Fog Likelihood Calculator
Fog forms when the air temperature cools to the dewpoint, reaching saturation and causing water vapor to condense as tiny droplets. The dewpoint depression (temperature minus dewpoint) is the primary indicator of fog potential: when this difference falls to 4 F (2 C) or less, fog becomes likely. Meteorologists and pilots check dewpoint depression routinely as a quick fog-probability check. Enter the current temperature and dewpoint to see the dewpoint depression, relative humidity, and fog likelihood assessment.
Fog likelihood formula
Dewpoint Depression = T - Td (F)
Fog Likely: depression <= 4 F (2 C)
Fog Possible: depression 5-9 F (2.8-5 C)
Fog Unlikely: depression > 9 F (5 C)
T is air temperature and Td is dewpoint temperature, both in degrees Fahrenheit. Relative humidity is computed using the Magnus formula approximation. The 4 F threshold is used by aviation meteorology and NWS operational forecasters. This is a necessary but not sufficient condition: fog also requires a cooling mechanism or moisture source.
Factors affecting fog formation
- Clear sky overnight: promotes radiative cooling, lowers temperature toward dewpoint.
- Light winds (2-10 mph): mix moist surface air gently; ideal for radiation fog development.
- Moist soil or nearby water: provides moisture flux to maintain high dewpoints.
- Cold ocean or lake surface: cools advected warm moist air, creating sea fog.
- Calm winds after rain: saturated surface evaporates moisture into stable surface air.
Fog likelihood: frequently asked questions
What dewpoint depression indicates fog is likely?
A dewpoint depression (temperature minus dewpoint) of 4 F (2 C) or less is the standard meteorological threshold for high fog probability. When temperature equals dewpoint (depression = 0), air is fully saturated and fog is essentially certain, assuming a surface or moisture source. Aviation weather uses this threshold to assess low-visibility potential.
What is radiation fog vs advection fog?
Radiation fog forms overnight when clear skies allow the ground to radiate heat away, cooling the surface air to the dewpoint. It typically burns off after sunrise. Advection fog forms when warm moist air moves over a cooler surface (land or ocean), cooling to the dewpoint. Advection fog can persist through the day and is common along coastlines and in the spring when warm air moves over cold ground.
How does wind affect fog formation?
Light winds (2-10 mph) mix the near-surface air gently and allow fog to deepen. Calm winds allow ground-level fog to form but limit vertical mixing. Strong winds (above 15 mph) typically prevent radiation fog by mixing the moist surface air with drier air aloft, but may not prevent advection fog if the moist air layer is deep.
What does NWS use to forecast fog?
The NWS considers dewpoint depression, sky condition (clear skies promote radiation cooling), wind speed, soil moisture, and the moisture flux from nearby water bodies. Dense fog advisories are issued when visibility is forecast to fall to one-quarter mile or less over a wide area. Patchy fog is expected when conditions are borderline.
Does relative humidity alone predict fog?
Relative humidity approaching 100% is a necessary but not sufficient condition for fog. The air must also be in contact with a cool surface or cooling mechanism. High humidity in a warm air mass aloft does not cause ground fog. Conversely, fog can form at relative humidity slightly below 100% if condensation nuclei are abundant (pollution fog).
Official sources
- NOAA/NWS: Dense Fog Safety.
- FAA Aeronautical Information Manual: AIM Chapter 7: Safety of Flight.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.