An emission of electricity from cloud to cloud, cloud to ground, or ground to cloud, accompanied by a flash of light. It results from variations of electrical charge on droplets within clouds and on the Earth’s surface. This variation may be caused by the break-up of raindrops, the splintering of ice crystals, or differences between splintered ice crystals and soft hail. As a cumulus cloud develops, the frozen upper layer becomes positively charged, and most of the cloud base negatively charged, with positively charged patches. The negative charges are attracted to the earth, which has a positive charge. When the electrical field strength gets to about 1 MVm−1, the electrical insulation of the air breaks down. The result is a leading stroke, or stepped leader, from cloud to ground, which creates a conductive path between them. The return stroke, from earth to cloud, follows the same pathway (only millimetres across), with a charge up to 10 000 amps. The intense heat of the stroke engenders light, and a violent expansion of the air, making waves, heard as thunder. Not all the negative charge may be released; there may be several return strokes, each prefaced by a downward dart leader which reactivates the channel. Where the path between ground and cloud is clearly visible, forked lightning is seen. The illumination of other clouds by a concealed fork is sheet lightning.
De Pablo and Rivas Soriano (2007) Monthly Weather Rev. 135, 7 report on an inverse correlation between lightning flash rates and index values of the North Atlantic Oscillation, and Gołkowski (2011) Acta Geophysica, 59, 1, 183 investigate variations in the global atmospheric electric circuit.
Ball lightning has been described as a sphere of glowing light meandering through the lower air. Little is known about it, but there are a few fascinating 19th-century observations accessible from the American Meteorological Society website.
http://www.ametsoc.org/ American Meteorological Society.