Which condition would require a site-specific visibility observation?

A site-specific visibility observation is necessary when there are rapid changes in visibility due to weather. This is critical because quickly changing weather conditions, such as incoming storms, fog development, or other atmospheric phenomena, can significantly impact visibility in a localized area. Understanding these fluctuations is essential for pilots and other individuals relying on accurate weather information, as it directly affects flight safety, navigation, and operational decision-making.

When visibility is stable, as noted in one of the other options, it does not require such specific observation since the conditions are consistent and predictable. Likewise, visibility over 3 miles or under 1 mile can often be assessed using standard observation techniques without needing to conduct site-specific measurements, unless specific local factors suggest otherwise.