The relative humidity measured on the ground (where the sponge is) doesnt reflect moisture levels miles above in the sky. Rain occurs when the rising air can no longer hold the water droplets that have formed clouds high in the sky. So 100 percent humidity might not mean rain, but it does mean dew.
At what percent humidity does it start to rain?
100 percent Rain often occurs when the relative humidity is below 100 percent (but it is always 100 percent in the clouds where condensation is occurring.)
Why is rain not 100% humidity?
The percentage of humidity is not measured by the ratio of water to air. Instead, it looks at how much water vapor the air can hold at a certain temperature. But in order for rain showers to occur, humidity has to be at 100 percent up where the clouds are (at ground-level, meanwhile, it can be at any percentage).
Does humidity lead to rain?
Rainfall certainly decreases humidity, but it is not stated at what percentage it starts to rain. Humidity will increase during a rainstorm as rain and puddles evaporate. Temperature will decrease toward the wet bulb temperature. As noted in the answer, there isnt a magic number of %humidity that causes rain to start.