This chapter considers the effects of refractive index variations on the propagation of radiowaves in the troposphere, and in particular those mechanisms which lead to propagation beyond the normal line of sight. Clear air implies that the effects of condensed water (clouds, rain etc.) are ignored, although gaseous absorption is included. The frequencies of interest are above about 100 MHz; below this frequency refractive index variations are not strong enough to cause significant effects, and the ground wave and ionospheric mechanisms dominate at transhorizon ranges. The emphasis is on the meteorological mechanisms that give rise to anomalous propagation, and the basic models that have been developed to predict the effects of refractive index variations on radiowave propagation.

Inspec keywords: tropospheric electromagnetic wave propagation

Other keywords: refractive index variations; meteorological mechanisms; ground wave mechanisms; ionospheric mechanisms; radiowave propagation

Subjects: Electromagnetic wave propagation and interactions in the lower atmosphere; Radiowave propagation