Lightning is a wonder of nature that occurs as a heavy flash within the same cloud or between two clouds or between the cloud and the earth. But most of them occur within the cloud. These lightning flashes are made by the thunder clouds due to the difference in the potential difference.
A thunder cloud is a type of cloud that has more number of water droplets and ice crystals in it, which are the prone elements for making lightning activities. These elements make the thunder clouds to appear in black in color, because more number of particles in it does not allow sunlight to pass through it, thus making the cloud to appear in a dark color. But the top portion of these thunder clouds will remain bright due to the reflection of sunlight. A potential difference is mainly a voltage that is developed by the various charges present in the clouds.
Charges can be classified into three types namely, positive charge, negative charge and no charge. Lightning occurs mainly due to the positive and negative charge build-up. When there is too much of positive or negative charge at one place, the other charges will make their move towards the place where there is less charge. For example, if there is more concentration of negative charge at one place and a more concentration of positive charge at another place, there will be an imbalance condition created. To balance the condition between these two places, the negative charge (electrons) will be get attracted towards the positive charge and the positive charges will also get attracted towards the negative charge thereby getting neutralized to each other and creating a balanced condition. This neutralizing effect of the positive and the negative charge creates the bombardment effect called lightning. This same effect also happens in the cloud.
This neutralizing effect in the cloud also creates a heavy sound called thunder. This thunder is created due to severe agitation of the air molecules present between two charges. When a lightning bolt is made, a large amount of heat is liberated. This heat in the flash makes the surrounding air molecules to agitate, which causes the sound to be heard as thunder. Actually, both the lightning flash and the thunder occur at the same time, but we hear thunder only after a certain time of seeing the lightning flash. This is because light travels faster than sound. As sound is the movement of air molecules and light is the movement of photons, the sound will always travel at a lesser speed than the light. A single lightning flash contains nearly 1 billion voltages and a current range of 10,000 to 200,000 amps, which is equivalent to the power delivered by a power station in a year. So a lightning strike occurs when there is an imbalance of the charge build-up in the sky or between the sky and the earth.
Mechanism of the lightning stroke:
In order for a lightning stroke to occur, there must be an imbalance in the charge build-up. This charge build-up in the clouds is mainly caused by heavy air currents, ice crystals, and water droplets in the cloud.
When the cloud has an excessive particle build-up like ice crystals and water droplets, it causes severe collisions between them. Usually, the thunder clouds are named as cumulonimbus clouds which will be very high in altitude. The top portion of these clouds remains cool while the bottom remains warm. When the warm air from the surface of the earth rises due to the heating of the sun, it begins to cool, to form clouds, small water droplets and ice crystals. The heavy air currents that flow in the upper area, flow into the clouds and begin to force the small water droplets in the clouds to the top portion. The water molecules begin to cool by colliding with the existing ice crystals and graupel. Graupel is a form of ice crystal that forms when supercooled water freezes on snowflakes. As the water droplets forced by air currents cool partially, the outer surface of the droplet begins to freeze while the inner water will still be warm. This warmer water inside the partially frozen droplet begins to swell up and splinters the ice causing the deposition of positive charge on the upper area and the negative charge on the lower area. The charges with negative matter settle down always due to its huge mass while the charges with positive matter stay afloat always due to its low mass. Thus the positive and negative charges are separated in the cloud.
 |
Positive and Negative charge buildup in the cloud |
When there is so much of charge build-up in the cloud, then there will be an occurrence of lightning within the cloud. But if any of the charges decrease outside the cloud, it causes lightning to occur outside. The negative charge accumulation under the cloud may be in the form of a charge centre also, which means more accumulation of charges at a single point. A cloud may have many charge centres.
At first, the excessive negative charges build- up in the cloud causes the electrons in the ground (Earth) to be get repelled causing it to go deeper, but the positive charges in the earth get attracted by the cloud and come to the upper surface of the ground. This occurs because the same charges always repel each other, while different charges attract each other. This causes a positive charge to build up, on the surface of the earth and a negative charge to build up in the cloud. When there is too much of negative charge in the cloud and too much of positive charge on the surface of the earth, an imbalance condition will be created. When the potential difference between the cloud and the earth is 10kv per cm, a breakdown will occur, thus causing the electrons from the cloud to move towards the ground. The electron movement from the cloud originates as a streamer also called as pilot streamer which you can see in the diagram below.
 |
Streamer |
 |
Ionization |
The produced streamer progress its step as a stepped leader by ionizing the air molecules in the way. Ionization is the process of losing or gaining an electron. During lightning too much of negative charge in the cloud causes the electrons in the air molecules to get separated. These multiple separated electrons from the atoms constitute a streamer. The current in the streamer will be in the order of 100A. As the streamer originates, it begins to remove the electrons from other atoms in the air molecules and makes it conductive. Thus the streamer develops into a stepped leader. The lightning pattern will always be seen in a zig-zag manner. This is because the electrons in the ionization path always took the least resistant path which indirectly means the current always flows where there is a least resistance. So the lightning will be in a zig-zag manner.
 |
Stepped leader |
 |
Positive charge propagation from earth |
As the stepped leader progress towards the ground, it causes the propagation of the positive charge from the ground thus making the conductivity a little faster. Now when the negative charge from the cloud and the positive charge from the earth meet, a return stroke will occur. This causes the positive charge from the earth to flow into the cloud through the ionization path and neutralizing the charge centre in the cloud. The return stroke is so powerful that the current in the stroke will be in the order of 1000Amps to 200000 Amps. This flow of positive charge neutralizes all the negative charges in the charge centre of the cloud which we see as a lightning flash. We see the lightning strike only during the return stroke and not during the stepped leader as it progresses towards the earth.
 |
First Return Stroke |
When a return stroke happens in the cloud one of the charge centre become neutralized; and another charge centre that is near to the neutralized charge centre will create a potential difference between the earth and causes a flashover through the ionized path. This stroke caused by the second charge centre is called dart leader also called as a continuous leader. The dart leader originates from the cloud and progresses towards the earth. When it reaches earth, another return stroke will occur and neutralize the second charge centre. That’s why flickering of the lightning can be observed sometimes, due to the consecutive return strokes.
 |
LEFT: Propagation of Dart leader from the cloud; and positive charge from the ground |
So if a cloud has more charge centres and if it makes lightning, the order will be in such a way that it starts as a streamer and then progress as a stepped leader and then ends as a return stroke. If there is another charge centre in the cloud then a dart leader from another charge centre and a return stroke for that dart leader occurs. The speed of the dart leader is more than that of the steamer and stepped leader because an ionized path is already provided for the charges to flow. Like this way, more number of charges in the clouds is neutralized. This repetition of strokes between the cloud and the earth is called multiple or repetitive strokes. The dart leader is also called a hot lightning stroke because it lasts for a long time even though it constitutes a very small current. But the return stroke lasts for a small duration so it is called a cold lightning stroke. A detailed diagram of the mechanism of lightning stroke is shown below,
 |
Mechanism of Lightning Stroke |
There are generally two types of lightning strokes that can discharge electrical charges. They are,
- Direct stroke
- Indirect Stroke
Direct Stroke:
Direct stroke is a type of stroke in which the lightning stroke from the cloud directly hits an object on the earth. This type of stroke mostly occurs on high buildings and on sharp-edged objects. That’s why lightning arresters are sharp-edged and placed on high buildings to protect the corresponding object from lightning. But the reason for the sharp point in the arresters is that the electric field will be higher in the sharp point.
 |
Direct Stroke |
Indirect Stroke:
Indirect Stroke is a type of stroke in which the lightning stroke from the cloud indirectly hits an object on the earth. Indirect stroke can be classified into two types, namely Side Flash and Ground Current. Side flash occurs, when a lightning bolt strikes an object and travels into another object through air, through the process of ionization. For example, if a human is standing near a tree or a pole, and lightning striking on that object will be meant to travel through that object and gets travelled through the air and attacks the nearby person. This is called side flash.
 |
Side Flash |
The ground current, which is also called as step potential occurs when the lightning strikes on the ground and travels through the ground for a long area. When a person or another object steps into that area the charge will flow into it and gets neutralized. So in both the types of indirect stroke, the person or the object is affected indirectly by lightning. This is how the wonderful and dangerous lightning works.
 |
Ground Current |
Comments
Post a Comment