How is fog typically formed?

Fog is typically formed when the air temperature cools to the dew point, which is the temperature at which air becomes saturated with moisture and condensation occurs. When the temperature drops to this level, water vapor in the air condenses into tiny water droplets, creating the visible phenomenon known as fog. This cooling can happen in various ways, such as through radiational cooling at night or as a result of warm, moist air moving over colder surfaces.

The other choices do not accurately describe the fundamental process of fog formation. For instance, increasing air temperature above the dew point leads to evaporation and drying, not fog. Warming air through solar heating generally prevents saturation and thus does not promote fog. Mixing moist and dry air masses may create clouds or precipitation under certain circumstances, but it does not directly lead to the conditions necessary for fog unless it results in temperatures dropping to the dew point.