Since the year 2000 the radio detection technique has become a well established method to probe cosmic-ray-induced air showers. In this work, we discuss the coherent transition radiation produced when the particle cascade crosses a boundary surface. Standard transition radiation is the emission induced due to a net charge crossing between different media. We show that a similar effect is expected for a net charge-neutral transverse current crossing a boundary. We discuss the implications of this transition radiation component for the coherent radio emission from a high-energy cosmic-ray-induced particle cascade hitting Earth. We show that this component cannot be neglected for balloon-based experiments such as ANITA, or detectors below the boundary surface such as ARA.