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