It would be handy to be able to have a detector that can give an audible
signal when it starts to rain.

I often have sports kit outside drying (Scuba, boat etc) and can't rely on
it staying dry long enough to dry kit.


I have found a link to one project
http://www.techlib.com/electronics/raindetectors.htm

Anybody anything better .... the key would seem to be the sensor ... large
enough surface are to catch first few drops of rain, stable enough not to
corrode in a couple of weeks.