This paper describes a refractometer for measuring small changes in the dielectric constant or index of refraction of atmospheric air. The method employed is that of measuring the change in phase of a 9375 megacycle per second radio wave along a path approximately three feet long caused by a change in the average dielectric constant of the air over the path.The test path may be confined to a wave guide through which air is drawn, or it may be the space between two antenna systems. Because of the greater rigidity and the elimination of reflections, the wave guide system is able to measure short time changes in the index of refraction with an error of less than one part in a million. The antenna arrangement usually has somewhat larger error because of the flexibility of the supports and the external reflections.