Long-Term Variability of Atmospheric Refractivity and Gradients from Tropics to Poles: A 31 Year Analysis

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 analyze vertical structures, seasonal patterns, year-to-year changes, and extreme conditions, and relate them to physical processes in the atmosphere that affect radio-wave behavior. 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. Year-to-year variations in tropical and arid regions are linked to ENSO events, while polar regions experience strong variability due to temperature inversions and Arctic warming. The vertical profiles indicate that the effect of moisture decreases with height, and refractivity values in the upper troposphere become similar across all climate zones, highlighting the layered structure of abnormal propagation. The study also finds that standard models, such as ITU-R P.453, tend to underestimate near-surface super-refraction in tropical areas and oversimplify conditions in arid and polar regions. 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.