Rain occurs when moisture in the atmosphere condenses on small particles of dust. This seeding of rain drops depends on the factors that effect condensation: temperature, humidity, pressure and surface chemistry. The latter hygroscopic effect may cause rain to initiate when relative humidity is less than 100%. Dust particles may, on occasions, have a lower temperature than the surrounding air and this can also cause precipitation. The seeded droplets are heavier than air and begin to fall, sometimes into air where the relative humidity is less than 100%.
Some sources suggest that the relative humidity is typically between 60% and 100% when it is raining. However, humidity is likely to increase as the air is cooled by the falling rain and the rain water is warmed by the surrounding air, increasing evaporation until equilibrium is reached or conditions change.
For more on cloud seeding try this link.