What atmospheric condition is essential for fog to form?

Fog forms when air becomes saturated with moisture, leading to countless tiny water droplets condensing out of the air. High humidity is crucial because it indicates that the air has a significant amount of moisture. When the humidity level reaches nearly 100%, the air can't hold any more water vapor, resulting in fog formation. Typically, fog occurs when the temperature drops, allowing the air to cool and reach its dew point, at which point saturation happens. Thus, high humidity directly facilitates the conditions necessary for fog to form.

While cool temperatures can contribute to fog formation, it is the humidity that is the fundamental requirement. In contrast, high temperatures and strong winds would generally disperse fog rather than support its formation. Therefore, high humidity stands out as the essential condition.