As long as the cables are properly insulated, jump starting a car in the rain does not add to the immediate danger of jump starting the car, but exposing the battery to rain or other forms of moisture causes corrosion and reduces the life of the battery. If bluish-white matter accumulates around the terminals following exposure to moisture, it is important to clean them thoroughly to avoid faulty connections.
Though jump starting a car in the rain is not dangerous, jump starting a frozen battery could lead to an explosion. To ensure the battery is not frozen, remove the caps to inspect for ice in the cells. Jump starting a cracked or leaking battery can also cause an explosion. Additionally, it is dangerous to attach jumper cables to incompatible vehicles. Connecting vehicles with 6-volt and 12-volt batteries together can cause damage to the electrical systems.
Attach red jumper cables to the positive sides of compatible batteries. Attach one black cable to the negative terminal of the good battery and the other black cable to some unpainted metal in the car with the dead battery. Connect the cables before starting the car with the good battery.
When jump starting cars or doing any other work with car batteries, it is important to wear gloves and safety goggles to avoid injury from possible explosions or battery acid burns. When batteries are frozen, cracked or leaking, they should not be jump started but replaced.