A capacitor microphone works by the electrical principal Q = CV, or charge = capacitance x voltage.
Electrical charge is the presence or absence of electrons compared to how many there normally would be. In a capacitor microphone the diaphragm and backplate are charged with respect to each other. Capacitance is the ability of a capacitor (such as the capacitor formed from the diaphragm and backplate) to hold charge.
If Q = C x V, then V = Q / C or voltage = charge / capacitance.
Vibrations in the diaphragm alter the capacitance, therefore in turn the voltage varies and this becomes the output signal.
However, the diaphragm and backplate need to be charged for this to work. So a 48 volt supply is connected, through a high value resistor so that the charge does not leak out.
The problem with capacitor mics is that they rely on good insulation to prevent charge leaking from the diaphragm. Any moisture that is present will degrade this insulation significantly.
The problem is worst when a capacitor mic is brought in from the cold to a warm studio environment. Condensation is likely to occur.
When this happens, the microphone will sizzle like a frying pan until the moisture dries out. This could take half an hour or more, after which the microphone should function perfectly once again.
Outside broadcast crews use a type of capacitor microphone in which the diaphragm forms part of a tuned radio frequency circuit. This is not susceptible to moisture problems.