What conditions need to exist for rain to fall?

Odd question, yes, but at present I have a rainfall indicator which relies on a raindrop hitting a sensor and creating a short circuit and thus closing vents in the glasshouse.

What I would like is a rain predictor so the vents close before rain starts.

I presume that temperature, pressure, and UV would all change in some way, but...
1. are the changes measureable
2. are these the only measurements that will indicate rain is about to fall

What I would like is a rain predictor ...
Wouldn't we all!! You could become millionair that way!!

:D

Look up in the sky. If it starts getting dark. Than it'll soon rain. Haha. [ moring/afternoon time]
[evening] if the wind suddenlystarts coming hardly. And you feel some wetness in the wind.

Before it will rain, there are several cloud types that can give hints, air pressure will start to decrease, temperature will also possibly decrease slightly with rainfall, depending on where you live.

I think a barometer type arrangement is going to be the best thing you can hope for. But there are so many variables, many you cannot measure from a greenhouse alone that you are never going to get perfect system.

My joints start hurting about 12 hrs before rain falls. I'm a real reliable forecaster. Ask for a prediction from anyone you know who has arthritis.