More Reading

The Violent Face of Nature: Lightning

The weather satellite 22,300 miles out in space reveals a beautiful but deceptive picture of earth. The view we see is of a placid world floating peacefully in the blackness of space, patterns of white clouds winding like ornaments across the planet's disk.

But we know how misleading that serene scene can be. Within those clouds, within the rest of the atmosphere so invisible, and beneath the surface of what we think of as a solid planet, lurk awesome energies that need only the proper conditions to unleash their havoc.

At any given moment, 1,800 thunderstorms are in progress over the earth's surface. Lightning is striking the earth 100 times each second. If the season is late summer, one or more of the some 60 hurricanes or typhoons that swirl into existence each year is likely to be moving toward a populated coastline. If the time is late afternoon, the odds are good that a tornado is raking across the center of the United States; 600 to 1,000 times a year they do so, and in the prime months can strike with a frequency of four or more a day. Somewhere at any given moment, people's homes or crops are under flood waters. Half a billion people live on floodplains, and the crops grown on floodplains supply food for a third of the world's population.

There is no comfort beneath the reach of the turbulent atmosphere either. More than 2,000 earth tremors strong enough to be recorded course through the planet every day. Twice a day somewhere in the world earthquakes strike with enough force to damage homes and buildings. From 15 to 20 times a year a quake strikes with enough energy to cause widespread death and destruction. And all the while there are 516 active volcanoes waiting to erupt. An eruption begins somewhere every 15 days.

There is no way to switch off the energy that continually feeds such violence. Storms derive their energy from the life-giving flow of heat from the sun. Earthquakes and volcanoes get their energy from the heat of radioactive decay of the material within the earth itself. This slow but unending release of heat from the interior to the earth's surface amounts to 10 times all the energy used by man. Even then, it is equivalent to only one five-thousandth the energy that the sun delivers to our planet.

Clearly we are talking about forces of nature that dwarf the puny efforts of humankind. A single thunderstorm three miles in diameter may hold half a million tons of water and contain energy equal to 10 atomic bombs. Larger ones carry the potential energy of several one-megaton hydrogen bombs. A single great earthquake of the size that occurs about four times each year releases the energy of a 10-megaton hydrogen bomb, or nearly twice the daily U.S. consumption of electrical energy. A large volcanic eruption produces almost exactly the same amount.

In one important sense there is no such thing as a natural disaster. A disaster is a social phenomenon. Across our planet for about four and a half billion years, the forces of nature have shaped, molded, and changed the earth. Oceans have come and gone, continents have assembled and split apart, mountains have grown and eroded. These are awesome changes indeed. But what we call a disaster requires the presence of humans, caught up as victims in the violence of nature. A huge volcanic eruption on an uninhabited ocean island may not be a disaster at all. But even a small earthquake beneath a densely populated urban area can be an unmitigated tragedy. Since populations nearly everywhere are on the rise and people are occupying vulnerable areas to a greater extent, the potential for most kinds of disasters is on a steady increase.

Surprisingly, the worst and most consistent killer of all natural phenomena over the long range in this country is not, as we might think, tornadoes, hurricanes, floods, or earthquakes, but lightning, which picks off its victims one by one, thus avoiding the attention and national publicity accorded the more mass-destructive hazards. And lightning can strike anywhere. Lightning is not only a solo killer, it is a technological incapacitator. It is capable of knocking out computer systems or electrical power systems our society has grown dependent upon.

In the roughly seven seconds it takes you to read this paragraph, lightning will have struck the earth some 700 times. This estimated 100-times-a-second rate of discharge worldwide represents something like 4 billion kilowatts of continuous power.

Some intense storms produce almost continuous lightning and thunder for considerable periods. A storm over London during the night of August 2, 1879, is said to have produced continuous thunder for 20 minutes. And Frank W. Lane tells of a storm in Hertfordshire, England, during the night of June 12, 1964, in which thunder was timed that lasted continuously for 28 minutes. During a great thunderstorm in England in July, 1923,6,924 flashes were recorded over London alone, including 47 in one minute. A great storm over Pretoria, South Africa, on Christmas Day, 1923, produced 100 flashes a minute for more than an hour.

Lightning can be a dangerous killer, and often is, far more often than many people realize. When a tornado, flood, or hurricane strikes, it gets enormous news attention because a particularly bad storm can cause fatalities by the dozens and injuries by the hundreds, and these understandably stick in the public's mind, even though they are relatively infrequent. Lightning, in contrast, picks off its victims by ones or twos, and that doesn't make the national news.

Hurricanes kill an average of 54 persons a year, floods 90, and tornadoes 132, according to a 34-year compilation of storm deaths in the United States. Over the same period, lightning killed 6,928 persons, an average of 204 a year. And it is generally concluded that the lightning-fatality figures are conservative because the statistics-gatherers may miss many deaths or there may have been a secondary cause of death, such as a fire resulting from lightning. Lightning is, without a doubt, a big killer, the greatest cause of death among all types of storms.

Unique with the lightning hazard is the relatively high probability of death if struck. Persons caught in hurricanes, tornadoes, and earthquakes are very likely to survive. Put with lightning, for every two persons injured, one is killed. No other natural hazard has such a high death-to-injury ratio.

Most people realize that lightning is a major cause of forest fires — at least 7,500 a year in the nation as a whole, or an average of 20 a day. But lightning has also caused many damaging industrial fires, such as the disastrous oil fire at San Luis Obispo, California, on April 7,1926. The fire lasted five days, spread over 900 acres, and burned nearly 6 million barrels of oil.

The most costly lightning strike in the United States is considered to be the one that hit an ammunition magazine at an army arsenal in northern New Jersey on July 10,1926. It set off a series of explosions. All buildings within 2,700 feet were destroyed and 16 persons were killed. Debris fell as far as 22 miles away. Property damage from this single lightning bolt was placed at $70 million.

The susceptibility of today's interdependent technological society to lightning was dramatically brought home to millions of people on the night of July 13,1977. At 8:37 P.M. lightning from one of a series of thunderstorms that had been sweeping through New York State struck an electrical transmission tower in Westchester County and short-circuited two 345,000-volt lines. There quickly followed a series of equipment malfunctions and at least one serious error by a system operator. When a second lightning bolt knocked out two more lines of the same voltage at 8:55 P.M., the electrical system serving New York City cascaded toward collapse, and by 9:36 P.M., the New York City metropolitan area was plunged into darkness. Subways halted. Elevators stopped. Air conditioners went off. Power was completely restored only after 24 hours.

An extraordinary case of a lightning bolt nearly incapacitating one of the most complex technological systems ever built happened November 14,1969, in Florida. This time the target was not on the ground but in the air. It was a rocket, a Saturn 5, and in a capsule at its top were nestled the three Apollo 12 astronauts, lifting off for the moon. Less than half a minute after the 11:22 A.M. launch, with Apollo 12 barely a mile and a half above the earth, a brilliant bolt of electricity appeared between the rocket and the ground. Virtually all the electrical equipment in the spacecraft suddenly shut down. Row upon row of abruptly opened circuit breakers glowed ominously green. The vital inertial platform, heart of the Apollo guidance system, began drifting. "I don't know what happened here. We had everything in the world drop out, " Apollo 12 Commander Charles Conrad radioed to the ground. Fortunately, within three minutes the crew managed to get all circuits back in operation, and they went on to a successful landing on the moon. But the incident brought about a series of measures to mitigate lightning hazards in space launches and stimulated broad new programs of lightning research.

Lightning is such a powerful and transitory phenomenon that it poses formidable obstacles to our attempts to better understand it. With the spectacular violence it unleashes from the heavens, no wonder the ancients viewed it in supernatural terms, a manifestation of the gods. The divine origin of lightning is a part of the mythologies of Indian, Greek, Roman, Egyptian, Japanese, Chinese, and Tibetan cultures, and of many tribal societies as well.

One of the long-recognized fundamental properties of a thundercloud is that its uppermost part tends to be positively charged, the lower part negatively charged. (There is also often a small area of positive charge within the large area of negative charge at the base of the cloud.) To this day, scientists are arguing about what exactly accounts for this distribution of charge within a storm. As a thunderstorm moves over the landscape, its negatively charged base induces a positive charge over the ground below and extending out for several miles beyond the storm. This induced positive charge follows the storm like an invisible electrical shadow. The stronger the cloud's charge, the stronger the induced charge below. The potential difference may reach as much as 100 million volts. The positive ground current flows up high objects such as trees, hills, and buildings in an effort to establish a connection with the opposite charge in the cloud above. Air, being a poor conductor, however, discourages the connection. Until something else is done, no current can flow.

What is done is invisible to the eye. The charged cloud finds a way to gradually lessen the electrical separation between it and the oppositely charged ground. Early electrical action prepares the atmosphere and forms, in effect, a bridge for the main surge of current to suddenly rush across.

First a small amount of current advances downward toward the ground. This pilot leader blazes the path for a larger but as yet still invisible series of charges that proceed downward in steps. One proceeds several hundred feet, pauses, then another follows down the path and extends it a few more hundred feet. Step after step the charges proceed downward. This step leader, as known, repeats the sequence perhaps as many as 60 times until a conductive path of ionized (electrified) particles is near the ground. Discharge streamers on the ground, usually from some high point, extend upward to complete the conductive channel between the cloud and ground.

With the circuit completed, a tremendous surge of electricity, called the return, or main stroke, leaps upward along the channel at speeds of one-third to half the speed of light. The enormous energy released causes the surrounding atoms and molecules in the air to flow, and this light brilliantly illuminates the previously formed, descending step leaders. It is the illumination coming from this intense, upward return stroke lighting up the downward-pointing zigzagged step leaders that we see as lightning. Thus, although the main electrical surge in a cloud-to-ground lightning flash is upward, our eyes perceive it as downward because the step leaders are pointing downward. The process is a curious trick on our senses, a turning of our perceptions upside down. (The rarer ground-to-cloud-initiated lightning produces step leaders pointing upward, branching toward the sky like a tree.)

Once the return stroke has firmly established the electrical path and itself dissipated, dart leaders may fork down out of the cloud to initiate usually three or four secondary return strokes (the most ever recorded is 26) along the same path. These continue until the electrical difference between the cloud and ground has diminished and brought the opposing sides back into temporary balance.

The whole process from initiation of a pilot leader until fadeaway of the final secondary return strokes may last a second. The visible discharge (the flash) itself, including the return strokes, lasts about two-tenths of a second, and the separate strokes within a discharge have faintly luminous phases each lasting several thousandths of a second and separated by intervals of a few hundredths of a second. The eye can just perceive such individual strokes, and that is why lightning appears to flicker.

The heat generated by a lightning stroke can be as great as 30,000°C (50,000°F) or five times the temperature of the surface of the sun. The sudden expansion of the surrounding air because of this intense heat creates a sound wave which we hear as thunder. A nearby strike of lightning produces a sharp crack of thunder. More distant strikes produce the distinctive low rumble of thunder. The lower pitch is due to the attenuation of higher frequencies. The rumbling results from the sound reaching you at slightly different times because of both the differing distances from you to parts of the lightning channel and the reflection of the sound waves off clouds, hills, and buildings.

For all practical purposes, the lightning is seen at the moment it occurs, but the sound travels just a little more than 1,000 feet a second. So, if thunder is heard five seconds after the lightning is seen, the lightning struck about one mile away.

We all have been startled by a tremendous crack of thunder from a nearby lightning strike, but there is no reason to fear it. If you heard the thunder, the lightning has already struck.

TOP