Physics
When the positive and negative charges within a cloud separate the electric field inside the cloud begins to grow in strength. This is result of the fact that electrical charges of a similar charge repel one another and oppposite charges attract. Due to the fact that the atmosphere around a cloud acts as a very strong insulator, a significant amount of charge must be built up before an electrostatic discharge(lightning) can occur. The reason most lightning occurs within the cloud itself is because the electric fields within the cloud are oftentimes much stronger than those between the bottom of the cloud and the surface of the ground.
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A lightning storm