Bending of radio waves near the surface of the Earth by temperature or humidity gradients

What is it about?

Radio waves do not travel exactly in straight lines through the air because the refractive index of air is around 300 parts-per-million greater than 1 near the ground, and depends on temperature, pressure, and humidity. Vertical gradients of humidity or temperature due to weather conditions make the rays bend up or down, which can seriously affect radio-communication or radar systems. These gradients generally become stronger approaching the surface, and a logarithmic model is conventionally assumed, but this assumes a neutral atmosphere. This paper expands the logarithmic model to cater for unstable atmospheres where temperature gradients encourage convection, and stable atmospheres where temperature gradients go the other way and discourage vertical movement of air. In the case of sub-refraction, where radio waves bend upwards, we show that non-linear refractivity profiles from our model can affect radio propagation more severely than commonly assumed linear refractivity profiles.

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Why is it important?

This paper provides a more general model of radio ducting than has been used to date, as it takes atmospheric stability into account, as well as duct height or strength. This provides greater flexibility in fitting a model to data from propagation experiments, which may lead to improved radio propagation predictions.