Lightning and Thunder

Lightning is the discharge of accumulated static electricity from a thundercloud as a gigantic electrical arc between the cloud and the ground or another cloud. It is accompanied by intense heating and explosive expansion of air, producing the sonic boom known as thunder. Lightning may cause property damage and death, but it also contributes to the nitrogen absorption in the soil necessary for plant life and may have even helped to start life on Earth.

Early Lightning Studies

The search for an understanding of lightning has been long and circuitous, and even in the twenty-first century, there are conflicting theories. In ancient civilizations, lightning and thunder were viewed as manifestations of the power and wrath of the gods. Lightning bolts were the favored weapon of Zeus, the leader of the Greek gods. His Roman counterpart, Jupiter, subdued monsters with thunderbolts forged by Vulcan, a lesser god believed to have his forge located below Mount Etna in Sicily. The numerous lightning discharges in the dust and ash clouds above volcanoes may have been the source of this belief. The book of Exodus in the Bible relates that before Moses climbed Mount Sinai to receive the Ten Commandments, “there was thunder and lightning, with a thick cloud over the mountain, and a very loud trumpet blast.”

A scientific understanding of lightning began in earnest around the year 1750, in part with the work of Benjamin Franklin. Three years earlier, Franklin had experimented with the electrical charging of pointed objects connected to the ground and found that such points could draw a charge from another charged object when placed near it. In 1749, he wrote a letter to the Royal Society in England, speculating on the electrification of clouds and suggesting that lightning is an electric discharge to the ground that favors trees, spires, chimneys, and masts. He also compared the snap of an electric spark with the thunder produced by lightning. In 1750, he wrote to the Royal Society suggesting that a pointed metal rod extending several feet above a building and connected by a wire to the ground could protect the building from lightning damage, even drawing a charge silently from the cloud.

Along with his idea of the lightning rod, Franklin proposed an experiment for collecting electricity from clouds by placing a properly insulated metal rod above a high tower or steeple and drawing sparks from its lower end. Georges-Louis Leclerc, Comte de Buffon, performed this experiment successfully in France in May 1752. However, when the German scientist George W. Richmann attempted to repeat it in Russia in 1753, he was struck and killed by a bolt of lightning. In 1752, Franklin performed his famous kite experiment with a pointed wire fastened to the top of the kite and a metal key at the lower end of the string, held by a silk ribbon for insulation. Standing under a shed to keep the silk ribbon dry, Franklin was able to draw sparks from the key and collected enough electric charge to show that it was the same as ordinary electricity. In experiments with a metal rod above his house, he found that the charge at the bottom of thunderclouds was usually negative but sometimes positive, suggesting the complex nature of cloud charging.

Theories of Thundercloud Charging

Lightning occurs when separate regions in a cumulonimbus thundercloud contain opposite electrical charges. Several theories have been proposed to account for this separation of charge. The precipitation hypothesis, first proposed by the German physicists Julius Elster and Hans Geitel in 1885, assumed that raindrops and ice pellets in a thunderstorm are pulled down by gravity past smaller water droplets and ice crystals suspended in the cloud. The hypothesis suggested that collisions between these particles cause the larger particles to gain a negative charge (much like when charge transfers to shoes from a rug), and the smaller ones become positively charged. Updrafts within the cloud then sweep the smaller positive particles up near the top, while the larger negative particles accumulate near the lower part of the cloud.

A second theory, the convection hypothesis, was developed independently by the French scientist Gaston Grenet in 1947 and American scientist Bernard Vonnegut in 1953. They assumed cosmic rays from outer space ionize the air above a cloud, separating positive and negative charges. At the same time, corona discharges around sharp objects on the land surface produce positive ions. Warm air then carries these positive ions upward by convection until they near the top of the cloud, where they attract the negative ions formed by cosmic rays. These negative ions enter the cloud and attach to water droplets and ice crystals, which are then carried by downdrafts at the edges of the cloud to the lower regions, charging the cloud negatively at the bottom and positively at the top.

A theory developed in the 1980s combines the effects of convection and precipitation with the microphysics of ice particles. This explains Franklin’s observations that the bottoms of clouds are sometimes positive. It assumes that charging occurs as millimeter-sized ice particles called graupel fall through a region of supercooled droplets (liquid water below 0 degrees Celsius) and ice crystals. When these droplets collide with the graupel, they freeze on contact, releasing latent heat. This heat keeps the surface of the graupel warmer than the surrounding ice crystals. Several studies at American and British universities have shown that negative charge is transferred to graupel below a critical temperature of about –15 degrees Celsius. When the graupel and crystals come into contact, positive ions are transferred to the ice crystals, which are then carried by updrafts to the top parts of the cloud. The graupel and larger hailstones fall, depositing a strong negative charge near the critical temperature level in the cloud. Below this level, the graupel acquires a positive charge, causing some weaker positive regions near the bottom of the cloud.

Cloud-to-Ground Lightning

The normal fair-weather atmosphere is characterized by a positively charged upper atmosphere, called the ionosphere, and a negative charge on the land surface. As a thundercloud develops with strong convection updrafts from warmer surface regions, its strong negative charge repels electrons in the ground below, causing it to become positively charged relative to the cloud. The positive charge becomes most concentrated on objects that protrude above the ground, such as trees, poles, and buildings. When the difference in charge between some point on the ground and the cloud builds up to an electric potential of about one million volts per meter, the air begins to ionize sufficiently to overcome the air’s normal electrical resistance, and current begins to flow. High-speed camera studies have revealed that the resulting lightning stroke is a complex series of events rather than a single straightforward electrical arc event.

In cloud-to-ground lightning, a local electric potential of more than 3 million volts per meter develops within the cloud along a path of some fifty meters. This causes a discharge of electrons to rush toward the base of the cloud and then toward the ground in a series of steps, carrying a negative current of a few hundred amperes downward. Each step in the discharge covers about fifty to one hundred meters, with stops of about fifty microseconds between steps as charge sufficient to achieve the next step accumulates. This initial discharge is called the “stepped leader” and produces only a faint glow nearly invisible to the human eye. As the stepped leader approaches to within about fifty meters of the ground, a region of positive charge moves up into the air through any susceptible object (usually elevated) to meet it.

When the stepped leader of electrons meets the upward-moving positive charge, an immense quantity of electrons flows to the grounding point, starting from the bottom of the ionized channel in a “return stroke” equivalent to an upward positive current of some ten thousand amperes. The luminous return stroke, an electrical arc several centimeters in diameter, generates heat and light in a bright flash lasting less than two-tenths of a millisecond, too fast for the eye to resolve the motion. This leader-and-stroke process is often repeated in the same ionized channel several times at intervals of about two hundredths of a second. These subsequent surges of electrons from the cloud, called “dart leaders,” proceed downward more rapidly because of the lowered resistance of the path, each followed by a less energetic return stroke. The process lasts less than half a second, and it is too short to distinguish individual strokes. Some dart leaders that do not reach the ground appear as lighter forked flashes.

Varied Forms of Lightning and Thunder

Lightning and thunder take on a variety of shapes and forms. Sometimes, a dart leader is met by a return stroke along a new conducting path from the ground, causing a forked lightning bolt that strikes the ground in more than one place. In fewer than 10 percent of cases, a positively charged leader emanates from the cloud; even more rarely, a leader starts from tall objects on the ground and meets with a return stroke from the cloud. If the wind moves the ionized channel fast enough between strokes, a broader streak called ribbon lightning can be produced, or a series of bright streaks called bead lightning results when the channel appears to break up. A rare phenomenon called ball lightning consists of an electric discharge in the form of a slowly moving basketball-size luminous sphere that has been variously observed to either explode or simply decay.

Studies have shown that about 80 percent of lightning occurs within clouds hidden from view and occasionally even strikes between clouds. Lightning inside clouds can illuminate them with flashes of light called sheet lightning. Lightning from distant thunderstorms that can be seen but are too far away for their thunder to be heard is called heat lightning. Since 1989, three new rare types of lightning have been videotaped, jumping from the tops of clouds into the atmosphere. These consist of bright bluish, cone-shaped bursts called “blue jets” that rise more than twenty kilometers from the cloud center, clusters of dim reddish bursts called “red sprites” rising to heights of more than fifty kilometers, and doughnut-shaped bursts of light called “elves” some four hundred kilometers wide and one hundred kilometers above the cloud tops.

Lightning usually produces thunder. A lightning discharge heats the air along the conducting path so quickly (a few microseconds) and to such a high temperature (several times that of the Sun's surface) that the air expands explosively. This produces a shock wave of compressed air that decays into an acoustic wave within a few meters. Because sound travels almost 1 million times more slowly than light, thunder is heard after the lightning is seen. Sound travels about 330 meters per second, so the distance to the flash can be estimated as 1 kilometer for every three seconds between the lightning flash and the thunder. Within about 100 meters of lightning, thunder sounds like a clap followed by a loud bang. At greater distances, thunder often rumbles as it emanates from sections of the lightning stroke (about five kilometers long) at different distances from the observer. Thunder is seldom heard beyond about twenty kilometers because of temperature differences and turbulence in the air.

Significance

Lightning has both destructive and beneficial effects. Scientists estimate that yearly, around 16 million thunderstorms and 1.4 billion lightning flashes occur worldwide. Tropical landmasses experience thunderstorms on more than one hundred days yearly, but they rarely occur in polar regions or dry climates. In the United States, the average number of thunderstorm days each year varies significantly depending on location, ranging from seventy to one hundred in Florida, from thirty to fifty in the Midwest, and from five to ten along the West Coast. Annually, in the United States, lightning causes an average of around twenty deaths and starts about 17,400 fires, resulting in property damage of around $451 million. Lightning can be most dangerous near elevated places and isolated trees. The best protection is secured inside automobiles and buildings, out of contact with conducting surfaces.

Lightning is essential for life on Earth in that it provides a source of plant fertilizer and ozone. Its electrical action combines nitrogen and oxygen in the atmosphere to form nitric oxide, which then dissolves in precipitation and is brought to the surface as a nitrate. Hundreds of millions of tons of nitrates are produced by lightning each year and absorbed by the soil, where the nitrates help to nourish plant life and food crops. Studies at the National Center for Atmospheric Research (NCAR), using a computer model called MOZART, showed that lightning is probably the main source of ozone in the upper atmosphere. The ozone layer protects life on Earth by blocking ultraviolet radiation from the Sun.

Lightning also helps to maintain the potential difference of some three hundred thousand volts between the surface and the ionosphere, which reflects the radio waves used for long-distance communication. This voltage causes a fair-weather current of about two thousand amperes to discharge from the surface at a rate of a few microamperes per square kilometer. Lightning balances this flow by returning electrons to the surface. Lightning may have even been the spark that stimulated the development of life through the formation of organic molecules on primitive Earth and was a likely source of the first fire used by humans for protection and survival.

Principal Terms

ball lightning: a rare form of lightning appearing as luminous balls of charged air

bead lightning: lightning that appears as a series of beads tied to a string

convection: vertical air circulation in which warm air rises and cool air sinks in a cyclic manner

corona discharge: a continuous electric discharge from highly charged, pointed objects that produces the luminous greenish or bluish halo known as St. Elmo’s fire

cumulonimbus: thunderstorm clouds that develop vertically by strong convection in the atmosphere and, due to high-altitude wind shear, often form a top shape reminiscent of an anvil

dart leaders: surges of electrons that follow the same intermittent ionized channel taken by the initial stepped leader of a lightning stroke

graupel: ice particles between 2 and 5 millimeters in diameter that form by a process of accretion in a cloud

ionosphere: an electrically active region of the upper atmosphere from about 80 to 800 kilometers above the surface of Earth that contains a relatively high concentration of ions (charged atoms or molecules) and free electrons

return stroke: the luminous lightning stroke that propagates upward from the ground surface toward the base of a cloud as electrons surge downward and a positive current flows to the cloud

stepped leader: an initial discharge of electrons that proceeds in a series of steps from the base of a thundercloud toward the ground

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