There was a previous subject like that months ago, plenty discussed with
several very good ideas about it.  The problem was to close a door over
an electronic device, and it should be done as fast as possible when
rain starts.

One of the best ideas was to use a large funnel to collect rain drops
and use a pair of led (emmiter/receiver) to "read" the drops running
down the funnel small end.  The funnel would "amplify" the rain drops
quantity and the operation is quickly.

I already used a simple solution but demand analog measurements.  Two
resistors (around 330 Ohms 1%) connected to +5VCC, one is encapsulated
and protected from the weather, another is also encapsulated in thin
epoxy and exposed to the rain drops.  Both resistors heat with the
current, the one exposed to the rain drops goes to a lower temperature
(water dissipation), creating a difference of current between both
resistors, what triggers the electronics "it is rainning".  When the
rain stops, the actual resistor heatting dryes it in less than a
minute.  Both resistors should be able to receive the same air and wind
temperature.