This project started as a complement to my cellphone remote control idea: I have opted for using a "GSM module" with standard AT commands, which allows me to receive and send SMSes. Since some info about the remote location can be fit into an SMS, I have been thinking of useful ambient parameters to be measured and transmitted.
I got stuck on how to detect an ongoing rainfall, and search engines didn't seem to help with DIY solutions, at least with keywords I used.
After some brainstorming, there seem to be two ways to detect rain, actually three: with a resistive sensor, with a capacitive sensor and with an infrared reflectometer.
Resistive sensor can be as simple as a copper clad board with two interlaced combs designed on it. Quick and cheap. How to protect it from the harsh environment? Use iron, Cu+Sn or Au (gold) as contacts.
The capacitance method does not require electrical contact, but it rather relies on the different dielectric characteristic or water vs air. It has to be defined how much C changes and then understand how hard is to measure it: an interesting challenge.
The infrared reflectometer is used in rain detection systems on car windshields, and it is mentioned here just in case you want to learn about it (in that case use a search engine).
I will first investigate the resistive method: if it lasts 10-12 months I can afford to change the sensor once a year. First step: build two interlaced copper combs and measure R when water bridges them.