Lightning—Awesome Force in the Sky!
CONSIDER, if you will, one of the most awesome displays of unbridled power in the world—the thunderbolt! Most people, at one time or another, have experienced an electrical storm, with all its frightening aspects: torrents of rain, blinding flashes of light, crashing thunder and that anxious anticipation of the next strike.
Would you like to learn more about what causes this mysterious electrical phenomenon in the sky? What goes on during a thunderstorm that generates such terrific awe-inspiring forces? Since lightning is an electrical manifestation in the atmosphere, we need to know something about the electrical properties of air to understand where lightning originates.
An Electric Atmosphere
We are not usually aware of it, but the atmosphere in which we live is highly charged electrically. The electric potential of the atmosphere is indeed astonishingly great. In clear weather, at the surface of the ground the potential goes up, on the average, about 150 volts per meter (45 volts per foot). The air is positive with respect to the ground, and the greater the altitude, the greater the voltage.
This means that if you are standing in the open, away from buildings and trees, the air at your head level may be 250 volts above that at the ground level. Why, then, do we not feel the effects of this voltage? A person could be electrocuted at a voltage this high, with sufficient current, but we do not feel even a tiny spark. The reason is that air is such a good insulator. Our skin is a comparatively good conductor of electricity, and it maintains our body at an even potential. Only with very sensitive instruments, carefully insulated and shielded from other things that might carry an electrostatic charge, can the atmospheric potential be measured.
If the potential continues to rise at this rate, up only a hundred meters (328 feet), it would be 15,000 volts. There is, however, a limit to the potential set by the circumstance that at high altitudes, above the stratosphere, air becomes a conductor. What causes this difference—that the same air that is such a good insulator at the ground becomes a good conductor high in the sky? The answer lies in the phenomenon of ionization.
Air molecules, either of nitrogen or of oxygen, ordinarily are neutral. This means that the positive charge on each atomic nucleus is exactly balanced by the negative charges of the electrons around the nucleus. But if one of the electrons is removed from its orbit, it leaves the molecule with a positive charge. Then we say the molecule is ionized. Or, for short, it is an ion.
This action of ionization may result from various causes, but in the clear, lower atmosphere the chief agent is the cosmic rays that bombard us from outer space. High-energy particles strike the air molecules with such force that electrons are knocked loose, leaving positive ions. The free electrons may attach themselves to other molecules, forming negative ions. At levels as low as fifty kilometers (30 miles), enough ions are produced to make the air a good conductor.
We call this conducting layer of air the electrosphere. This has sometimes been included in the ionosphere, but this latter name is properly applied to the higher layers, above a hundred kilometers (60 miles), which reflect radio waves.
Now, the ground is also a good conductor. In this case, the current is carried by ions in solution in groundwater. Any mineral in solution in water is in the form of ions. Thus, common salt gives positive sodium ions and negative chloride ions. Gypsum forms ions of calcium and sulfate. All groundwater contains more or less dissolved mineral, and even fairly dry earth still has some moisture. So even though a small clump of earth might not carry much current, the earth’s crust is so vast that, all together, it is an excellent conductor.
All parts of a good conductor must be electrostatically at the same potential. If something happens to raise the potential at one point, current will flow from there to parts of lower potential until it is equalized. This is true of the earth. It is also true of the electrosphere. But the lower atmosphere is an insulator that separates the two. This makes it possible to maintain the great potential difference between them. In fact, this system forms a giant electric condenser, in which the earth is negative and the electrosphere is positive. The potential across the atmosphere averages about 300,000 volts. It varies considerably from this figure from hour to hour during the day, and from month to month during the year.
Nothing is a perfect insulator. With sufficiently sensitive instruments, a tiny current can be detected even in the lower atmosphere. It is slightly conductive because of the few cosmic rays that penetrate to the ground. The earth has a surplus of electrons, and these are constantly leaking away from a multitude of points on the surface. Such point discharges occur at the ends of leaves on trees, at the tips of blades of grass, and even from sharp corners on grains of sand. Man-made structures, which stand higher in the air, compress the electric field around their peaks and roof corners, and the discharge of electrons is concentrated at such points. Earth wide, these tiny discharges add up to enough total current that they could completely discharge the earth to the electrosphere in less than an hour. There must be, then, some charging mechanism to maintain the surplus electrons on the earth. And this is where lightning comes into the story.
The Thunderstorm as a Generator
We see many types of clouds in the sky. Most of them are more or less flat and horizontal. But those that most excite our admiration are the beautiful white cumulus clouds, billowing up high into the blue sky like giant cauliflowers. Under the right weather conditions, a large cumulus cloud keeps on growing, rising thousands of meters toward the stratosphere at the same time that it broadens its base. Thus it becomes a cumulonimbus, or thunderhead. When fully developed, its top is blown out into a plume forming the familiar anvil head. It is still beautiful at a distance, but to someone below the thunderhead, it is now a dark, threatening cloud mass. Soon torrents of rain, sometimes with hail, drench the earth beneath.
This is the kind of cloud that generates lightning bolts and peals of thunder. It is like a gigantic electric generator in the sky, towering up from eight to eighteen kilometers (5 to 11 miles) high, and covering an area as much as 3,000 square kilometers (1,150 square miles). There are violent updrafts and downdrafts within the cloud, driving water drops and ice crystals along at speeds of forty to a hundred kilometers (25 to 60 miles) per hour. Innumerable particles of rain, ice, sleet and hail are heaving up and down, while the cloud twists and turns, billows and swells.
Of course, gravity keeps tugging at the water and the ice, and somehow, in the friction so generated, electrons and ions are torn apart at the interfaces between air, water and ice. The charges are separated by the rushing winds. These carry positive charges to the top of the cloud while raindrops with negative charges slip through to the bottom. The potential difference between top and bottom keeps increasing as the cloud matures. Finally it is “bursting at the seams” with a tremendous excess of charge. Madly the cloud seeks some way to get rid of the hundreds of millions of volts it has churned up within itself. The insulating quality of the air can withstand only so much electrical pressure. It finally breaks, and a blinding flash of lightning dramatically relieves the stress.
At any given time, it is estimated that there are around 3,000 thunderstorms in progress all over the earth. Most of these take place over the land.
Much of the lightning occurs within the cloud itself, but the negative charge built up at the bottom of the cloud so overwhelms the normal potential of the earth that lightning flashes also to the ground, carrying electrons to the earth. When the cloud dissipates, the positive charge in its top finds its way into the electrosphere. Then, in fair weather, positive ions leak through the atmosphere to the earth to neutralize its negative charge, and negative ions rise into the electrosphere to neutralize it. So the cycle is completed.
How a Lightning Bolt Forms
It is difficult to study lightning within the cloud; it is not a very comfortable environment for the scientist and his delicate instruments. But lightning to the ground can be seen and photographed with high-speed cameras, and from this scientists have learned much about the progressive build-up of a lightning flash. Here is the picture that emerges.
From laboratory studies of electrical breakdown of air, it is known that a lightning flash begins when the electric field reaches a strength of about three million volts per meter (75,000 volts per inch). What happens is that the few electrons that are always being freed by cosmic rays are pushed hard enough at this voltage that they knock other electrons out of the neutral molecules that they strike. These, in turn, are accelerated, collide with new molecules, and ionize them. Thus a veritable avalanche of electrons builds up, moving away from the negative charge in the cloud and leaving a trail of positive ions behind. This weakens the resistance of the air and pierces a path for the developing stroke of lightning through the insulating blanket.
Cameras designed to stop the action in times measured in millionths of a second (microseconds) show that this is a step-wise action. A “stepped leader” breaks out the cloud at a point where the air is momentarily weaker in resistance, and the electron avalanche advances some fifty meters (160 feet). Then it “runs out of breath,” so to speak, and pauses momentarily while the potential builds up at its tip. After some fifty microseconds, it again breaks out, perhaps in a different direction, according to the local resistance of the ionized air. Thus, step by step, the successive leaders open a path of strongly ionized air, one to ten meters (3 to 30 feet) wide, toward the earth.
The air being more ionized in some places than in others, the surging path of the leader twists and turns to take advantage of every favorable variation. That is how lightning gets the familiar forked appearance, as it shoots out in one direction or another, exploring various branches, always seeking the easiest path to the earth. When it comes closer than fifty meters (160 feet) to its goal, a streamer reaches up from a favorable point on the ground to meet it. Now the circuit is complete! The cloud has a pipeline through which to unload its unbearable burden of surplus electrons.
First, those electrons in the channel nearest the ground surge through, followed immediately by those pressing above. So the return stroke, now brilliantly glowing, reaches up toward the cloud with a speed approaching that of light itself. Whereas it might have taken the leaders 20,000 microseconds to reach the ground, the return stroke makes the trip in a mere seventy microseconds. Now for perhaps forty microseconds, the cloud discharges a current of 10,000 to 20,000 amperes or more. For this brief moment it generates a power of thousands of millions of kilowatts—more power than all the electric power plants in the earth combined. Truly, it is an awesome display of power!
The stroke quickly dies away, but that is rarely the end of the action. The path of the lightning bolt through the air remains, still intensely ionized. Other parts of the cloud that are still highly charged flash across to the region that has discharged, and this continues down the channel still open to the earth. Thus there are usually three or four successive strokes, repeated so fast that they look like a single flash. Sometimes it takes more than a dozen strokes to drain the cloud of its charge.
Now, in only a fifth of a second, the lightning flash has finished its work. ‘It’s all over but the shouting,’ as the saying goes. The shouting, in this case, is the thunder. You may hear a clap, a roll, or a rumble, depending on how far you are from the lightning. A narrow, tortuous cylinder of air only a few centimeters (about an inch) thick in the path of the lightning has been heated to more than 30,000 degrees Celsius (55,000° F.). As soon as the current dies away, this superheated column of air expands explosively, at supersonic speed. The shock wave from this expansion makes the thunder, which may be heard up to twenty-five kilometers (15 miles) away.
You may wonder why the Creator saw fit to make lightning in the clouds. Does it do any good? Indeed it does. It plays a paramount role in the nitrogen cycle in nature. Nitrogen is essential to life, and there is a vast reservoir of it in the atmosphere. But living creatures cannot use it directly. In the lightning bolt, however, the intense heat splits both nitrogen and oxygen molecules into atoms, and as they cool, many combine to form oxides of nitrogen. These compounds dissolve in rain and are carried into the soil. There, converted into nitrates, they provide a vital fertilizer for growing plants. This is a major process for the natural fixation of nitrogen. It is estimated that hundreds of millions of tons of nitrate are provided every year by thunderstorms.
Living with Lightning
You really do have reason to feel uneasy when lightning is on the loose. It has tremendous destructive potential. Lightning splinters trees and telephone poles, punches holes in roofs and walls, and starts many forest fires and building fires. Often, in a tree, the electric current is so intense that it instantly vaporizes the moisture in the wood and the superheated steam literally blows the tree to bits.
Obviously, too, lightning can kill. Animals seeking shelter under a tree during a thunderstorm are often electrocuted when lightning strikes the tree. People frequently suffer the same fate, especially on beaches and golf courses. Lone trees in such locations offer likely targets to lightning. If you are caught out in a thunderstorm, do not seek shelter under an isolated tree. In the woods, stay away from tall trees. And avoid wire fences, pipelines and railroad tracks. You are safer in a valley than on a hilltop.
If you live in an area of frequent electrical storms, you may be wise to protect your house with lightning rods. To be effective, they must be well grounded. Pointed rods connected through heavy wire (insulated from the building) to a well-buried metal cable or plate will attract the lightning and conduct it harmlessly to the ground. Television antennas and electric power lines leading into the house can be protected with lightning arrestors.
If you are in an automobile or on a train during a thunderstorm, you have nothing to worry about. The metal body of the car around you distributes the electric current and conducts it to the ground. Likewise, the occupants of an airplane are safe from lightning. Planes are struck not infrequently, sometimes emerging with small holes pierced in the metal skin, but no instance has been reported of an airplane crash being caused directly by lightning. Of course, the violent wind turbulence of thunderstorms presents a peril to which a pilot wisely gives wide berth.
By taking these precautions, the next time a thunderstorm strikes your area you can relax and enjoy this magnificent display of the Creator’s power. And knowing something about how lightning works should heighten your appreciation of that awesome force in the sky.
[Chart on page 20]
A Typical Lightning Bolt
Length: 3 miles (5 kilometers)
Strokes per Bolt: 3 or 4
Peak Current: 20,000 amperes
Voltage: 100,000,000 volts
Peak Power: 2,000,000,000 kilowatts
Duration: 1/5 second
[Diagram on page 17]
(For fully formatted text, see publication)
EARTH-ATMOSPHERE, ELECTRICAL CYCLE
ELECTRON FLOW CYCLE
STRONGLY POSITIVE (DEFICIENCY OF ELECTRONS)
STRONGLY NEGATIVE (SURPLUS OF ELECTRONS)
FAIR WEATHER IONIC CURRENT
SLIGHTLY POSITIVE (ELECTRONS REPELLED BY CLOUD)
SLIGHTLY NEGATIVE (SURPLUS OR ELECTRONS)