Lightning

INTRODUCTION

Lightning, form of visible electric discharge between rain clouds or between a rain cloud and the earth (see Electricity). 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.

THE ELECTRICAL CHARGE

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 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. Flashes of lightning are thought to travel at one-third the speed of light, or about 100 million meters per second (62,000 miles per second). 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.

A possible new class of lightning has been discovered, consisting of at least three types of lightning associated with severe thunderstorms. All three confirmed types occur far above the cloud layer, jumping from the tops of the clouds into the stratosphere, and are much rarer than normal lightning. The first type, called a red sprite, is a dim, reddish-colored burst that lasts only a few thousandths of a second and can be many kilometers wide. Red sprites appear suddenly, usually in clusters of two or more, and rise to heights of about 50 to 90 km (30 to 50 mi) above the cloud layer. The second type, a blue jet, is a blue, cone-shaped burst, brighter than a red sprite. Blue jets erupt from the center of a thunderstorm at speeds of up to 6000 km/h (3300 mph), rising to heights of about 20 to 50 km (10 to 30 mi) above the cloud layer. Red sprites and blue jets were first photographed in 1989 in Minnesota by American physicist John R. Wincklyer. Since then, more than a thousand pictures and video images of red sprites have been taken and numerous blue jets have been documented. A third type of cloud-to-stratosphere lightning, called Elves, was announced in 1995. Elves are saucer- or doughnut-shaped bursts of light about 400 km (about 250 mi) wide that occur about 100 km (about 60 mi) above the cloud tops. They are thought to be greenish, but they last such a short time (less than a thousandth of a second) that scientists have not yet determined their color.

THUNDER

Thunder is the explosive sound produced by an ordinary lightning discharge. The lightning bolt heats the air around it so quickly (within a few millionths of a second) and to such a high temperature (about 10,000° C, or about 18,000° F) that the air molecules are pushed apart with great force, much like in an explosion. A wave of compressed air (a sound wave) moves out from the lightning bolt.

A lightning strike seems to be over very quickly, but thunder can last much longer, changing in pitch and loudness (see Sound). This happens for several reasons: the lightning bolt has an irregular shape; the air expands in all directions at once; lightning bolts overlap; and objects on the ground interfere with the sound. Because the lightning bolt is not straight and is at an angle to the vertical, not all parts of the bolt are the same distance from the listener, so sound from different places on the bolt reaches the listener at slightly different times. Also, sound from the far side of the lightning reaches the listener after sound from the near side. Lightning often occurs in groups of several bolts very close to each other, and sound waves from different bolts mix to form a continuous sound. Echoes from hills or other reflecting objects contribute to the rumbling effect.

Because sound travels more slowly than light, thunder is heard after the lightning is seen. The distance between an observer and the lightning bolt can be estimated by counting the number of seconds between the lightning and the thunder. The light reaches the observer almost instantaneously, but the sound travels at about 1.6 km (about 1 mi) every 5 seconds. Thunder can seldom be heard from more than 24 km (15 mi) away.

PROTECTIVE MEASURES

Buildings are protected from lightning by providing them with metallic lightning rods extending to the ground from a point above the highest part of the roof. These rods form a low-resistance path for the lightning discharge and prevent it from traveling through the structure itself. Power lines and radio sets with external aerials are protected against lightning by lightning arresters that consist of a small gas-filled gap between the line and ground wire. This gap offers a high resistance to ordinary voltages, but a lightning discharge, which has a potential of tens of millions of volts, causes the gas in the gap to ionize, providing a low-resistance path to earth for this discharge.

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 is 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, actually dangerous during violent electric storms. 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. 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 percent of lightning strokes.

In the U.S. about 100 persons are killed and many injured by lightning each year, more than by tornadoes or hurricanes. Forty percent 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.