Actually does sound like a problem a-brewing...
The voltage drop at 650 rpm (idling) is normal. It "typically" takes about 1000 rpm for the alternator to really kick-in. However, if you are now registering about 8 volts no matter what the rpm's are, you've got a problem. This means your charging system is NOT doing it's job, and you'll eventually have a dead battery as it won't be able to keep up with the electrical demands in the car. You should have (ideally) between 13.8 volts and 14.2 volts at the battery terminals when the engine is at approx. 1000 rpms.
Since you mentioned rain--I'm thinking you may have a possible corrosion problem with your connections--BUT FIRST make sure your voltmeter you have in the car has ITS connections good, and the thing is working properly.
Have an assistant rev the engine a couple times with accessories off, then on. Get a separate (known good) voltmeter and measure at the battery terminals. If it still looks suspecious, clean and inspect battery wires/terminals/etc. Then, if that doesn't do it,try replacing the voltage regulator, if that doesn't do it (or you don't have one) replace the alternator.