I never said the circuit is not working: I've said that's very critical. I mean that if you're going to create more-than-just-one circuit, you'll have to re-tune each one, because the inherent error, tolerance and whatever will lead to (even to) dramatic different results.
A typical resistor is given at 5%, but you might find at 1% as well. The capacitance of a comb plate is due to many factors: exposed surface area, dielectric, and distance. Thus, upon a temperature range of, let's say, 0 to 40°C, you'll have a very different C, despite the same rain fall.
Have a try, and you'll face the problem. However, if you're going to create a single one, okay, you can tune it perfectly, table its trans-characteristics on software and plot a real behavior upon rain and temperature.
I really didn't want to fire discussions: just to alert other readers that the problem isn't simple!
Cheers