The main cause of fog is water vapor or water in it's gaseous form condensing.
During condensation, the molecules of water vapor combine to make tiny liquid water droplets which hang in the air.
And because of the tiny water droplets you can see the fog.
The water vapor itself is a gas that is invisible.
And steam fog forms when cold air moves over warm water.
When the cool air mixes with the warm moist air over the water, the moist air cools until its humidity reaches 100% and fog forms.
Warm air, air blows in from the south and if there is snow or cool moisture on the ground it will come in contact with the warm, moist winds.
This contact between the air and ground will cause the air blowing in to become cool.
Then dew point rises and creates high humidity and forms fog.
Fog normally occurs at a relative humidity near 100%.
This occurs from either added moisture in the air, or falling ambient air temperature. However, fog can form at lower humidity, and can sometimes fail to form with relative humidity at 100%.