You can use the Cypress Capacitive sensing solution to detect the rain.
The main underlying principle of Cypress CapSense is to detect the change in capacitance of the sensor and behave accordingly. In your application you have to detect the change in capacitance in your application due to droplets of rain.
Please go through the documents "Getting started with CapSense" and "PSoC 4 CapSense design guide" obtained from the links below for more information regarding the working of Capsense
Once you have gone through that you will get an overall idea of how Cypress CapSense works.
For your application we recommend you to use the guard sensor to detect the presence of big water droplets. Please refer above documents for information related to guard.
Please note that since the parasitic capacitance of the sensor varies with temperature, humidity etc the application will be very sensitive.
For this application you may need to have manual tuning of CapSense sensors. Please go through the attached document which helps you to tune the sensors manually.
Thanks and Regards
tuning.pdf 2.4 MB