yes, rain causes more loss on the radio path:
-on air interface, cell coverage is reduced (max -3dB, in very heavy rain)
– on abis, if microwaves are used, links are lost intermittently
* water in the antenna feeders/connectors -> it reduces the coverage even more
* a statistical effect : more calls are made indoor or in-car, less calls are made outdoor -> rxlev is lower -> call drops occur more frequently.