In the context of visibility measurements, what does "greatest distance visible into total obscuration" refer to?

The phrase "greatest distance visible into total obscuration" refers specifically to vertical visibility. Vertical visibility is defined as the maximum distance that can be seen vertically through an obscuring phenomenon, such as fog or snow, that may affect one's ability to see upwards. In meteorological terms, when total obscuration occurs, such as during thick fog, the vertical visibility provides a measure of how far one can see vertically into that obscured environment.

This distinction is critical because while the other options refer to various aspects of visibility, they do not specifically pertain to the context of vertical measurements in obscuring conditions. Runway Visual Range pertains to horizontal visibility available for pilots during takeoff and landing, prevailing visibility is the maximum visibility in a particular direction or across a specified area, and variable visibility refers to fluctuating visibility conditions over time or space. However, none directly apply to measuring visibility into an obscured vertical perspective as described in the question.