Ask the meteorologist: What causes fog?

Question: What causes fog? — Qwest Cockman

Answer: Fog is essentially a cloud on contact with the ground, and occurs when a layer of air in contact with the ground either cools to the dew point temperature or below, thus becoming saturated with respect to water vapor, or when a layer of air touching the ground has moisture added it to it so that the dew point is raised to match the temperature, again saturating the air and leading to condensation of liquid water onto tiny particles in the air called condensation nuclei.

There are different types of fog based on the conditions present when the fog forms.

When the ground cools, giving off warm air, that is radiation fog.

Air flowing up hill and thereby cooling through expansion is upslope fog.

Moist air flowing from a warm location to a cooler location where the cooler surface reduces the temperature of the air moving over it is advection fog.

Rain falling through a layer of air near the surface and causing it to both cool down and become more moist is precipitation fog.

Moisture evaporating from a warm water body into cooler air above to a point where condensation of fog droplets occurs is steam fog.