How can a Surface Weather Observer determine if fog is present?

A Surface Weather Observer can determine if fog is present primarily by observing visibility and humidity levels. Fog is characterized by the presence of tiny water droplets suspended in the air near the ground, which significantly reduces visibility. Specifically, fog is typically defined as occurring when visibility drops below 1 mile. Additionally, fog often forms in conditions of high humidity, usually when the relative humidity approaches 100%, as this indicates that the air is saturated with moisture.

By combining visibility observations with humidity measurements, the observer can confirm the presence of fog. For instance, even if fog is not physically visible, if the visibility is detected to be low and humidity is evaluated to be high, it is reasonable to conclude that fog may be affecting the area. This approach is more directly associated with fog discernment than measuring temperature changes, checking atmospheric pressure, or noting changes in wind direction, which do not provide the necessary information for fog identification.