I have experienced several times that heavy rain seems to jam the signal a radio of a bus receives. The only explanation I have is that the rain drops also become weak emitters, if a radio wave hits the rain drop. The stronger the noise, the lower the SNR becomes.
I had another thought, in which the frequency of the signal is altered depending on the density of the rain in the air. I would ignore the distance from emitter to receiver as it should be about the same all the time. At least as long as you are not moving directly towards or away from it. I don't really understand what process is happening there, so I would be very happy, if someone could help me.