Long version:
The rain gauge was another thing I wanted to use to trigger sprinklers. I have a Rain Bird sprinkler setup and had considered a project using a RPi as the controller. Actually, someone wrote something for HG that was pretty slick and might work well for a sprinkler controller. It was on the old forum so it may have been lost.
I actually have several metering devices that communicate over 433MHz but I never got the radio reliable enough. I had the goal of reading my power usage and temperature from a Blueline meter reader. I was going to track the power usage to detect trends and possibly trigger things off of it at some point. I wanted to use the Acurite 5-in-1 weather station (temperature, humidity, rain, wind speed, wind direction) to modify sprinkler schedule as well as thermostat trigger times. That meant I needed to change my Honeywell thermostat to the WiFi version, but they changed the model to the 8000 series which is not compatible with my heat pump for some reason (was missing one pin that was mandatory as I recall). I also wanted to use the Acurite wireless thermometer to trigger an email in HA that would alert me when the temp inside the kennel was above or below some limits (say 100°F and 20°F). I was thinking I could even add a relay (maybe Sonoff) to turn on a light or other heat source if it was really cold and maybe a fan or sprinkler/mister if it was too hot (both of these are really overkill in Seattle though).
Anyway, all of that hinged on the ability to utilize 433MHz wireless. I had all the protocols decoded, but the stupid Arduino couldn't stay working reliably. I was going to switch over to an ESP solution, but they are 3.3v and the libraries for the RFM69 were complicated enough I never figured them out.
TL;DR: I spent months getting 95% of the way to a wireless 433MHz solution and it all failed when deployed so now I don't use weather to trigger anything.