What is the required observation for vertical visibility if there is fog present?

When fog is present, a special observation is required for vertical visibility. This type of observation is necessary because fog significantly reduces visibility, which in turn can affect aviation safety. Vertical visibility specifically pertains to how far an observer can see directly upwards, which is crucial for determining if aircraft can safely operate under the prevailing weather conditions.

Special observations are utilized in situations where the weather conditions change rapidly or when specific phenomena, such as fog, present unique challenges. These observations ensure that updated and accurate information is available to pilots and air traffic controllers, allowing for informed decision-making regarding flight operations.

Routine observations may not adequately capture the conditions of reduced visibility caused by fog, as they follow a standard schedule without the urgency necessitated by deteriorating weather. Automated observations can offer valuable data but may not always account for the nuanced impacts of fog on visibility. Daily observations, while important, do not provide the timely updates needed in situations where weather is fluctuating rapidly due to fog.