Atmospheric discharges are one of the most impressive phenomena of nature, which is why, since ancient times, our ancestors, amazed by this phenomenon, associated its origin with an expression of the power of the deities. Thousands of years later, man has been able to unravel the physical processes that give rise to this natural phenomenon, which has allowed the design of systems that mitigate its impacts.

* This article was published in Geociencias SURA Journal | Issue 5 | September 2019.

Throughout human history, different cultures have attributed atmospheric electrical discharges to manifestations of anger or power of their respective deities. This is the case, for example, of the ancient Greeks, Vikings, Buddhists and some ancestral indigenous communities, who associated this type of phenomenon with divine punishments sent by Zeus, Thor, Buddha or certain mythical figures.

It was not until the second half of the 18th century that, thanks to the work of Benjamin Franklin, a physical explanation could be given to this phenomenon, which has the potential to affect material goods and cause the loss of human lives..

What is the origin of electrical discharges?

Electrical discharges generated in the Earth's atmosphere can be caused by volcanic eruptions, extremely intense forest fires, snowstorms or even dust storms. However, most of these phenomena occur in cumulonimbus clouds, also known as thunderclouds. 

These types of clouds are the cause of many natural atmospheric phenomena that represent a threat to material goods and to the lives of people and animals. Hurricanes, tornadoes, torrential rains, hailstorms and electrical discharges originate in storm clouds, which are characterized by having a great vertical development.

Cumulonimbus clouds have strong ascending and descending wind currents inside them, caused by the temperature differences between the Earth's surface and the upper atmosphere. These currents have the ability to move at high speed the ice particles, snow and water droplets contained within the cloud structure, also known as hydrometeors. 

Hydrometeors move chaotically inside the cloud, colliding with each other, which causes them to become electrically charged, as explained by Ph. D. Silverio Visacro, professor and head of the Atmospheric Discharge Research Center at the Federal University of Minas Gerais (UFMG) in Brazil. During these collisions, the largest particles become negatively charged and, due to their size, remain at the base of the cloud due to the force of gravity, while the smallest particles become positively charged and remain in the highest layers of the cloud. 

This charge distribution causes the bottom of the cloud to be negatively charged and the top to be positively charged. The Earth's surface, on the other hand, has a slight negative charge. However, when a storm cloud forms, the negative charge at the base of the cloud is large enough to repel the negative charges on the ground. 

Therefore, the ground and any object that is under or near the storm becomes positively charged, generating an electric field that is the beginning for the discharge to be generated. Given the conditions described, the next step necessary to generate an electric discharge is to establish a connection channel between the opposite charges, since the air is a poor electrical conductor. However, when the difference between the opposite charges is very large, this potential overcomes the resistance of the air, which begins to become electrically charged through a process known as ionization.

During the ionization process, the negative charges, seeking to establish a connection channel with the positive charges, break the insulating capacity of the air and begin to flow freely, forming branches and generating a main path of passage in a process known as stepped leader, in English. 

When the main passage route is about 50 meters from establishing connection with the positive charges, the latter are attracted and also overcome the air resistance, establishing the connection channel and giving rise to the electric discharge, which is also known as return current. 

It may happen that after the main electrical discharge or return current, excess negative charges remain in the cloud, which flow through the same channel established initially, generating a second, third or fourth consecutive discharge, depending on the excess charges in the cloud. In these cases, no ramifications are observed during the process.

Key concepts to understand the phenomenon

• Lightning: name commonly assigned to atmospheric electrical discharges.
• Lightning: refers to the visible energy associated with atmospheric electrical discharge.
• Ceraunic level or ceraunic level: refers to the number of days with storms per year in a certain geographic region. It is used where direct measurements of direct discharges to land are not available. 
• Thunder: is the sound caused by atmospheric electrical discharge. It occurs as a consequence of the heating of the air surrounding the discharge channel.

What are the main types of atmospheric discharges?

Cloud-ionosphere 

Studies carried out in the 80s and 500s identified a range of atmospheric discharges that occur from the top of cumulonimbus clouds towards the ionosphere. This is a layer of the Earth's atmosphere that extends between XNUMX and XNUMX kilometres in altitude and is characterised by large ionisation processes that allow the concentration of free electrons.

Intraclouds 

These are the most common type of electrical discharges in the atmosphere. They occur between two opposite charges in the same cloud. Although they generally occur within the physical limits of the cloud, it is also possible that they sometimes go beyond its surroundings; this is when the branching of the lightning can be seen, as observed in cloud-atmosphere discharges.

Cloud-cloud 

It occurs when there is an electrical discharge between two opposite electrical charges, present in two clouds that are at a certain distance.

Cloud-ground 

They occur when there are transfers of electrical charges between the atmosphere and the earth. Most of these discharges occur from clouds to the earth (downward discharges), but it is also possible for them to occur from the earth to clouds (upward discharges). Although cloud-to-earth discharges are not the most common, they do represent a greater danger to people and property than other types of discharges.

"The geographic distribution of cloud-to-ground discharges is closely linked to the orography and local storm dynamics. However, tropical regions are characterized by a higher frequency of downward discharges, while in extratropical areas, with lower temperatures, there are more upward discharges than in the tropics.". 

Ph. D. Silverio Visacro, expert in atmospheric electrical discharges and research professor at the Federal University of Minas Gerais, Brazil.

What is the global distribution of electrical discharges? 

While there are several measuring instruments to determine the location of an atmospheric electrical discharge, recent advances in remote sensing such as satellites have broadened the spectrum to better determine the global distribution of this phenomenon. 

Satellite sensors detect electrical activity at the top of the cloud using optical or temperature sensors, which measure internal cloud discharges. However, they have the disadvantage of not detecting cloud-to-ground discharges. For the latter type of discharges, there are other more precise instruments such as electromechanical devices, as explained by Ph.D. Silverio Visacro.

Based on satellite sensor records, it is possible to conclude that electrical discharges occur anywhere on Earth. However, the areas where the highest density of discharges is concentrated are located in the tropical region., located between the Tropics of Cancer and Capricorn. 

This region is more exposed to solar radiation than the rest of the globe, which is why its surface temperature is higher. This creates a large difference in temperature with the upper atmosphere, which favors the convective processes that give rise to thunderstorms.

Although tropical areas have a higher number of atmospheric electrical discharges than the global distribution, there are other additional factors - such as regional topography, the shape and length of the coastline or shorelines and the dynamics of local rainfall - that influence the fact that within tropical areas there are regions with a higher density of discharges than others.

An example of the local effects on the density of electrical discharges occurs at the confluence of the Catatumbo River with Lake Maracaibo., in Venezuela, which is known as one of the geographical points where the most activity is recorded, with approximately 250 lightning bolts per square kilometer each year, which is why this phenomenon is widely known in Venezuela. like the “Catatumbo Lightning”.

What are the impacts of electrical discharges?

Atmospheric electrical discharges can have significant impacts on both the safety of people and property. Although it is difficult to establish a reliable figure due to the lack of information and reporting, It is estimated that this phenomenon causes the death of an average of 2000 people each year, according to a report by National Geographic.. 

As for the material costs associated with this phenomenon, it is estimated that in the United States alone, economic losses can exceed eight million dollars per year, according to figures reported by the National Lightning Safety Institute of the United States. There are many ways to classify the material damage that an electric discharge can cause. However, It is possible to group the major impacts into three main categories:

Fire damage: They represent perhaps the greatest threat due to their severity, since, in addition to causing material losses, they can compromise the life and safety of people. Fires associated with electrical discharges can occur after a lightning strike on wooden installations or other types of flammable materials and are generally started by what is known as “hot lightning”. This type of electrical discharge presents a direct current, in which the electricity flows for a longer period of time than a normal electrical discharge, generating the heat necessary to start the fire.

Another source of fires associated with atmospheric electrical discharges occurs when lightning strikes near or directly on structures containing flammable materials. These types of events, in which explosions or release of hazardous substances into the environment occur as a result of a natural phenomenon, are known as Natech (Natural Hazard Triggering Technological Disasters). According to research, about 61% of these events are triggered by atmospheric electrical discharges, with the petrochemical and oil industries being the most affected.

Damage from power surgesLightning strikes rarely directly impact electrical or electronic devices. This type of damage, usually caused by impacts on power lines, causes an increase in the voltage of transmission lines. It can also be caused by induced overvoltages, which produce a drop in voltage. 

In both cases, devices that are connected to a power source at the time of impact may be damaged. Damage to electronic devices can cause other types of damage that result in much greater economic losses when there are industrial processes that depend on the continuous operation of this equipment, such as monitoring systems, ventilation, telecommunications, among others. For this reason, it is important to have adequate protection systems and auxiliary equipment that guarantee continuity in operations. 

Damage associated with shock waves: Atmospheric electrical discharges generate shock waves by heating the air. We perceive these waves as thunder. When they occur at very close distance from a structure, they can be destructive, fracturing concrete, brick, cinder block and plaster walls. In addition, these waves can break glass and create cracks and ditches in the ground.

Protection systems 

Atmospheric electrical discharges can impact structures and facilities and cause damage to nearby areas and connected services. In this regard, Ph. D. Silverio Visacro highlights that, although these phenomena can affect the operation and continuity of businesses, there are external and internal protection systems that can mitigate their impact on structures, facilities and equipment:

External protection systems: This type of system seeks to dissipate or channel the energy of a lightning bolt safely, minimizing damage to people, equipment and structures. 

Internal protection systems: They are used to mitigate the risks that could arise as a result of lightning energy. They consist of devices that regulate overvoltages, which divert the energy that may enter the structure through the conductive elements that supply services, such as communications or metal pipes.

Lightning rods are the most well-known types of protectionIts function is to attract electrical discharges and channel their energy to the ground through a set of elements such as collector points, down conductors and a grounding system. In this way, it is prevented that the discharges directly impact the structure causing damage to it or its contents.

Tall, isolated trees act as natural lightning rodsThis, combined with a phenomenon called “lateral discharge” in which lightning “bounces” and strikes nearby objects, is the reason why one should not seek shelter under these elements during a storm, as explained by Ph. D. Silverio Visacro.

Lightning can occasionally strike wired service networks, inducing overvoltages that can cause damage to electrical and electronic devices if the corresponding internal protection systems are not in place.

Recommendations during a thunderstorm

Atmospheric electrical discharges can represent a latent danger for people when they are caught in a storm while carrying out activities outdoors. Activities commonly associated with the impact of atmospheric discharges on people are:

• Practicing sports in open fields: soccer, golf, mountaineering, cycling, camping, among others.
• Open water activities: sailing in small boats, fishing, swimming.
• Work carried out in open fields: with agricultural machinery, on roads, among others.
• Talking on phone.
• Repair or use of electrical devices.

There are a series of recommendations that should be taken into account before and during a thunderstorm. to reduce the risk of damage from discharges produced:

• Postpone outdoor activities, such as sports practices. 
• Remove loose or dead branches from trees that could fall during the storm and cause damage. 
• Seek shelter inside houses, buildings or cars if you are in the middle of an open field. 
• Avoid taking showers during a thunderstorm, as plumbing and some bathroom fixtures can conduct electricity. 
• Avoid using landline phones. 
• Unplug electrical and electronic devices, such as computers. 
• Avoid approaching natural lightning rods, such as tall, isolated trees in an open area. 
• Do not travel on hills, high points, open fields and beaches. 
• Avoid sheds or small isolated structures in open areas. 
• Do not operate or drive tractors, farm machinery, motorcycles, golf carts and bicycles. 
• If you are near forests, it is recommended to seek shelter in areas where there are small bushes. 
• In open areas, it is advisable to move towards low areas of the terrain, such as small valleys. 
• If you are in open water, it is advisable to go to dry land and seek shelter immediately.

Fonts

• Juan Pablo RestrepoCivil engineer and specialist in Hydraulic Resources from the National University of Colombia. 
• Silverio Visacro SonElectrical Engineer, M. Sc. from the Federal University of Minas Gerais (UFMG), Belo Horizonte, and Ph. D. from the University of Rio de Janeiro.