Fog Formation Likelihood Calculator
Fog forms when air cools until it reaches its dew point and becomes saturated, so the closer the air temperature is to the dew point, the more likely fog or low cloud becomes. This calculator takes the air temperature and dew point in degrees Celsius, computes the dew point depression (their difference), estimates the relative humidity from the standard Magnus saturation vapor pressure approximation, and reports a plain-language likelihood band. Use it as a thermodynamic screening indicator alongside your official local forecast rather than as a substitute for it.
Fog likelihood formula
Dew point depression = temperature - dew point
Saturation pressure e_s(T) = 6.112 * exp(17.62 * T / (243.12 + T))
Relative humidity = 100 * e_s(dew point) / e_s(temperature)
Margin = threshold - dew point depression
Fog likely when depression <= threshold (about 2.5 C)
The Magnus coefficients 17.62 and 243.12 degrees C are the WMO-recommended values for vapor over water. A depression at or below the threshold with relative humidity near 100 percent indicates near-saturation and a high chance of fog or mist.
Fog context
- Radiation fog forms on clear, calm nights as the ground and air cool toward the dew point.
- A dew point spread of about 2.5 degrees C (4 degrees F) or less is a common forecaster screening rule for fog or low cloud.
- Light wind helps mix moisture into a shallow layer; strong wind tends to disperse fog into low stratus.
- Relative humidity here is derived from temperature and dew point, so it tracks how near the air is to saturation.
- This is a thermodynamic indicator only and does not replace an official National Weather Service forecast.
Fog formation: frequently asked questions
How does the temperature and dew point spread predict fog?
Fog forms when the air temperature cools to near the dew point, so the air becomes saturated and water vapor condenses into suspended droplets. As a rule of thumb used by forecasters, when the spread between temperature and dew point is about 2.5 degrees Celsius (about 4 degrees Fahrenheit) or less, fog or low cloud becomes likely. The smaller the spread, the higher the likelihood.
What is the dew point depression?
The dew point depression is simply the air temperature minus the dew point temperature. It is a direct measure of how close the air is to saturation. A depression of zero means the air is saturated (100 percent relative humidity) and fog or cloud is present. A large depression means dry air and clear skies.
Does relative humidity alone tell me if fog will form?
Relative humidity is closely related: fog usually requires relative humidity at or very near 100 percent. This calculator estimates relative humidity from temperature and dew point using the standard Magnus approximation of saturation vapor pressure. Humidity above roughly 95 percent combined with a small dew point spread strongly favors fog.
Why is this a likelihood and not a forecast?
Fog formation also depends on wind speed, mixing, radiative cooling, terrain, and the presence of condensation nuclei. This tool only evaluates the thermodynamic closeness to saturation. Treat it as a screening indicator, not an official forecast. Always check your National Weather Service local forecast.
What temperature units does this use?
Enter temperature and dew point in degrees Celsius. The Magnus formula constants used here are defined for Celsius. If you have Fahrenheit values, convert them first: Celsius equals (Fahrenheit minus 32) divided by 1.8.
Official sources
- NOAA National Weather Service: Weather Calculators and formulas.
- American Meteorological Society: Glossary of Meteorology.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.