Fog point

(strictly fog-point temperature): is the air temperature (as measured in a standard thermometer screen) at which fog is expected/does form. Its calculation (before an event) is usually based on empirical work which employs either the surface air temperature/dew point at some time earlier in the day, or by construction on a thermodynamic diagram. The fog point is lower than the air-mass dew point, because as air cools through the evening and night, moisture is condensed out on contact with the chilled land surface, and this lowers the dew point from afternoon values.