Clear night + calm air + temperature within ~2.5 °C of the dew point = expect fog.
Radiative cooling drops the surface air to its dew point and water condenses out. The smaller the spread, the less cooling is needed.
Those conditions make radiation fog, which is only one kind. Advection fog forms in strong wind when warm moist air crosses a cold surface — the calm-air condition is exactly wrong for it. Sea fog rolling in at 20 knots breaks the rule while proving the mechanism.
Estimate with the rule, then check it against the calculator that models it properly.
Open Psychrometer →Clear night + calm air + temperature within ~2.5 °C of the dew point = expect fog. Radiative cooling drops the surface air to its dew point and water condenses out.