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SESK: Stuff Everyone Should Know · sesk.info/aero-fog-near-dew-point

Which conditions make fog most likely?

Temperature near the dew point and light wind favor fog.

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  1. Dry air and strong wind
  2. Warm air far above dew point
  3. Temperature near dew point with light wind ✔

Why: Air near its dew point is nearly saturated. With light wind, condensed droplets can remain gathered near the ground instead of mixing away.

Huh, didn't know that: A common rule of thumb: fog forms when temperature and dew point are within five degrees and winds are five knots or less.

The dad joke
Fog loves a close race because temperature and dew point finish together.
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References

Answer: Temperature near dew point with light wind. Air near its dew point is nearly saturated. With light wind, condensed droplets can remain gathered near the ground instead of mixing away.

  1. National Weather Service: Fog Ingredients www.weather.gov/source/zhu/ZHU_Training_Page/fog_stuff/forecasting_fog/FORECASTING_FOG.htm
    An afternoon shower can cause the dewpoint depression to be near zero in the evening. If skies then clear and wind is light, fog is very likely.
    Checked 2026-10-10.
  2. National Weather Service Fog Forecasting Guide www.weather.gov/media/zhu/ZHU_Training_Page/fog_stuff/fog_guide/fog.pdf
    Fog forms when the temperature and dew point of the air approach the same value (i.e., dew-point spread is less than 5°F)
    Checked 2026-10-10.

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