Chapter 7- Tornados

Chapter 7 Tornados - Chapter 11 Lightning Thunder and Tornadoes About 80 percent of all lightning is cloud-to-cloud lightning or sheet lightning

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Chapter 11 Chapter 11 Lightning, Thunder, Lightning, Thunder, and Tornadoes and Tornadoes
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About 80 percent of all lightning is cloud-to-cloud lightning , or sheet lightning , which occurs when the voltage gradient within a cloud, or between clouds, overcomes the electrical resistance of the air. The result is a large and powerful spark that partially equalizes the charge separation.
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Cloud-to-ground lightning occurs when negative charges accumulate in the lower portions of the cloud. Positive charges are attracted to a relatively small area in the ground directly beneath the cloud establishing a large voltage difference between the ground and the cloud base. The positive charge at the surface is a local phenomenon; it arises because the negative charge at the base of the cloud repels electrons on the ground below. Farther away, the surface maintains its normal negative charge relative to the atmosphere.
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All lightning requires the initial separation of positive and negative charges into different regions of a cloud. Most often the positive charges accumulate in the upper reaches of the cloud, negative charges in lower portions. Small pockets of positive charges may also gather near the cloud base.
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The actual lightning event is preceded by the rapid and staggered advance of a shaft of negatively charged air,called a stepped leader .
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When the leader approaches the ground, a spark surges upward from the ground toward the leader (top). When the leader and the spark connect, they create a pathway for the flow of electrons that initiates the first in a sequence of brightly illuminated strokes, or return strokes (bottom).
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Another leader (the dart leader ) forms within about a tenth of a second, and a subsequent stroke emerges from it. This sequence of dart leaders and strokes may repeat itself four or five times. Because the individual strokes occur in such rapid succession, they appear to be a single stroke that flickers and dances about. We call the combination of strokes a lightning flash , the net effect of which is to transfer electrons from the cloud to the ground.
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Lightning
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A bizarre type of electrification called ball lightning appears as a round, glowing mass of electrified air, up to the size of a basketball, that seems to roll through the air or along a surface for 15 seconds or so before either dissipating or exploding.
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St. Elmo’s fire is a rare and peculiar type of electrical event. Ionization in the air, often just before the formation of cloud-to-ground lightning, can cause tall objects such as church steeples to glow as they emit a continuous barrage of sparks producing a blue-green tint to the air, accompanied by a hissing sound.
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Sprites are very large but short-lived electrical bursts that
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This note was uploaded on 11/19/2008 for the course CJC 101 taught by Professor Brown during the Spring '07 term at Ball State.

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Chapter 7 Tornados - Chapter 11 Lightning Thunder and Tornadoes About 80 percent of all lightning is cloud-to-cloud lightning or sheet lightning

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