When a car drives through dry air it is often charged due to friction with the air molecules. But when it is humid there is more water vapor in the air and this causes that the car get discharged again (or perhaps it prevents it from getting charged?). So this is the common explanation how this happens.
But what I don't understand is that actually pure water is a good insulator and only due to some other atoms dissolved in it makes water a good conductor. So the question is does humid air contain, besides more water molecules, also other atoms which can make a conductor of humid air?
Perhaps someone knows too when a car drives through dry air if it loses or gains electrons to get charged?