This drawing shows a cloud before lightning strikes. Notice that the positive (+) and negative (-) signs are mixed.
Click on image for full size
Image courtesy of Windows to the Universe
The sky is filled with electric charge. In a calm sky, the positive (+) and negative (-) charges are mixed up.
Inside a thunderstorm, the + and - charges are separated into two sections. All of the + charges are at the top of the thunderstorm cloud. All of the - charges are at the bottom of the thunderstorm cloud.
During a thunderstorm, the ground has a + charge, and + and - charges are attracted to each other. The - charge at the bottom of the thunderstorm cloud wants to meet the + charge of the ground.
The - charge of the cloud
starts to rush toward the + charge at the ground. At the same time, the + charge rushes up toward the - charge. It happens so fast that all we can see is a line of light. This is called lightning
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