When should a special observation be made regarding prevailing visibility?

A special observation regarding prevailing visibility is required when there is a significant change that could impact flight operations. The criteria for when to make these observations are set to ensure that pilots have access to updated and relevant visibility information.

The correct choice is when prevailing visibility changes from 1/2 to 1 statute mile. This threshold represents a substantial alteration in visibility that could influence a pilot's ability to operate an aircraft safely. The change from half a mile to one mile is considered significant because it can affect takeoff, landing, and other flight operations.

Changes in visibility that do not meet this threshold – such as from 0 to 1 statute mile, 2 to 3 statute miles, or 1 to 2 statute miles – do not present the same level of immediate concern for aviation safety. While they indicate that visibility is improving, they do not represent the same magnitude of concern as the shift from 1/2 to 1 mile, thus they do not necessitate a special observation. Ensuring that pilots have the most accurate visibility information helps them make safer decisions while flying.