Lightning |
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Apart from regions around the edges the base of a rain cloud is normally negative. The most common form of cloud to ground lighting is initiated by a negative leader moving towards the ground in a series of (~50 m) steps, ionizing the air as it proceeds. Once contact is made with a short positive streamer close to the ground a positive pulse travels upward. Subsequent negative dart leaders may then each initiate a number of return strokes. Lightning flickers. Positive leaders from the edges of a storm may initiate a single negative return pulse that carries more charge and potentially does more damage. Very often lightning is cloud to cloud, or within a cloud with no visible conducting path. The flash is then called sheet lightning. Thunder is a shock wave radiating from the rapid expansion of a column of suddenly heated air (at ~30 000 °C). |
| The initiating potential differences may be ~100 million Volts. People and planes are often struck by lightning without harm because of the skin effect. The Ben Franklin story about the kite in the thunder storm may be true, but consider this video clip before making a final judgment. |