How does a rain alarm work?

There are many common moisture & water detectors readily available. They work like this: 2 conductors are placed close together but aren't touching. When water (which is electrically conductive, but not as conductive as most people think) contacts the conductors, the circuit is completed, and whatever is designed to happen next, happens. It could be an alarm, light, or a signal to an overall supervisory system such as in a factory basement, or tunnel.