How is lightning produced?

If we apply a high voltage between two electrodes, they will be closer together. When two electrodes are close to a certain distance, there will be an electric spark between them, which is called "arc discharge".

The lightning generated by thunderstorm clouds is very similar to the arc discharge mentioned above, except that lightning is fleeting, but the spark between electrodes can exist for a long time. Because the high voltage between the two electrodes can be artificially maintained for a long time, it is difficult to replenish the charge in the thunderstorm cloud immediately after discharge. When the accumulated charge reaches a certain amount, a strong electric field is formed between different parts of the cloud or between the cloud and the ground. The average electric field intensity can reach several thousand volts/cm, and it can be as high as 10000 volts/cm in some areas. Such a strong electric field is enough to break through the atmosphere inside and outside the cloud, so dazzling flashes are excited between the cloud and the ground or between different parts of the cloud and between different clouds. This is what people often say about lightning.

Seeing lightning with the naked eye is very complicated. When the thunderstorm cloud moves somewhere, the middle and lower part of the cloud is the center of strong negative charge, and the underlying surface opposite the cloud bottom becomes the center of positive charge, forming a strong electric field between the cloud bottom and the ground. With more and more charges and stronger electric field, a section of air column with strong atmospheric ionization first appears at the bottom of the cloud, which is called cascade leader. This ionized gas column extends to the ground step by step. The leader of each step is a dim light beam with a diameter of about 5 meters, a length of 50 meters and a current of about 100 ampere. It extends to the ground step by step at an average high speed of about 150000 m/s. When it was about 5-50 meters from the ground, the ground suddenly counterattacked. The channel of counterattack is from the ground to the bottom of the cloud, along. The return stroke galloped from the ground to the bottom of the cloud at a higher speed of 50,000 km/s, emitting an extremely bright beam, which lasted for 40 microseconds, and the current passed through exceeded 10000 amps. This was the first lightning strike. A few seconds later, a dim light beam from the cloud, carrying huge current, flew to the ground along the path of the first lightning strike. This is the so-called direct channeling pilot. When it is about 5-50 meters away from the ground, the ground strikes back again, forming a bright beam, which is the second lightning strike. Then, like the second time, there were the third and fourth lightning strikes. Usually 3-4 lightning strikes constitute a lightning process. A lightning process lasts about 0.25 seconds. In this short time, a huge amount of electric energy will be released in the narrow lightning channel, which will form a strong explosion, produce shock waves, and then form sound waves to spread around. Is this thunder or "thunder"

The structure of lightning

What has been studied in detail is linear lightning, so let's take it as an example to talk about the structure of lightning. Lightning is a pulse discharge phenomenon in the atmosphere. Lightning consists of multiple discharge pulses, and the interval between these pulses is very short, only a few hundredths of a second. Pulse after pulse, and the subsequent pulse follows the path of the first pulse. Now it has been clearly studied that each discharge pulse consists of a "leader" and a "counter-attack". Before the first discharge pulse bursts, there is a preparation stage-"step-by-step" discharge process: under the impetus of strong electric field, the free charge in the cloud moves quickly to the ground. During the movement, electrons collide with air molecules, which makes the air slightly ionized and glow. The leader of the first discharge pulse spreads down step by step, like a glowing tongue. At first, the smooth tongue was only a dozen meters long. After a few thousandths of a second or even less, the smooth tongue disappeared. Then, on the same paragraph, a longer light tongue (about 30 meters long) appeared and disappeared in the blink of an eye; Then a longer smooth tongue appeared ... the smooth tongue approached the ground step by step in a "gnawing" way. After many times of discharge-disappearance, the smooth tongue finally landed. Because the leader of the first discharge pulse propagates from the cloud to the ground step by step, it is called "step leader". On the channel of the light tongue, the air has been strongly ionized, and its conductivity has been greatly increased. The process of continuous ionization of air only occurs in a narrow channel, so the current intensity is very high.

When the first pilot, the ladder pilot, reached the ground, a large amount of charge immediately flowed from the ground to the cloud through the highly ionized air channel. This current is so strong that the airway fires and a winding and slender light beam appears. This stage is called the "counter-offensive" stage, and also called the "main force expulsion" stage. The ladder pilot and the first counterattack constitute the whole process of the first pulse discharge, which lasts only one hundredth of a second.

740) this.width = 740 "border = undefined > After the first pulse discharge process, the second pulse discharge process only happens after a very short time (4 seconds). The second pulse also starts from the pilot and ends at the return trip. However, after the first pulse discharge, "the ice has broken and the route has been opened", so the pilot of the second pulse will not go down step by step, but directly reach the ground from the cloud. This kind of pilot is called "direct channel pilot". After the direct pilot reaches the ground, it takes about a few thousandths of a second to counterattack and end the second pulse discharge process. Then the third and fourth ... straight lead and return strokes, completing multiple pulse discharge processes. Because each pulse discharge will consume a lot of accumulated charges in the thunderstorm cloud, the future main discharge process will become weaker and weaker, and the pulse discharge will not stop until the charge reserve in the thunderstorm cloud is exhausted, thus ending a lightning process.

The cause of lightning

The atmospheric electric field in thunderstorm is obviously different from that in sunny days. The reason for this difference is the accumulation of charges in thunderstorm clouds, which forms the polarity of thunderstorm clouds, produces lightning and causes great changes in atmospheric electric field. But how does thunderstorm cloud get electricity? That is to say, what are the physical processes in the thunderstorm cloud that cause it to be charged? Why can so many charges accumulate in thunderstorm clouds and form a regular distribution? This section will answer these questions. As we said before, the macroscopic process of thunderstorm cloud formation and the microphysical process in thunderstorm cloud are closely related to cloud electrification. Scientists have made a lot of observations and experiments on the charging mechanism of thunderstorm clouds and the regular distribution of charges, accumulated a lot of data and put forward various explanations, some of which are still controversial. To sum up, the power-on mechanism of the cloud mainly includes the following types:

A. the hypothesis of "ion flow" in the initial stage of convective clouds

There are always a lot of positive ions and negative ions in the atmosphere. On the water droplets in the cloud, the charge distribution is uneven: the outermost molecules are negatively charged, the inner layer is positively charged, and the potential difference between the inner layer and the outer layer is about 0.25 volts. In order to balance this potential difference, water droplets must "preferentially" absorb negative ions in the atmosphere, which makes water droplets gradually negatively charged. When the convection begins, the lighter positive ions are gradually carried to the upper part of the cloud by the updraft; However, the cloud droplets with negative charges are relatively heavy and stay in the lower part, which leads to the separation of positive and negative charges.

B. Charge accumulation in cold clouds

When the convection reaches a certain stage and the cloud reaches a height above 0℃, there are supercooled water droplets, graupel particles and ice crystals in the cloud. This cloud, which is composed of water vapor condensate with different phases and the temperature is lower than 0℃, is called Leng Yun. Leng Yun's charge formation and accumulation process is as follows:

A. Friction and collision charging between ice crystals and graupel particles

Polonium particles are composed of frozen water droplets, which are white or milky white and have a brittle structure. Because supercooled water droplets often collide with it and release latent heat, its temperature is generally higher than that of ice crystals. Ice crystals contain a certain amount of free ions (OH- or OH+), and the number of ions increases with the increase of temperature. Because of the temperature difference between the contact part of graupel and the ice crystal, the free ions at the high temperature end must be more than those at the low temperature end, so the ions must migrate from the high temperature end to the low temperature end. In the process of ion migration, the lighter positively charged hydrogen ions are faster, while the heavier negatively charged hydroxide ions (OH-) are slower. Therefore, in a certain period of time, the phenomenon of excess H+ ions at the cold end appeared, which led to negative polarization at the high temperature and positive polarization at the low temperature. When ice crystals come into contact with graupel particles and separate, the graupel particles with higher temperature are negatively charged, while the ice crystals with lower temperature are positively charged. Under the action of gravity and updraft, the lighter positively charged ice crystals are concentrated in the upper part of the cloud, while the heavier negatively charged haze particles stay in the lower part of the cloud, resulting in the Leng Yun being positively charged in the upper part and negatively charged in the lower part.

B, supercooled water droplets collide with graupel particles to freeze and generate electricity.

There are many water droplets in the cloud that will not freeze when the temperature is below 0℃. This kind of water drop is called supercooled water drop. Supercooled water droplets are unstable. Just shake it a little and it will freeze into ice particles at once. When supercooled water droplets collide with graupel particles, they will freeze immediately, which is called collision freezing. When the collision occurs, the outside of the supercooled water droplets immediately freezes into an ice shell, but the inside of the supercooled water droplets temporarily remains liquid. Because the latent heat released by the external freezing is transferred to the inside, the temperature of the liquid supercooled water inside is higher than that of the external ice shell. The temperature difference makes the frozen supercooled water droplets positively charged outside and negatively charged inside. When the inside also freezes, the cloud drops expand and split, and the outer skin breaks into many small positively charged ice chips, which fly to the upper part of the cloud with the airflow. The core of the negatively charged frozen drops adheres to the heavier graupel particles, so that the graupel particles are negatively charged and stay in the middle and lower part of the cloud.

C. Water drops are charged because they contain thin salt.

In addition to the above two electrification mechanisms in Leng Yun, it has been suggested that the electrification mechanism is due to the thin salt contained in water droplets in the atmosphere. When the cloud drops freeze, the crystal lattice of ice can accommodate negative chloride ions (Cl-) but repel positive sodium ions (Na+). Therefore, the frozen part of water droplets is negatively charged, and the unfrozen outer surface is positively charged (water droplets are frozen from the inside out). In the process of falling, the graupel particles frozen by water drops fall off from the surface water before freezing, forming many small clouds with positive charges, while the frozen core is negatively charged. Due to the separation of gravity and airflow, positively charged water droplets are carried to the upper part of the cloud, while negatively charged polonium particles stay in the middle and lower part of the cloud.

D. charge accumulation in warm clouds

The above mentioned some main mechanisms of power generation in Leng Yun. In the tropics, some clouds are above 0℃, so they only contain water droplets and no solid water particles. This kind of cloud is called warm cloud or "water cloud". Lightning can also appear in warm clouds. In the thunderstorm clouds in the mid-latitude region, the part of the cloud below the 0℃ isotherm is the warm region of the cloud. There is also an electrification process in the warm area of the cloud.

In the process of thunderstorm cloud development, the above mechanism may play a role in different development stages. However, the main electrification mechanism is still caused by the freezing of water droplets. A large number of observation facts show that only when the cloud top presents a fibrous filament structure can the cloud cluster develop into a thunderstorm cloud. Aircraft observation also found that there are a large number of cloud particles mainly composed of ice, snow crystals and graupel particles in thunderstorm clouds, and the accumulation of a large number of charges is the rapid charging mechanism of thunderstorm clouds, which can only occur through collision, freezing and friction during the growth of graupel particles.

Strange lightning.

Lightning has several shapes: the most common linear (or dendritic) lightning and flaky lightning, and spherical lightning is a very rare lightning shape. If carefully distinguished, it can also be divided into strip lightning, beaded lightning and rocket lightning. Linear lightning or dendritic lightning is a lightning shape that people often see. It has dazzling light and very thin light. The whole lightning is like a branch hanging horizontally or downward, and it is like a river with many tributaries on the map.

The difference between linear lightning and other discharges is that its current intensity is particularly large, reaching tens of thousands of amperes on average and 200,000 amperes in a few cases. Such a large current intensity. It can destroy and shake trees and sometimes hurt people. When it comes into contact with buildings, it often causes "lightning strike" and fire. Linear lightning is mostly cloud-to-ground discharge.

Flake lightning is also a common lightning shape. It looks as if there is a flash of light on the cloud. This kind of lightning may be the background light of the invisible spark discharge behind the cloud, or the diffuse light generated by the lightning in the cloud being blocked by the cloud droplets, or it may be a cluster or flickering independent discharge phenomenon appearing in the upper part of the cloud. Flake lightning often occurs when the intensity of clouds weakens and precipitation tends to stop. It is a weak discharge phenomenon, mostly in the cloud.

Although spherical lightning is a very rare lightning shape, it is the most striking. It is like a fireball, and sometimes it is like a blooming "hydrangea" chrysanthemum. It is about the size of a human head, and occasionally it is several meters or even dozens of meters in diameter. Spherical lightning sometimes swims slowly in the air, and sometimes it hangs completely still in the air. It sometimes emits white light, and sometimes it emits pink light like a meteor. Ball lightning likes to make holes. Sometimes, it can enter the house through chimneys, windows and cracks, turn around the house and then slip away. Ball lightning sometimes hisses and then disappears with a muffled sound; Sometimes it just makes a faint crack and disappears unconsciously. After the ball lightning disappears, some unpleasant gas smoke may be left in the air, which is a bit like ozone. The life history of ball lightning is not long, about a few seconds to a few minutes.

Strip lightning. It consists of multiple continuous discharges. Between each lightning, due to the influence of the wind, the lightning path moves, making each individual lightning close to each other and forming a band. The belt width is about 10 meter. If this lightning strikes a house, it will immediately cause a large area to burn.

Beaded lightning looks like a connecting line that slides on the cloud curtain or throws it to the ground through the clouds, and it also looks like a sparkling pearl necklace. Some people think that bead lightning seems to be a transitional form from linear lightning to spherical lightning. Beaded lightning often follows linear lightning with almost no time interval.

Rocket lightning is much slower than other kinds of lightning, and it takes L- 1.5 seconds to fully discharge. Its activities are easy to track and observe with the naked eye.

People can observe various shapes of lightning with their own eyes. However, to observe lightning carefully, it is best to take pictures. High-speed camera can not only record the shape of lightning, but also observe the development process of lightning. Using some special cameras (such as mobile phone cameras), we can also study the structure of lightning.