A special observation is necessary when the prevailing visibility changes from what to what statute miles?

A special observation is required when there is a significant change in prevailing visibility, specifically when it changes from 2 statute miles to 3 statute miles. This interval is important because a change in visibility within this range can materially affect the safety and operations of aircraft, especially in terms of approach and landing procedures.

Visibility is a crucial factor in aviation as it determines how far a pilot can see and, therefore, how well they can navigate, identify other aircraft, and make safe landing decisions. When visibility decreases or increases in this range, it signals a noteworthy change in the atmospheric conditions that could impact flight safety and effectiveness. Therefore, reporting such changes is essential to keep all personnel informed and ensure proper management of flight operations during those conditions.

The other options represent visibility changes that are either too minor to warrant a special observation or do not fall within this key range that directly affects operational decisions.