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SESK: Stuff Everyone Should Know · sesk.info/ifr-fog-temp-dewpoint

Fog seldom forms when the temperature and dewpoint spread is wider than about what?

About 2 degrees C (4 degrees F).

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  1. 10 degrees C (18 degrees F)
  2. 5 degrees C (9 degrees F)
  3. 2 degrees C (4 degrees F) ✔

Why: Fog forms when temperature and dewpoint become identical or nearly so. That can happen by cooling the air to its dewpoint or by adding moisture. A spread wider than about 2 degrees C usually means fog is unlikely.

Huh, didn't know that: Radiation fog is favored by a shallow moist layer under dry air, clear skies and light surface winds.

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Why did the fog get bad grades? Because it was too dense.
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References

Answer: 2 degrees C (4 degrees F). Fog forms when temperature and dewpoint become identical or nearly so. That can happen by cooling the air to its dewpoint or by adding moisture. A spread wider than about 2 degrees C usually means fog is unlikely.

  1. FAA Aviation Weather Handbook (FAA-H-8083-28B) www.faa.gov/sites/faa.gov/files/FAA-H-8083-28B.pdf
    Fog seldom forms when the temperature-dewpoint spread is greater than 2°C (4°F).
    Checked 2026-10-11.
  2. FAA Aviation Weather Handbook (FAA-H-8083-28B) www.faa.gov/sites/faa.gov/files/FAA-H-8083-28B.pdf
    radiational cooling reduces the air temperature to or below its dewpoint.
    Checked 2026-10-11.
  3. FAA Aviation Weather Handbook (FAA-H-8083-28B) www.faa.gov/sites/faa.gov/files/FAA-H-8083-28B.pdf
    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.
    Checked 2026-10-11.

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