  Terahertz (THz) communication channels, integral to outdoor applications, are
critically influenced by natural factors like rainfall. Our research focused on
the nuanced effects of rain on these channels, employing an advanced rainfall
emulation system. By analyzing key parameters such as rain rate, altitude based
variations in rainfall, and diverse raindrop sizes, we identified the paramount
significance of the number of raindrops in the THz channel, particularly in
scenarios with constant rain rates but varying drop sizes. Central to our
findings is a novel model grounded in Mie scattering theory, which adeptly
incorporates the variability of raindrop size distributions at different
altitudes. This model has displayed strong congruence with our experimental
results. In essence, our study underscores the inadequacy of solely depending
on a fixed ground-based rain rate and emphasizes the imperative of calibrating
distribution metrics to cater to specific environmental and operational
contexts.
