Short review on the refractive index of air as a function of temperature, pressure, humidity and ionization

TL;DR

This paper reviews how the refractive index of air varies with temperature, pressure, humidity, and ionization, emphasizing the limitations of classical laws and the importance of wavelength-dependent effects for high-precision applications.

Contribution

It provides a comparative analysis of various models and highlights the significance of humidity and ionization effects across different wavelengths for accurate refractive index estimation.

Findings

Hygrometry has a small effect at visible wavelengths.

Humidity effects are significant at infrared and radio wavelengths.

Ionospheric curvature critically affects astronomical refraction at radio wavelengths.

Abstract

The empirical law of Gladstone-Dale is insufficient for high-precision studies using the refractivity of a gas: this is not exactly proportional to its density, and the gas may not be properly described as perfect. An optical Mariotte temperature allows making a comparative analysis of the results given by various authors. The effect of hygrometry on the refractivity at visible wavelengths is historically traced and its small effect on the astronomical refraction angle numerically shown. Finally at infrared and radio wavelengths, the effects of the humidity in the lower atmosphere can be strong; as for the ionosphere, its curvature plays an essential role for the astronomical refraction angle unlike in the visible.