New constants are derived for the equation relating the radio refractive index, n, at frequencies below 20 GHz to the pressure, temperature, and humidity of the air. The new equation is
where Pα is the partial pressure of dry air (mb), T is the absolute temperature (K), e is the partial pressure of water vapor (mb), and Zα−1 and Zw−1 are the inverse compressibility factors for dry air and water vapor, respectively. Improved values of the constants are obtained by considering the relationship between the radio and optical portions of the electromagnetic spectrum. With the use of the full three-term equation including non-ideal gas law effects, the accuracy of the new radio refractive index equation ranges from 0.018%, or about 0.05N, for dry air to 0.048%, or 0.21N, for extremely moist air.