HOW LIGHTNING FORMS
As a towering cumulus develops, a large electrical charge separation builds up within the cloud. Lightning results when this electrical charge becomes strong enough to jump from the cloud to the ground, to another cloud, or to an opposite electrical charge within the same cloud.
In the early stages of development, air acts as an insulator between the positive and negative charges in the cloud and between the cloud and the ground. When the opposite charges build up enough, this insulating capacity of the air breaks down and there is a rapid discharge of electricity that we know as lightning.
"Between the tops of two cumulonimbus clouds." Incorrect.
An equalisation of charge only occurs between one positively charged and one negatively charged area. The tops of the clouds have the same charge, therefore no lightning bolt develops.
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