This question is related to the following link and Longley-Rice Propagation Model
https://www.itu.int/dms_pubrec/itu-r/rec/p/R-REC-P.453-11-201507-S!!PDF-E.pdf
In the Longley-Rice Propagation Model the surface refractive index is taken into account and it also discussed how it is dependence on height. The question is: how does it affect different frequencies? I could not find any literature explaining its characteristic related to frequency? Any suggestion how to understand the effect of earth surface refractive affect RF propagation,?