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Have you done any testing to see if rain mimics  the density of heavy fog? Do they care?

One set of tests  indicates Mie scattering causes more reflectivity with particle sizes near wavelength

Effects of Fog

• UV:        Transmissivity  <5–15%, reflectivity 60–80%.
• Visible:   Transmissivity  10–25%, reflectivity 40–70%.
• Near-IR: Transmissivity  30–60%  reflectivity 20–40%.
• Mid-IR:  Transmissivity   <10%,       reflectivity <20%.

I have made optical test equipment in my career.

 I would choose a reflectivity method into a clear view and use pulse mode for both UV and IR to differentiate between fog and rain, the latter which will modulate the results more.