Source : link
How is lightning formed?
Lightning is formed basically because of the electric charges in the clouds. The clouds are one of the most fascinating objects of the Mother Nature to observe at. They do not have a precise form. It is also interesting to watch them move. What makes them move? It’s the wind on the earth’s troposphere that makes the clouds move.
What is a cloud exactly?
A cloud is nothing, but is full of water in all of its three states i.e.
Water droplets (liquid)
Water vapor (gas)
Now, what is a graupel? Meteorologists believe that the graupel is the major contributing factor to the charges present in the clouds. Graupel is water in its frozen state or in other words water in its solid state. Then why not ice or snow, why is it given a special name? That is because, the meteorologists believe that these frozen particles in the clouds measure less than 50um. Though they are a solid matter, they cannot be seen even with a microscope. They are visible only under a powerful electron microscope. Hence they cannot be categorized as a form of ice or snow and hence the special name GRAUPEL. In the process of water cycle, moisture gets accumulated in the atmosphere. This accumulated water in all its three states forms the cloud. The water particles in the cloud are at constant motion and get collided with each other when they move rapidly. During their collisions, electrons are knocked off and thus creating a charge separation. It is also observed that the lower portions of the clouds are always negatively charged and the upper portions are always positively charged. This is primarily because of the presence of the graupels. But how do these graupels create such a standard charge separation with the negative charges always at the bottom still remains a mystery!!!
Cloud – A Giant Capacitor
As the lower portions of the clouds become negatively charged, equal positive charges are created at the earth’s surface (because of induction of charges). Hence, now the positive charges on the earth’s atmosphere and the negative charges in the clouds are now separated by air. Hence, it’s like a capacitor with air as dielectric. The charges keep building. When the electric field created by these charges is strong enough to break the air, the negative charges from the clouds move toward the positive charges on the earth’s surface. In other words, a lightning has stuck!!
Ionization of air
When the negative charges are formed at the lower surface of the clouds, the elements of the air immediately next to the cloud particles gets ionized i.e get separated into electrons and positive ions. Then the elements next to these ions get ionized. This is called induction of charges and the end result of this process is the creation of positive charges on the earth’s surface. Now, the air between the clouds and the earth’s surface i.e. the air between the charges are said to be ionized. Ionization of air means that the electrons are now freer to move than the previous state, the neutral state, i.e. the state before ionization. The ionized air is known as plasma. The air is the conductive path for the electrons to descend from the clouds to the earth’s surface.
Once the ionization process is begun and plasma is formed, a path is not formed right away. In fact, many paths of ionized air stem out from the cloud. there are usually many separate paths of ionized air stemming from the cloud. But the path with the least resistance wins. The electrons flow in that path which has least resistance. The path can be from cloud to cloud or cloud to ground. The path chosen by the electrons is called step leaders. It is because of this reason we do not see lightning as a single line but it looks like root. The electrons choose their paths in all direction, with the only criteria which has the least resistance. The air is not ionized equally in all directions. Dust or impurities (any object) in the air may cause the air to break down more easily in one direction, giving a better chance for the step leader to reach the earth in that direction.
As the step leaders are being developed and are in the way approaching the earth’s surface, the electric potential on the earth’s surface starts to increase. And therefore, electric lines of flux start from the earth’s surface too as the step leaders start from the clouds. These lines of fluxes are called Streamers. When the step leader and the streamer meet the lightning is said to have been reached the earth’s surface or in other words lightning is stuck !!! Also, it has to be noted that streamers do not occur in all the lightnings striking the earth’s surface
Source : link
It occurs only with the heavily charged clouds. In the above figure a streamer can be seen clearly. The bright line from the earth’s surface to the cloud the streamer and a little less bright line from the cloud to the earth’s surface is the step leader.
Why do we hear a sound (thunder) when lightning occurs?
Heat is always associated with electric current. Since there is an enormous amount of current in a lightning strike, enormous amount of heat is also associated with it. The heat generated is so large that a bolt of lightning is hotter than the surface of the sun.The white-blue flash that we see, is actually because of the heat generated. This heat energy makes the air around the step leader extremely hot. Immensely hot, that the air explodes as the heat energy expands the air. This explosion is what we hear as thunder.
Why is thunder heard after lightning?
Lightning and thunder actually occur at the same time. Light is very faster than sound. The velocity of light is 300,000,000 m/sec and that of sound is 331 m/sec. Hence, light reaches first and is followed by the sound(the thunder)
The lightning, results in heating up the atmosphere. The lightning can actually heat the area in the general vicinity to 20,000 degrees Celsius! (This is 3 times the temperature of the surface of the sun). Hence, the buildings on the earth’s surface can easily be damaged and can even cause death to the living beings. Hence, we have to protect our self against lightning. The device used to protect us against lightning is called lightning arrester.