Is there a reason an at-home occupant wouldn't notice it raining, or wouldn't hear the thunder?
The challenge here is translating the "analog" decision that its raining into a digital world. You know its raining by a combination of three senses:
1) You might feel pressure on your arm/head/back as a raindrop hits it
2) You might see a raindrop fall
3) You might here some noise, and in combination with 1 + 2, decide its raining.
Alternatively, you might see a weather report that calls for rain, or you might look at weather radar and see that its "green" over your house.
I don't think irrigation timers meet your definition. "Instant" shutoff for irrigation (note: still takes 2-5 minutes)
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How about something decidedly different:
-- a sensitive microphone installed on/near/under something metalic that's located outside.
When it starts to rain, the "noise" of the raindrops is picked up from the microphone and amped throughout the house. Alas, you might pick up the errant bird
Perhaps a humidity sensor tied to a relay would reduce errant noises. If you have enough money, you might research an extremely sensitive pressure plate as well.
Alternatively:
1) you can install a few LCD screens around the house that show local weather radar. You can even use an image diff routine to determine if the radar has painted a "green" picture near your house.
2) you can automate off of the prediction of rain (eg: weather forecast automation)
My opinion on the most reliable sensor??
Get a dog that's afraid of rain.