You can't do what you want using that sensor - The sensor is just a simple on/off type sensor that tells you if the sensor plate is wet or not, it does not measure how much rain is falling. Even if you timed how long it was activated how would you know between heavy rain and a light shower? You need a 'tipping bucket' type sensor to give you an estimate of how much rain there has been, but even then you need to do some work with experimentation to get reasonable accuracy.

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Just for others coming here in the future: I changed the connections as suggested by @mfalkvidd , restarted the node and now the radio runs from batteries and the Arduino is powered from USB (phone charger). Unfortunately this doesn't seem to have improved the radio communication, but this is another story... Stefano