What is the primary cause of fog formation according to its definition?

Fog formation primarily occurs when the temperature and dew point are close or identical. This situation leads to a saturation of air with moisture, resulting in condensation. When the air temperature decreases to a point that is equal to the dew point, the air becomes saturated and cannot hold all the moisture in vapor form. As a result, tiny water droplets form, leading to the visibility-reducing phenomenon we recognize as fog.

The presence of high humidity is generally a contributing factor to fog, but it must be paired with a temperature that reaches the dew point to create the conditions necessary for fog formation. Other choices, such as low humidity, clouds, or significant drops in air pressure, do not directly pertain to the primary mechanism that leads to fog. Low humidity is more likely to inhibit fog formation rather than promote it, while clouds and changes in air pressure are related to weather systems but do not specifically define fog formation.