FAA-H-8083-28B, ch. 18

Types of fog and how they form

Fog is a cloud on the ground, and the FAA names each type by how it forms. Know which ones need wind and which ones need calm air.

Part 107, explained › Weather · Last reviewed October 25, 2026 against the eCFR and faa.gov. UAV Campus is not affiliated with the FAA.

What are the types of fog, and how do they form?

Fog types are named for how they form. Radiation fog forms over land when the ground cools at night; advection fog when moist air moves over a colder surface; upslope fog when moist, stable air cools as it climbs terrain; frontal fog when precipitation saturates cold air; steam fog when cold air crosses warm water.

Fog forms when the temperature and dewpoint of the air become identical or nearly so, either by cooling the air to its dewpoint (radiation, advection and upslope fog) or by adding moisture (frontal and steam fog). The favorable conditions for radiation fog are a shallow layer of moist air, clear skies and light surface winds. It is restricted to land, and stronger winds disperse it.

Advection fog and upslope fog need wind. Advection fog deepens as wind speed increases up to about 15 knots, and stronger wind lifts it into low stratus. Upslope fog is favored by winds of 5 to 15 knots and, unlike radiation fog, can form under cloudy skies. Steam fog brings convective turbulence, and low-level turbulence and icing are commonly associated with it.

For a Part 107 flight, fog cuts visibility, and the 3 statute mile minimum is explained on our Part 107 visibility and cloud clearance page. The temperature-dewpoint spread is the clue that fog may form.

Sources: FAA-H-8083-28B, p. 222 · FAA-H-8083-28B, p. 223 · FAA-H-8083-28B, p. 224 · FAA-H-8083-28B, p. 226 · FAA-H-8083-28B, p. 227 · FAA-H-8083-25C (PHAK), p. 299

What the FAA says

FAA-H-8083-28B, p. 222

Fog forms when the temperature and dewpoint of the air become identical (or nearly so).

FAA-H-8083-28B, p. 223

Factors favoring the formation of radiation fog are: 1) a shallow surface layer of relatively moist air beneath a dry layer, 2) clear skies, and 3) light surface winds.

FAA-H-8083-28B, p. 224

Advection fog (see Figure 18-5 and Figure 18-6) forms when moist air moves over a colder surface and the subsequent cooling of that air to below its dewpoint.

FAA-H-8083-25C (PHAK), p. 299

Steam fog, or sea smoke, forms when cold, dry air moves over warm water. As the water evaporates, it rises and resembles smoke. This type of fog is common over bodies of water during the coldest times of the year. Low-level turbulence and icing are commonly associated with steam fog.

Sources: FAA-H-8083-28B, p. 222 · FAA-H-8083-28B, p. 223 · FAA-H-8083-28B, p. 224 · FAA-H-8083-25C (PHAK), p. 299

Exam traps

  • Radiation fog forms with clear skies and light winds, while advection fog and upslope fog both need wind to form.
  • Radiation fog is restricted to land because water surfaces cool little from nighttime radiation.
  • Steam fog, not radiation fog, is the type associated with low-level turbulence and icing.
  • Advection fog is usually more extensive and much more persistent than radiation fog, and it can move in rapidly at any time of day or night.

Quick quiz: 3 questions

Similar to the FAA test, not the actual test questions. Each answer shows its FAA source.

Question 1 of 3

What types of fog depend upon wind in order to exist?

  1. A Radiation fog and ice fog.
  2. B Advection fog and upslope fog.
  3. C Steam fog and ground fog.
Show the answer
Answer: B. Advection fog and upslope fog.

Advection fog requires wind since the moist warm air mass needs to be pushed in land. Upslope fog also requires wind since the air mass must be pushed upslope to be cooled off.

FAA sources: FAA-H-8083-25C ch. 12, Fog · FAA-H-8083-25C ch. 12, Fog

Question 2 of 3

Low-level turbulence can occur and icing can become hazardous in which type of fog?

  1. A Rain-induced fog.
  2. B Upslope fog.
  3. C Steam fog.
Show the answer
Answer: C. Steam fog.

One of the characteristics of steam fog is the possibility of icing and low-level turbulence.

FAA source: FAA-H-8083-25C ch. 12, Fog

Question 3 of 3

An air mass moving inland from the coast in winter is likely to result in

  1. A rain.
  2. B fog.
  3. C frost.
Show the answer
Answer: B. fog.

If an air mass moves inland in the winter, it is likely a moist air mass, and warmer than the land. In which case, it will be cooled off when it moves inland, resulting in fog.

FAA source: FAA-H-8083-25C ch. 12, Fog

When does radiation fog clear?

Radiation fog is generally a nighttime occurrence and often does not dissipate until after sunrise. Ground fog usually burns off rather rapidly after sunrise, and other radiation fog generally clears before noon unless clouds move in over the fog.

Sources: FAA-H-8083-28B, p. 222 · FAA-H-8083-28B, p. 223

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