Lightning, form of visible electric discharge between rain clouds or between a rain cloud and the earth. The discharge is seen in the form of a brilliant arc, sometimes several kilometers long, stretching between the discharge points. The discharge also sets up a sound wave that is heard as thunder.

How rain clouds become charged is not fully understood, but most rain clouds are negatively charged at the base and positively charged at the top.

The various hypotheses that explain how the polarization occurs may be divided into two categories: those that require ice and those that do not. Most meteorologists believe, however, that ice is a necessary factor, because lightning is not usually observed until ice has formed in the upper layers of thunderclouds. Experiments have shown that when dilute solutions of water are frozen the ice gains a negative charge but the water retains a positive charge. If, after freezing has started, rising air tears small droplets of water away from the frozen particles, the droplets are concentrated in the upper part of the cloud and the larger ice particles fall toward the base. On the other hand, experiments have also shown that large, swiftly falling drops of water become negatively charged whereas small, slowly falling drops become positively charged. The polarization of a thundercloud may thus be due to the rates at which large and small raindrops fall. However formed, the negative charge at the base of the cloud induces a positive charge on the earth beneath it, which acts as the second plate of a huge capacitor. When the electrical potential between two clouds or between a cloud and the earth reaches a sufficiently high value—about 10,000 volts (V) per centimeter (cm) or about 25,000 V/in. (a volt is a measure of electrical potential; for comparison, the potential supplied by an ordinary electrical outlet in the United States is 110 V)—the air becomes ionized along a narrow path and a lightning flash results. Many meteorologists believe that this is how a negative charge is carried to the ground and the total negative charge of the surface of the earth is maintained.

A new theory, suggesting that the electrical polarization in a thundercloud may cause precipitation rather than be a consequence of it, postulates that the electrical potential existing between the ionosphere, the highest layer of the atmosphere and the earth initiates the polarization in a thundercloud. According to this theory, the upward flow of warm air through a thundercloud carries with it positively charged particles. These accumulate at the top of the cloud and attract negative charges from the ionosphere. The negative charges are carried to the base of the cloud by powerful downdrafts at the periphery of the cloud, thus preventing oppositely charged particles from neutralizing each other. Perhaps 90 percent of all lightning discharges, known as bolts or strokes, from cloud to ground are negative; the remainder are positive flashes. Rarely, strokes may move from ground to cloud, particularly from mountain peaks and from tall objects such as radio towers. Studies with high-speed cameras have shown that most lightning flashes are multiple events, consisting of as many as 42 main “strokes,” each of which is preceded by a “leader” stroke. All strokes follow an initial ionized path, which may be branched, along with the current flows. The average interval between successive lightning strokes is 0.02 sec and the average flash lasts 0.25 sec. Because the duration of one powerful stroke is no more than 0.0002 sec, the intervals between strokes account for most of the duration of a lightning “flash.” So-called sheet lightning is simply the reflection of an ordinary lightning flash on clouds. Ball lightning is a rare phenomenon in which the discharge takes the form of a slowly moving, luminous ball that sometimes explodes and sometimes simply decays.