All bodies (the car, the human body and so on.) Have a tendency to lose or to accumulate electric charges through friction or even for a trivial contact (for example, the car is traveling “rubbed” by the wind). When you touch the car, the electricity accumulated on the body passes through the body of the passenger (and the car is “discharged”), from there the electrical shock. This phenomenon is called triboelettricità – electricity through friction – and there is more in dry days. In fact in those days conductivity (the tendency to transmit electric charges) in air, is lower. So when a charge accumulates on a body (eg car) has a tendency to remain there. In a humid day, however, the charges will disperse more easily in the air and you feel a lot less shock touching objects.