This paper describes a seven‐pound device that indicates changes in the refractive index of a gas. Because it is capable of detecting refractivity fluctuations that are as small as one part per million, it is useful in studying the irregularities in the refractive index of the troposphere that are important in short radio‐wave transmission. The refractometer is self‐contained, so that it may be used in controlled or in free ascent through the atmosphere. The device is essentially a high frequency oscillator whose frequency is governed by the capacitance of an air sensing condenser. Special provision is made for reliable operation over the wide range of temperatures normally found in the troposphere.