Introduction dew point

Fog can be a dangerous weather phenomena.
Fog may form either when there is a lot of moisture near the ground, or when the air near the ground is cooled to its dew point.
But what is the dew point? Scientifically speaking, the dew point is the temperature to which an air parcel must be cooled, assuming that the pressure and water vapor content are constant, in order for saturation to occur.
In simple terms, at the dew point the air contains as much moisture as it can hold.
When the air near the ground cools to this temperature, water vapor from the air will become visible as dew on the ground or as fog in the air.