Why is the sky blue?
What is lightning?
Lightning is a kick-butt jolt of electricity that can travel inside a cloud, between clouds, or from a cloud to Earth. There's no real consensus on how clouds accumulate such huge charges. The Why Files asked David Martin, a scientist at the University of Wisconsin-Madison's Space Science and Engineering Center to explain the most plausible theory.
Photo ©Daniel Robinson
Martin told us that differences in temperature cause updrafts in thunderstorms. These fast vertical winds contain large and small particles of ice, and supercooled water (water that's cold enough to freeze, but has not yet frozen). As the water and ice collide, positive and negative charges separate. The updrafts then move the charges, forming the cloud into a sandwich with negative charges in the middle, and positive ones at the top and bottom.
When enough charges accumulate, the resulting electrical potential, or voltage, overcomes the air's electrical resistance and starts a lightning bolt. The first step is a small "leader" stroke that ionizes the air in its path. (Ions are molecules or atoms with an electric charge that can flow to form an electric current). With the ions in place, a much larger return stroke travels up from the Earth to the cloud. Most lightning bolts last less than 1/10 second.
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