When the ground is cooler than the air you get fog.
Ground is cooling faster than the air.
The water is cooler than the land and air sinks over the water
the water is cooler than the land, and air sinks over the water. ~Apex
Warm air is less dense than cooler air.
Water vapor in the air condenses when the ground cools at night. the cooler ground (often due to radiation into space when it is clear) cools the air near the ground. This cooler air cannot hold as much moisture as warm air and the moisture condenses as fog.
it decreases.
Fog would most likely occur when an air mass is resting on cooler ground. This is because the ground cannot absorb the moisture.
it sinks
it decreases.
Convection currents in the air cause winds to form. As warm air rises, cooler air rushes in to fill its space near the ground. The rising warm air becomes cooler and begins to fall. The cooler air near the ground becomes warmer and begins to rise.
It tends to decrease as it mixes with cooler air underneath. (Meanwhile it warms up the cooler air it blends into).
The cool air will sink and the warm air will rise.