A lithographic apparatus includes a radiation system for providing a beam of radiation, and a first support for supporting a patterning device. The patterning device serves to pattern the beam of radiation. The apparatus includes a second support for supporting a substrate, a projection system for projecting the patterned beam onto a target portion of the substrate, and an interferometer measurement system for providing an interferometric measurement beam extending along an axis in an elongated volume of gas extending below the projection system. The apparatus also includes a gas conditioningstructure for providing a conditioned gas flow in the volume. The gas conditioning structure includes a plurality of gas guiding vanes disposed at an outlet of the structure for guiding the gas flow to the volume. The gas guiding vanes are shaped continuously and are oriented diverging away from the axis of the volume.