Plain-language answer: Atmospheric refraction is gradual bending of radio waves as air refractive index changes with height.
Why it changes the path
Normal refraction slightly extends the radio horizon, while unusual temperature and humidity gradients can enhance, reduce, or duct VHF/UHF coverage.
What to observe
Record weather and time with unexpected paths, and avoid treating a temporary enhancement as normal system range.
The common mistake
Using the same effective-Earth-radius assumption in every atmosphere.
Planning boundary
Propagation models and field regions depend on frequency, geometry, terrain, atmosphere, polarization, antenna pattern, and measurement distance. Use them to plan and compare—not to guarantee a contact or installed range.
