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Source: Wikipedia.org article, adapted under https://creativecommons.org/licenses/by-sa/3.0/ license.

Atmospheric electricity is the pattern of electrical charges in the Earth's atmosphere (or less commonly, that of another planet). The normal movement of electric charges among the Earth's surface, the various layers of the atmosphere, and especially the ionosphere, taken together, are known as the global atmospheric electrical circuit. Much of the reasoning required to explain these currents lies within the field of electrostatics, but also requires understanding of other disciplines within Earth science.

Eliminating, for the moment, consideration of the extremely dense charge populations that exist in the upper reaches of the atmosphere, a region called the ionosphere, filled with hot, dense, plasma gas whose ions give the ionosphere its name, we note that there is always some amount of unbound positive and negative, but net positive, electric charge in the atmosphere closest to the surface of the negatively charged Earth on a 'fine day'. When days are not so 'fine', the net unbound charge that exists in the clouds of thunderstorms can be exceedingly negative.

The 'fine day' net positive charge sets up an electric field between the negative Earth and the net positive charge in the air, and this electric field stores electrical energy. The positive charge acts by induction on the earth and electromagnetic devices.

Experiments have shown that the intensity of this electric field is greater in the middle of the day than at morning or night and is also greater in winter than in summer. In 'fine weather', the potential, aka 'voltage', increases with altitude at about 30 volts per foot (100 V/m), when climbing against the gradient of the electric field. This electric field gradient continues up into the atmosphere to a point where the voltage reaches its maximum, in the neighborhood of 300,000 volts. This occurs at approximately 30–50 km above the Earth's surface. From that point in the atmosphere up to its outer limit, nearly 1,000 km, the electric field gradient produced in the lower atmosphere either ceases or has reversed.

Global daily cycles, with a minimum around 03 UT and peaking roughly 16 hours later, were researched by the Carnegie Institution of Washington in the 20th century. This Carnegie curve variation has been described as "the fundamental electrical heartbeat of the planet".

The phenomena characterizing atmospheric electricity are of at least three kinds. There are thunderstorms, which create lightning bolts that 'instantaneously' discharge huge amounts of atmospheric charge to ground in a rapid release of energy stored in the electric field that built up to a particularly extreme degree in the storm clouds. There is a related phenomenon of continual electrification (re-charging) of the air in the lower atmosphere. A third phenomenon is that of the polar auroras.

Most authorities agree that whatever may be the origin of the net unbound positive charge in the atmosphere, the generation of enormous currents (flow of electrons, negative charges), that flow between clouds and ground during a lightning discharge, begins with condensation of water vapor within the clouds; each minute water droplet moving through the air collects upon its surface a certain amount of negative charge by collecting 'free' electrons. As these tiny drops coalesce into larger drops, and still larger drops, there is a corresponding decrease in the total exposed surface upon which the collected electronic charges can be carried, raising the negative voltage as droplets combine. This is because an object's potential rises as the electrical capacitance of the object holding the charge is decreased. The combined negative electric potential of all the coalescing water drops rises until it overcomes the breakdown voltage of the, usually non-conductive, air, and jumps to earth as a lightning bolt. The similarity of lightning to the discharge of accumulated electrons developed on an electrical machine was demonstrated by Franklin in his memorable kite experiments.
atmospheric science, how the atmosphere works

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