Here's a wild one - what about using one of those
simple X-band, Gunn-diode Doppler units that are
used as motion detectors?

Experimenting with a low-power Gunn-diode based,
X-band Doppler RADAR (police speed RADAR) unit a
few years ago during the onset of rain I noticed
was able to 'hear' (they supply the recovered Doppler
shift to a speaker on the unit) the 'precip' as it
fell ... sort of a whistling sound ...

This was done by looking 'up' through the roof of
my garage ... the roof was covered with the usual
'composition shingles' and the actual ceiling in
the garage was composed of sheet rock (gypsum
board).

RF Jim

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