This is What Science Explains about Atmospheric Electrical Lightning

Much of the knowledge about our Earth’s atmosphere is due to the results of geophysical research carried out during the International Year of Geophysics, which took place in 1957 and involved seventy countries. But before that, it was already known that lightning was an electrical phenomenon and that storms moved in the atmosphere.

The electrical properties of the atmosphere is a subject whose treatment is very complex and requires a multidisciplinary approach, involving sciences such as Geology, Physics and Chemistry.

Let’s address some of that here and clarify the mechanism of lightning formation with an emphasis on the physics of the process.

1️⃣️ The Electrical Conductivity of the Atmosphere

Much that is known today about the electricity of the atmosphere was due to the contribution of Chalmers whose theories led to the understanding of the mechanism of separation of charges in the atmosphere and of atmospheric discharges. The model that is currently accepted is:

• That the earth’s atmosphere is endowed with electrical conductivity;
• Combined with this conductivity, it also demonstrably presents an electric field and a global electric current, the latter maintained with the contribution of storms across the planet.

🌩️The Electric Field of the Atmosphere
For those unfamiliar with the term, an electric field is a physical quantity that mathematically corresponds to the negative of the electric potential gradient.

The potential gradient can be conceptually understood based on the expression,

Figure 1. Distribution of atmospheric electrical potentials. This is a diagram and shows equipotential surfaces resulting from the electric field detected in the Earth’s atmosphere.

Figure 1 represents in a very schematic way the characteristics of the electric field of the Earth’s atmosphere. Each dashed horizontal line represents an equipotential surface. The Earth’s surface represents zero potential. The same figure also shows that the electric field points downwards. At ground level, the potential is considered to be zero (zero volts).

Let’s take an example. For the situation of the lines indicated by points C and D, in the diagram of figure 1, we have:

onde,

In this example, it is important to emphasize that the gradient estimated here is established in the vertical direction. Analyzing Figure 1, it is observed that the potential gradient points upwards (increases upwards), where the electric field, E, is the negative value of the potential gradient, its intensity will be

and vectorally pointing down. This leads to the conclusion that the Earth’s surface is electrically negative.

It is said that the confirmed presence of an electrostatic field led to the scientific conclusion that there is a configuration of electric potentials produced by the superficial distribution of electric charges in the atmosphere, easily understood from Gauss’s law,

Gauss’s law, which can be expressed in a simplified form,

Concomitantly with the presence of an electrostatic field, the presence of an electric current in the atmosphere is also observed. The presence of electric current presupposes the condition of electrical conductivity in atmospheric air. There are therefore two issues in focus:

• what causes its electrical conductivity? and,
• what is its consequence in the presence of the atmospheric electric field?

🌩️The Electrical Conductivity of the Atmosphere
Electrical conductivity is a property that indicates that a certain material has free electrical charges, and that can establish an electric current when subjected to an electric field.

To talk about the origin of the conductivity of the atmosphere, that is, the presence of electrical charges, we first have to talk about cosmic rays

Atomic nuclei produced by stellar explosions with a speed close to the speed of light, when penetrating the Earth’s upper atmosphere, produce, according to physics, effects identical to those produced by particle accelerators, that is, they collide with atoms in the air, removing electrons, producing free electrons.

The atoms reached, in this process, have either a positive resultant electrical charge or a negative resultant electrical charge, that is, they become ions. These atoms, once charged, begin to group into larger particles that move under the effect of the electric field existing in the atmosphere, characterizing the ionization of atmospheric air.

This process is the explanation for the cause of what at the beginning of the study of electricity, in the thirteenth century, was known for the not very well explained effect of when electrically charged spheres discharged spontaneously at a time when air was thought to be an insulator. electric.

With the discovery of X-rays, by Röntgen, a window of knowledge about this behavior in the Earth’s atmosphere was opened, leading physicists, already at the beginning of the 20th century, to compare their electrostatic effect similar to that of X-rays, in terms of ionization of a gas in a glass tube.

As new discoveries were made, physicists concluded that the effect of the ionization of atmospheric air was more intense in the upper layers, leading to the deduction that it was caused by cosmic radiation, that is, coming from outside planet Earth.

Full proof of these cosmic radiations was given by the Pioneer III lunar probe in 1958, which provided data for the discovery of the Van Allen radiation particle belts, consisting of electrons and protons generated in the interaction of cosmic particles with the Earth’s magnetic field (Alonso, et al., 1997 p. 67).

2️⃣️The Global Electrical Circuit

We already know that free electric charges in the presence of an electric field are accelerated, originating an electric current. Once the origin of the conductivity of the air and the presence of a vertical electric field are known, what would be an estimate of the value of the global electric current in the atmosphere?

Figure 2. Schematic diagram for global electrical circuit.

An electric current established by the electric field of the atmosphere would drive the positive ions in the air to move towards the surface of the Earth, which is negative.

According to Feynman (2008 pp. 9–3), the total potential difference from the Earth’s surface to the top of the atmosphere is something around 400000 V (four hundred thousand volts), and the resulting global electric current, given the conductivity of the atmosphere, is approximately 1800 amps.

Researches have shown that, at the same time that an electric current of ions circulates globally from the top to the surface, if, somewhere on Earth a storm is occurring, then in that place, a phenomenon of electricity generation will also be occurring, recharging the top of the atmosphere with positive charges. It is the mechanism of atmospheric storm cells.

🌩️The Atmospheric Storm Cell
It can be said that a tempest, or storm, is a weather state of short duration characterized by strong winds.

According to Feynman (2008 pp. 9–5), a storm is made up of a number of “cells” tightly packed together, but almost independent of each other.

This cell concept has to do with the idea that it comprises a region in which all basic processes take place. The analysis using storm cells led to the characterization of three stages in the storm formation process, which will be described below:

• Stage of development of the cell,
• Stage of maturation; and,
• Advanced stage.

Due to the complexity of the subject and aiming to offer a panoramic view of the general mechanism that occurs in atmospheric clouds, we present a conceptual map for each stage of the process.

🌩 The First Stage of the Storm Cell
This corresponds to the storm cell development stage.

Figure 3. 1st stage mind map. The balloons in the drawing represent the physical agents and the arrows indicate how they combine and clump together to form the initial conditions of the storm.

Due to the warm air, a process of convection and diffusion of warm, humid air is established by physical action.

This, being lighter, rises and, on its way to the top, cools, leading to the formation of vapor droplets (condensation) and, at the same time, drags such drops, increasing their concentration, and at higher altitudes these drops they turn into snow.

This cloud cell is not a closed enclosure and therefore suffers from the contribution of ambient air affluent from the lateral regions of the cloud (storm cell), influencing the process of mitigating the cooling of the ascending air, always keeping it lighter than the adjacent air increasing the upward convective process.

🌩🌩 The Second Stage of the Storm Cell
Corresponds to the mature phase of the storm cell. After the upward movement of air in the storm cell, expansion of its upper part occurs. Its top reaches a height above the general cloud bank (Feynman, 2008 pp. 9–7).

In this phase, the humid steam, brought in the ascending, starts to freeze in the form of ice particles. These particles then fall under the effect of their weight and consequently form a downward air current. As soon as the air descends, the formation of rains begins near the base of the storm.

The characteristic, in this phase, is the cold breeze that disperses when the relatively cold descending air reaches the Earth’s surface and thus, before the presence of rains on the surface, a cold breeze is a harbinger of an approaching storm.

Figure 4. 2nd stage mind map.

At this stage, electrical charges develop within the storm cell, as highlighted in the diagram in figure 4.

⚡THE DEVELOPMENT OF CHARGES WITHIN THE STORM CELL
The existence and behavior of these charges were detected after numerous researches and experiments in the atmosphere. When formed, they acquire such a configuration that it is possible to identify load centers,

• at the top of the storm, with positive charge,
• at the bottom with a negative charge.
• at the bottom, a small region has a positive charge, possibly due to the downward flow of drops from this second stage.

A schematic of this configuration is shown in figure 5.

Figure 5. Distribution of electrical charges in the storm cell. Adapted from Feynman (2008 pp. 9–6).

As can be seen in the drawing in Figure 5, despite the complexity, the charge configuration, predominantly negative at the bottom and the positive charge at the top, form an electric battery with correct polarity to condition the path of negative charges to the Earth’s surface.

The positive charges settle 6 or 7 km up in the air, where the temperature is around -20℃ while the negative charges settle 3 or 4 km high, where the temperature is between zero and -10℃.

The charge at the bottom of the cloud is large enough to produce potential differences of 20 or 30 or even 100 million volts between the cloud and the ground — far greater than the 400000V from the top of the atmosphere to the ground in a cloud at good weather (Feynman, 2008 pp. 9–5).

These large voltages break down the dielectric strength of the air and create massive arc flash discharges. When deterioration of the air insulation occurs, the negative charges present at the bottom of the storm cell are carried to the earth’s surface by lightning.

Due to the accumulation of charges, large differences in potentials appear in the cloud cell that lead to the breakdown of the dielectric strength of the air, causing the formation of independent electrical discharges interclouds and intraclouds, or between a cloud and the Earth’s surface.

In each of the independent discharge sparks, those that are perceived visually, contain about 20 or 30 coulombs of drained charge.

Analyzed from the point of view of electrical circuits, the temporal cell has a characteristic similar to an electrical capacitor, functioning in cycles of charging and discharging.

Taking into account the amount of charge in the range of 20 to 30 coulombs, it takes around five seconds to condition the cloud for a discharge process. Thus, there are approximately 4 amps of current in the lightning generating mechanism (Feynman, 2008 pp. 9–5).

🌩🌩🌩 The Third Stage of the Storm Cell
Corresponds to the advanced phase of the temporal cell.

After the formation of rains carried by the downdraft, the updraft stops, since there is no longer enough warm air to support it.

The downward precipitation continues for a while, the last small drops of water fall and the storm’s churning ends — although there are small ice crystals in the air (Feynman, 2008 pp. 9–8).

Because the winds at very high altitudes are in different directions, the top of the cloud usually spreads out in an anvil shape. The temporal cell then completes its life cycle.

Figure 6. Mind map of the advanced stage of the temporal cell.

Table 1 presents a comparison between the different phases and the corresponding altitudes. The reader will be able to observe in it the progression of the height of the top of the storm cell that expands towards the top of the atmosphere. It is also possible to observe the low temperatures that cause the drops to freeze, which favors the process of rain formation.

Table 1. Synoptic table of the temporal cell according to its stages.

3️⃣Electric Charges and Lightning

The issue of the origin of atmospheric lightning is not something completely closed, but there are explanations in the scientific literature of mechanisms for producing charges and discharges in the Earth’s atmosphere

In general, it is already known that lightning rays in the atmosphere come from the ionization processes that occur there in storms and that thunder comes from part of the thermal energy (kinetic energy) released by lightning rays.

The lightning formation mechanism depends on the following elements:

• atmospheric air current;
• Ions;
• Drops of water;
• Ice particles.

These elements, once present in the storm cell process environment, interact in such a way that leads to the following mechanism:

1. Positive charges are carried upward to the top of the cloud;
2. Negative charges are dumped on the ground in lightning;
3. The positive charges leave the top of the cloud, enter the higher altitude layers of more highly conductive air and spread across the earth;
4. The positive charges leave the top of the cloud, migrate to the upper layers of the atmosphere of highly conductive air and spread from there throughout the planet Earth;
5. In clear weather regions (good weather), the positive charges in this layer are slowly conducted to the ground by air ions — ions formed by cosmic rays, the sea and man’s activities.

⚡C.T.R. WILSON’S THEORY
This theory describes a mechanism of separation of electric charges in water droplets that applied in the situation of storm clouds provides the explanation for the development of electric charges in storms.

In the schematic of figure 7, the sphere represents a droplet of water moving downwards in the storm cloud. Such a drop receives the effect of the electric field E, the one present during good weather, explained earlier.

Figure 7. Schematic configuration of water drop on storm cloud in atmosphere.

The electric field E will induce a state of electric polarization in the drop, the effect of which is the appearance of a distribution of charges, with the top part negative and the bottom positive.

These charges, given that they have a distance, are called electric dipoles, remaining stable in this configuration due to the electrostatic forces induced by the electric field E.

Combined with this, this drop will approach particles present in the atmosphere ionized by cosmic rays, which can be both positively charged ions and negatively charged ions.

Two situations can then occur.

🔎 In the first situation, shown in figure 8, ion A will suffer a repulsion, due to the equal signs of positive charge. But, it can be accelerated to the top of the drop attracted by its negative pole and then abandoned due to the continuous movement of the drop downwards. In this way, this positively charged ion is directed towards the top of the atmosphere.

Figure 8. Falling drop approaching ionized particles. Notes: A — positive ion; B — negative ion. G — water droplet.

🔎 In another situation, the drop approaches the negative B ion, it will be attracted and captured by the drop, as suggested by the drawing in figure 9. The drop will acquire a negative charge — a consequence of the original potential difference across the planet Earth.

Figure 9. Falling drop after negative ion capture.

In this situation, the negative charge will be lowered to the bottom of the cloud by the droplets, and the positively charged ions that are left behind will be blown to the top of the cloud by the various air currents.

⚡THE ELECTRICAL DISCHARGE BRUSH
Once charge separation begins, very large electric fields are established, and at the position of these large fields there can be places where the air will become ionized (Feynman, 2008 pp. 9–11).

Figure 10. Electric discharge brush.

The drawing in figure 10 shows this in a schematic and simplified way: an electron and being pulled by the strong electric field at P, where:

1. The C branch is a path where the electron will collide with air atoms;
2. The C’ branch accumulates more electrons as a result of stronger and chain collisions.

The process is interrupted by the redistribution of the generated positive charges. This process contributes to the stock of ions that will undergo the capture process or electrostatic repulsion.

🌩️ The Lightning
So far we have described the accumulation of electrical charges that conditions the formation of lightning.

Lightning, as we know it, is identified by the flash and the characteristic noise that accompanies it a few seconds after it is perceived.

The flash is due to the light energy released after the electrons in the air are excited in the various collisions that occur in the various stages during the duration of the lightning.

🌩The Lightning Discharge
In describing the dynamics of lightning discharge here, we will give priority to the case of the cloud with a negatively charged base hovering over a flat area.

Due to the accumulation of charge, the electrical potential of the thundercloud becomes much more negative than the potential of the ground below, so the accumulated electrons will be accelerated towards the surface of the ground.

🌩 The Step Leader Electrical Discharge
In the literature on the subject, the knowledge that everything starts with a discharge known as a step leader, shown in the drawing of figure 45, whose light ray is not as bright as the lightning ray, is already widespread.

Figure 11. Formation of step leader discharge components. Notes: B — cloud base. S — ground surface below the cloud. I — Start of step leader unloading. R — secondary discharge branches.

Physical experiments photographing lightning detected that the beginning of its formation takes place from the appearance of a small bright point in the cloud, which moves at very high speed.

Continuing step by step in distances of only about 50 meters with a profile in steps (like a ladder), so that the path thus made resembles the one drawn in Figure 11.

There is evidence that, given the influence of the high intensity of the electric field reached at the level of 100 meters from the ground, the subsequent ionization gives rise to a brush discharge that rises from the ground to reach the ionized channel of the step leader discharge, preceding the contact of this to the ground.

Figure 12 illustrates this situation, the high electric potential gradient in L induces in P a condition for a brush discharge. The displacement of electrons in an avalanche produces a partial flash.

Figure 12. Brush discharge in step leader discharge path. Notes: P — high gradient point induced by the L point. L — walking end of the step leader discharge. Brush discharge at P speeds up the conductive connection between B and S.

⚡THE NEGATIVE CHARGE PRESENT IN THE ION CHANNEL
As a consequence of the movement of negative charges, the air around the step leader discharge path becomes ionized and acquires the property of conducting electricity.

The moment the step leader discharge reaches the ground, we have a negatively electrified connection that goes to the cloud. Condition for downloading the step leader cloud-download system, as described below.

⚡THE RETURN DISCHARGE
Still referring to figure 12, it should be noted that I₂ represents the electrons already at the bottom of the step leader discharge, which are the first to be drained to the surface in S, leaving a positive electric charge behind as shown in the drawing in Figure 13.

Figure 13. Return electrical discharge.

This last effect draws more negative charges from above towards the center of the step leader discharge, which in turn flows into the air towards the surface. Then, finally, all the negative charge in one part of the cloud is depleted along the column, quickly and witheringly.

Thus, the bright ray of light that is seen runs upwards from the ground, as indicated in Figure 13 (legend C).

In fact, this impulse of charges is the main one, it stands out from afar in terms of brightness and is called a return electrical discharge. It is what produces the very bright light and heat that, by causing a rapid expansion of the air, causes the sound waves of thunder (Feynman, 2008 pp. 9–8).

Note that the term return applies more to the visual effect of the lightning flash being drawn in the air from below upwards. That is, the first electrons that go to the ground, and that are close to this, are the first ones that emit the light.

In the charge equalization process, the secondary branches can randomly develop another path to the ground not simultaneously, depending on which branch is closer to the ground, or even very nearly simultaneously and this will be noticed during the lightning flash. If this does not occur, the secondary branch will have its load drained to feed the return discharge.

Figure 14. Motion of negative charges flowing in the lightning path indicated by the trace of light left in the air. Notes: Symbols with letter I represent directions of loads being drained. The thickest stroke represents the part of the bright light that characterizes lightning. Secondary branches also contribute to this unloading step by contributing their own local loads.

The bold line in Figure 14 represents the advance of the discharge lighted line. Given the situation of the charges during the storm that we consider to be present, there is movement of negative ions towards the ground and consequent atomic emission of light waves visually from the bottom upwards in the atmosphere, as the electrons are moving towards low.

4️⃣ How to avoid lightning

The primary way to avoid lightning during thunderstorms is to be in a sheltered location and never under a tree or in a telephone booth.

If, for example, there is a sharp object, such as a building with a spire, then as a step leader discharge approaches that point, the electric fields are so intense that a discharge starts from the sharp point and reaches it (Feynman, 2008 pp. 9–13). Then the lightning bolt tends to reach such a point and this is the working principle of lightning arrester systems.

Lightning arrester systems generally consist of metallic rods of high electrical conductivity, these are interconnected to electrodes, also metallic, buried in the ground. Engineering standards require the use of such protection systems, especially in buildings.

Another very common situation involves urban electrical networks. Generally, power utilities use network protectors to protect themselves from the action of lightning, such as:

• Electric zinc oxide resistors connected in parallel to the grid — they short-circuit the grid directly to the ground when they receive the voltage impulse from the lightning discharge;
• Line filters connected inter-conductors — electronically short-circuit the electrical network, making the lightning dissipate energy in the network itself, avoiding the burning of energy transformers, however, the lightning energy, in this case, can reflect to the residences.

📚 Bibliography

📖Alonso, Marcelo and Finn, Edward J. 1997. Fundamental University Physics. São Paulo: Edgard Blücher Ltda, 1997.

📖Feynman, Richard P. 2008. The Feynman Lectures on Physics: The Definitive and Extended Edition. Porto Alegre : Bookman, 2008.