I chanced upon the following rain sensor http://www.rainsensors.com/ which has the usual relay output which you can configure to trigger on the first drop, etc. But the great thing about it is that they have in their support link the drawing to interface the device via RS232 and get far more data. It's a sealed unit that uses internal reflected IR against its transparent dome, much as done with car windshield moisture detectors. I'm going to get me one and try it out. For 59$, hard to imagine its possible to sell such a nifty device for so little. It has a heater for ridding itself of condensation and presumably the rain, but not ice and snow. Here's what it looks like (hoping no copyright issues - Bob?);