Does it react when the moisture is from dew formation? I wouldn't want that; only droplets falling on it...
No. There is a heater in the sensor to avoid condensation. Plus there's a calibration procedure where you put tiny droplets on the sensor and adjust the sensitivity to where you want it.
With normal calibration, a few misty specks of water will not be detected. But if even a light mist continues, it will build up on the sensor and be detected. Larger drops like you'd experience in a light drizzle are detected very quickly. This is unlike most weather stations which use a teeter-toter collection bucket that needs to collect a fair amount of water before it tips and registers a count.