The sky is filled with electric charge. In a calm sky, the + and - charges are evenly interspersed thoughout the atmosphere. Therefore, a calm sky has a neutral charge.

Inside a thunderstorm, electric charge is spread out differently. A thunderstorm consists of ice crystals and hailstones. The ice crystals have a + charge, while the hailstones have a - charge. The ice crystals are pushed to the top of the thunderstorm cloud by an updraft. Meanwhile, the hailstones are pushed to the bottom of the thunderstorm by its downdraft. Thus, the thunderstorm's + and - charges are separated into two levels: the + charge at the top and the - charge at the bottom.

During a thunderstorm, the Earth's surface has a + charge. Because opposites attract, the - charge at the bottom of the thunder cloud wants to link up with the + charge of the Earth's surface.

Once the - charge at the bottom of the cloud gets large enough to overcome air resistance, a flow of - charge rushes toward the earth. This is known as a stepped leader. The + charges of the Earth are attracted to this stepped leader, so a flow of + charge moves into the air. When the stepped leader and the + charge from the earth meet, a strong electric current carries + charge up into the cloud. This electric current is known as the return stroke of lightning and is visible to the human eye.

Back to Thunder and Lightning

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