How thunderclouds become charged is not fully understood, but most thunderclouds 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 towards 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 V per cm or about 25,000 V per in), 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.
One theory suggests that the electrical polarization in a thundercloud may cause precipitation rather than be a consequence of it, and 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 per cent of all 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” of lightning, 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 by 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.
Three common and erroneous ideas about lightning ought to be mentioned.
One is that lightning never strikes twice in the same place. Photographic evidence shows that skyscrapers and other tall structures may be struck many times in the course of a single storm.
A second idea used to be that the safest place to stay in a thunderstorm is under a tall tree! Trees, because of their height, are apt to be struck by lightning and are, therefore, highly dangerous during violent electric storms.
A third misconception is that lightning is always clearly associated with thunder. Observers who listen for thunder as a cue may miss up to 40 per cent of lightning strokes.
The safest places for a person outdoors in a thunderstorm are inside a metal-bodied car or lying flat on the ground in the open.
In the United States alone, about 100 people are killed and many injured by lightning each year, more than by tornadoes or hurricanes. Forty per cent of all farm fires and 75,000 forest fires a year are started by lightning. Lightning is not all bad, however. The soil is enriched with nitrogen that is released from the atmosphere by lightning and carried to the ground by raindrops. Some scientists believe that lightning may have been a key element in the origin of life on earth, creating from simple elements complex chemical compounds that gave rise to living matter.