A sensor apparatus includes a cylindrical sensor window defining an axis oriented substantially vertically, an air nozzle positioned below the sensor window and aimed upward, and a sensor-housing-top cap positioned above the sensor window and including a sensor-housing top and a circumferential cap. The sensor-housing-top cap is a single piece. The circumferential cap includes a topside and an underside. The topside and the underside are disposed radially outward from the sensor window. The underside includes a groove having a cross-section elongated circumferentially around the axis. The cross-section of the groove curves from a lower end upwardly and outwardly to an upper end.