially on a clear, calm night.
Sources (checked 2026-11-14): FAA Aviation Weather Handbook (FAA-H-8083-28B, 2026).
What are the types of fog?
| Type | How it forms | Wind and timing | Persistence |
|---|---|---|---|
| Radiation fog | Ground cools at night, air in contact cools to its dew point | Clear sky, light wind; typically a night and morning event, over land only | Usually burns off before noon; ground fog (very shallow) clears fast |
| Advection fog | Moist air moves over a colder surface | Needs wind; deepens up to about 15 kt, lifts into stratus above that; common on coasts | Can persist for days, can arrive any time of day |
| Upslope fog | Moist, stable air cools as it climbs sloping terrain | Winds of about 5 to 15 kt; can form under cloudy skies | Dense, extends to high altitude, may not burn off with the sun |
| Steam fog | Very cold air moves over warmer water | Cold autumn mornings over lakes and rivers | Shallow, with convective turbulence; can be dense |
| Frontal fog | Rain from a front saturates the cold air below | Mostly with warm fronts | Can last a long time and cover large areas |
The Pilot's Handbook adds ice fog (ice crystals instead of droplets, usually when it is -25 degrees F or colder) and notes that radiation fog less than 20 ft thick is called ground fog. The Aviation Weather Handbook also covers freezing fog, where supercooled droplets freeze on contact with surfaces at or below freezing. For drone pilots that is a surface-icing problem on top of a visibility problem.
Sources (checked 2026-11-14): FAA Aviation Weather Handbook (FAA-H-8083-28B, 2026); FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C).
Which fog is most likely where I fly?
- Inland valleys and low ground on clear nights: radiation fog, with cold air draining into valleys. The Handbook says valley fog is most common in autumn and spring and densest around sunrise.
- Coasts and large lakes: advection fog, especially when a moist onshore wind moves over a cooler surface.
- Eastern slopes of mountains: upslope fog.
- Rivers and lakes on cold mornings: steam fog.
Rain that soaks the ground followed by clearing skies is a classic setup for radiation fog the next morning.
Sources (checked 2026-11-14): FAA Aviation Weather Handbook (FAA-H-8083-28B, 2026).
How can I tell fog is coming from the METAR?
Watch the trend, not only the number. If the temperature is falling toward the dew point after sunset and the wind is light, visibility can drop within an hour. You can also estimate where cloud bases form. The PHAK method for unsaturated rising air: temperature falls 3 degrees C per 1,000 ft and dew point 0.5 degrees C, so they converge by 2.5 degrees C per 1,000 ft, or about 400 ft of cloud-base height for each degree of spread.
Worked example
Reading a spread
Say a METAR shows KXYZ 150553Z 00000KT 1 1/2SM BR OVC004 11/10 A3004. The spread is 1 degree C, the wind is calm and visibility is 1 1/2 SM in mist. Estimated cloud base from the spread alone would be around 400 ft, and the report shows overcast at 400 ft. It is below the Part 107 minimum and the trend points to fog. If the sky is clearing at dawn and the wind stays calm, expect the worst visibility near sunrise.
Also look at the TAF for BR, FG and TEMPO groups, and at AIRMET Sierra or the G-AIRMET IFR areas, which cover ceilings under 1,000 ft or visibility under 3 miles.
Sources (checked 2026-11-14): FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C); FAA Aviation Weather Handbook (FAA-H-8083-28B, 2026).
What is the difference between mist (BR) and fog (FG) in a METAR?
| Code | Meaning | Visibility |
|---|---|---|
| FG | Fog | Less than 5/8 SM |
| BR | Mist | 5/8 SM or more, up to under 7 SM |
| HZ | Haze | Tiny dry particles; coded when visibility is under 7 SM or when operationally significant |
| FU | Smoke | Combustion particles |
| VCFG | Fog in the vicinity | Between 5 and 10 SM of the station |
The AIM key lists BR as mist with visibility 5/8 SM or more and FG as fog with less than 5/8 SM. Mist is wetter than haze: mist has relative humidity of 95 to 99 percent and a gray veil, haze is dry and looks bluish or yellowish depending on the background (AWH Chapter 18). Obscurations are coded in the body of the report when surface visibility is below 7 SM or when operationally significant, and shallow (MI), partial (PR) and patchy (BC) fog are special cases. See the METAR and TAF cheat sheet and how to read a METAR.
Sources (checked 2026-11-14): AIM Chapter 7, Section 1: Meteorology; FAA Aviation Weather Handbook (FAA-H-8083-28B, 2026).
How does the 3 SM rule apply in fog and haze?
14 CFR 107.51(c) requires minimum flight visibility, observed from the location of the control station, of no less than 3 statute miles. The regulation defines flight visibility as the average slant distance from the control station at which prominent unlighted objects may be seen and identified by day, and prominent lighted objects by night. Part 107 also requires the drone to stay 500 ft below and 2,000 ft horizontally from clouds, and fog is a cloud on the ground.
An airport report of 2 1/2 SM HZ means conditions are below the minimum at that airport and a warning for your site. A report of 10 SM at an airport 8 miles away does not give you 3 SM in a valley fog pocket at your job. You judge the visibility at the control station. The AWH notes that haze can have a definite top with good air-to-air visibility above it, while slant range to the ground is poor, so a drone looking down through haze may lose sight of the landing area sooner than the pilot expects.
Rule of thumb for the test: visibility under 3 SM at the control station means you cannot operate under Part 107 without a waiver, whatever the cause: fog, mist, haze, smoke, dust or precipitation.
Sources (checked 2026-11-14): 14 CFR 107.51 (eCFR); FAA Aviation Weather Handbook (FAA-H-8083-28B, 2026); FAA: Part 107 Waivers.
How long does fog last?
It depends on the type. The Handbook says ground fog usually burns off rather rapidly after sunrise, and other radiation fog generally clears before noon unless clouds move in over it. Winds up to about 5 kt tend to deepen radiation fog by spreading the cooling through a deeper layer, while stronger winds disperse it or lift it into stratus. Advection fog is usually more extensive and much more persistent, and it can move in rapidly at any hour. Upslope fog is often quite dense and extends to high altitudes. The Pilot's Handbook says upslope and advection fog may not burn off with the morning sun and can persist for days.
For planning: a clear, calm night with a small spread means a morning window that closes as the fog forms around sunrise and opens later in the morning. A coastal fog bank that has been sitting offshore may return after dark.
Sources (checked 2026-11-14): FAA Aviation Weather Handbook (FAA-H-8083-28B, 2026); FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C).
What else should I know about frost and dew on the aircraft?
On cool, clear, calm nights the surface of a drone parked outside can fall below the dew point. If the dew point is below freezing, moisture deposits as frost. The Pilot's Handbook says frost disrupts airflow and can drastically reduce lift, and that aircraft must be cleaned of it before flight. Treat propellers and the airframe the same way.
Sources (checked 2026-11-14): FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C).
What do I do if fog rolls in during a flight?
Remember you are the one who must see the aircraft. If visibility at your control station drops under 3 SM or fog approaches the flight area, bring the drone back and land. Use return-to-home only if you understand how it behaves and do not rely on it as a substitute for visual contact. If you cannot see the aircraft, 14 CFR 107.31 requires visual line of sight, and the other pages on operating rules and weather cover the rest.
Sources (checked 2026-11-14): 14 CFR 107.31 (eCFR).
Free practice questions on this topic
1 original questions from the Sectional Cue bank. Try each one, then open the answer.
Frost formation
1. Under what conditions does frost form on a drone left outside overnight?
- The surface cools to a dewpoint that is below freezing
- Rain falls onto a surface that is colder than freezing
- Humid air touches a surface that stays above freezing
Show answer and explanation
A. The surface cools to a dewpoint that is below freezing
Frost forms when a surface is at or below the dewpoint of the adjacent air and that dewpoint is below freezing, so water vapor deposits directly as ice. Rain freezing on contact forms glaze ice, not frost. If the surface stays above freezing, moisture condenses as dew rather than frost.
Source: PHAK (FAA-H-8083-25C), Chapter 12
Related guides
- How to read a METAR for the Part 107 test
- METAR and TAF abbreviations cheat sheet for the Part 107 test
- Part 107 operating rules: altitude, speed, visibility and clouds
- Weather for drone pilots: what the Part 107 test expects
- All Part 107 guides
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Sources
- FAA Aviation Weather Handbook (FAA-H-8083-28B, 2026), checked 2026-11-14
- FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), checked 2026-11-14
- 14 CFR 107.51 (eCFR), checked 2026-11-14
- AIM Chapter 7, Section 1: Meteorology, checked 2026-11-14
How this page was made: written by Certloom Learning from the sources above, with the rules checked against the current regulation text. Practice items are original questions from the app's reviewed bank. Spot an error? certloomlearning@gmail.com.