humidity.
How does fog form?
* Fog forms when the air near the ground becomes saturated and condensation occurs on tiny particles suspended in the air.
* Fog forms in stable air with light winds, high relative humidity and conditions which bring the air to its dew point.
in other words: Fog is cloud in contact with the ground. It occurs when moisture from the surface of the Earth evaporates; as this evaporated moisture moves upward, it cools and condenses into the familiar phenomenon of fog. Fog differs from cloud only in that fog touches the surface of the Earth, while clouds do not.