<p>Atmospheric refractivity and its vertical changes play an important role in how radio waves travel, but detailed studies across different climate regions are still limited. This study provides a long-term analysis of refractivity, its vertical gradients, and k-factor changes across five major climate zones: tropical, arid, temperate, subarctic, and polar. Using several decades of atmospheric data, we analyse vertical structures, seasonal patterns, year-to-year changes, and extreme conditions, and relate them to physical processes in the atmosphere that affect radio-wave behaviour. The results show that tropical regions have the strongest negative refractivity gradients near the surface (– 54.6 N/km) and the highest occurrence of super-refraction, mainly due to the combined effects of high humidity and temperature, especially during wet-season convection. In contrast, arid and polar regions are mainly influenced by temperature, showing more frequent sub-refraction and weaker seasonal changes. Temperate and subarctic regions show mixed characteristics, influenced by both moisture and temperature layering. In addition, we provide a direct evidence for the influence of large-scale climate variability, particularly El Niño, on refractivity gradients. Specifically, we examined the relationship between the refractivity gradient and the Niño 3.4 Index, and a clear correspondence between the temporal variations of the refractivity gradient and the Niño 3.4 Index is observed, providing independent support for our interpretation and strengthening the link between ENSO-related climate variability and refractivity gradient changes. We also demonstrate the effect of surface-pressure masking on refractivity gradient estimates. These findings highlight the need for climate-specific refractivity models and provide useful guidance for improving radio-wave prediction and communication system design in different environmental settings.</p>