What airport condition is reported by the tower when more than one wind condition at different positions on the airport is observed?

Wind shear is the correct choice when describing an airport condition in which more than one wind condition is observed at different positions on the airport. Wind shear refers to a sudden change in wind speed or direction over a relatively short distance, which can occur both horizontally and vertically. This phenomenon is critical to understand as it can greatly affect aircraft performance, particularly during takeoff and landing phases.

When the control tower reports wind shear, it indicates that pilots may experience varying wind conditions as they maneuver around the airport, possibly facing unexpected changes that could impact their approach or departure. Understanding the presence of wind shear helps pilots to anticipate and respond effectively to potential hazards associated with these changing wind conditions.

The other options do not accurately represent the scenario of experiencing multiple wind conditions across different areas of the airport. A light and variable wind condition would imply little to no significant change in wind direction or speed. A frontal passage suggests the advance of a weather front that typically brings more uniform weather changes rather than localized wind discrepancies. Calm winds indicate no significant wind activity at all, which would not account for the variability described in the question.