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.

[Diagram on page 17]

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EARTH-ATMOSPHERE, ELECTRICAL CYCLE

ELECTRON FLOW CYCLE

STRONGLY POSITIVE (DEFICIENCY OF ELECTRONS)

STRONGLY NEGATIVE (SURPLUS OF ELECTRONS)

LIGHTNING

FAIR WEATHER IONIC CURRENT

SLIGHTLY POSITIVE (ELECTRONS REPELLED BY CLOUD)

SLIGHTLY NEGATIVE (SURPLUS OR ELECTRONS)