National Weather Service United States Department of Commerce

Air is generally a poor conductor of electricity, normally preventing charges from moving freely through the atmosphere. However, if the charge difference between positively and negatively parts of the cloud, or if the charge difference between the cloud and the ground becomes too large, the insulating capacity of the air starts to break down the air starts to ionize. This breakdown occurs on a molecular level and allows negative charges to start moving more freely.

Leaders generally move downward from the cloud (by far, the most common) or upward from very tall objects ground (much less common). Leaders can be negatively charged (more common) or positively charged (less common).

An estimated 90% of the cloud-to-ground lightning flashes originate from negatively-charged leaders that develop downward from the cloud, while about 10% of the cloud-to ground lightning flashes originate from positively-charged leaders that develop downward from the cloud. Lightning flashes originating from upward-developing leaders are relatively rare.

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