Components for making a simple rain alarm?

Try a pair of 2n1613 NPN transistors arranged as a darlington pair, collectors common, and Q1e tied to Q2b. input is on Q1b, output from + to the collectors. Ckt has hi input Z, and the slight conductive nature of rain water from + to Q1base will forward bias the switch.

Lo tech- a bargan basement smoke alarm- bridge the test contacts with a wet finger and it should go off. If that works, just solder wires and run them outside close together on a board to detect rain.