A car may not start when it rains because of moisture or dirt interference. Other reasons lie within the car's battery system, especially in cars with weak and corroded batteries.
Plug wires have plastic coating that can wear out with time. When the car ignition is turned on, an electric arc can reach the surrounding metal but not the spark plug. Battery terminals that are brown with a coating of rust may also fail to provide an electric charge to the spark plugs.
Extremely high air humidity during a rainstorm can cause the distributor cap to become wet. This generally happens to older cars with worn distributor caps.