Lightning

Cloud Electrification

An understanding of lightning formation begins with an understanding of electrical charge in the atmosphere. In the absence of clouds and major weather systems (i.e., “fair-weather” conditions), Earth's atmosphere contains a preponderance of positively charged ions (an ion is simply a charged particle), while Earth's surface contains a preponderance of negatively charged ions. This global-scale distribution of charge is largely the result of thunderstorms, which act to remove negative charge from the atmosphere and deposit it on Earth's surface. Since air is a poor conductor of electricity, an exceptionally large electrical field must be established for lightning to occur. Under fair-weather conditions, Earth's electrical field is relatively weak, but in a thunderstorm environment, the strength of the electrical field increases [Page 1780]by many orders of magnitude until an electrical current can move freely between the cloud and another conducting surface.

Time-lapse photography captures cloud-to-ground lightning during a nighttime thunderstorm in Norman, Oklahoma.

Source: National Oceanic and Atmospheric Administration.

The most popular theory for the development of electrical charge in thunderstorms is the noninductive charging mechanism. Since thunderstorms grow to altitudes where temperatures are below freezing, they become composed of liquid water, ice crystals, and hailstones. Each of these particles has a slightly different arrangement of electrons, meaning that as they collide with each other in the turbulent air currents found within a thunderstorm, electrons will be transferred between them. This transfer of electrons results in an accumulation of positive charge near the upper portions of the thunderstorm (where ice particles are kept aloft) and an accumulation of negative charge in the middle and lower portions of the thunderstorm (where small, but heavier hailstones settle). Secondary charging mechanisms help strengthen the electrical field as electrons are continuously transferred between colliding thunderstorm particles. On the ground, an accumulation of positive charge builds as negative charge near the lower portion of the thunderstorm repels the negative charge on Earth's surface that existed under fair-weather conditions. When a lightning flash occurs between the cloud and the ground, negative charge is transferred back to the ground.

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