In article ,
(cpu) wrote:

Yesterday I flew a cessna 172 in the hard IFR. When I penetrated
apparently a heavy cumulonimbus rain cloud area, the VSI and altimeter
started to oscillate and bounce +/- 250 FPM (ALT oscilated 200~300 ft
up and back). The rate of bounces was about 3 to 4 Hz (3 to 4 times
per second). It lasted for about 10 minutes until I passed that area.
The AI and airspeed was relatively stable in such light to moderate
chops condition.

Can anyone explain the possible cause of the oscillation? And did
anyone experience this before? If it was the heavy rain that caused
this, I still don't understand why. (please explain how can rain cause
this?)

Water in the static line?